Oxidation Kinetics, Phase Evolution, and Surface Characteristics of Ti-35Nb-6Mo β-Titanium Alloy at Elevated Temperatures

Oxidation Kinetics, Phase Evolution, and Surface Characteristics of Ti-35Nb-6Mo β-Titanium Alloy at Elevated Temperatures | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Oxidation Kinetics, Phase Evolution, and Surface Characteristics of Ti-35Nb-6Mo β-Titanium Alloy at Elevated Temperatures Jarnail Singh, Vicente Amigó Borrás, Rajat Dhawan, Amarjit Singh This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9319174/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract This present study investigates the high-temperature oxidation behavior and microstructural evolution of Ti-35Nb-6Mo β-titanium alloy at 600°C, 700°C, and 800°C for exposure durations from 0.5 to 72 hours. A comprehensive experimental approach combining gravimetric analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), focused ion beam (FIB) cross-sectional analysis, optical profilometry, and contact angle measurements was employed. The oxidation kinetics exhibited a strong temperature dependence. At 600°C, oxidation followed near-linear kinetics with low mass gain, indicating the formation of a thin and protective oxide layer. At 700°C, the kinetics transitioned to diffusion-controlled behavior, consistent with parabolic trends, associated with the development of a multilayered oxide scale comprising rutile TiO 2 and sub-stoichiometric Ti 3 O 5 . At 800°C, oxidation became non-protective, showing sustained linear kinetics and significantly higher mass gain due to oxide scale cracking, porosity, and spallation. Microstructural analysis revealed progressive oxide thickening and increased crystallinity with temperature, with rutile TiO 2 dominating at higher temperatures. Surface roughness increased markedly with oxidation severity, while contact angle measurements indicated enhanced hydrophilicity due to combined effects of oxide chemistry and surface topography. The results demonstrate that Ti-35Nb-6Mo exhibits stable oxidation resistance up to 700°C, while degradation at 800°C defines its upper thermal limit. These findings highlight the alloy’s potential for both high-temperature engineering applications and surface-engineered biomedical applications. Beta-titanium alloy oxidation resistance high temperature surface characterization oxide kinetics Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 1. Introduction Titanium and its alloys have garnered significant attention in various advanced engineering fields, including aerospace, biomedical, chemical processing, and energy industries, primarily due to their exceptional strength-to-weight ratio, outstanding corrosion resistance, and biocompatibility [ 1 – 3 ]. These superior properties make titanium alloys a material of choice for applications demanding high mechanical performance under harsh environmental conditions [ 4 , 5 ]. Among the different classes of titanium alloys, β-titanium alloys have emerged as particularly promising due to their enhanced plasticity, lower modulus of elasticity, and excellent corrosion resistance, which result mainly from the stabilization of the β-phase by alloying elements such as niobium (Nb) and molybdenum (Mo) [ 6 – 8 ]. The Ti-35Nb-6Mo alloy specifically attracts attention for combining excellent mechanical properties with superior biocompatibility, making it highly suitable for orthopaedic implants and other biomedical devices requiring long-term durability and minimal adverse reactions in the human body [ 9 ]. Extensive research has been devoted to understanding the oxidation behavior of titanium and its alloys, which is critical for predicting their performance and longevity in high-temperature and oxidative environments [ 10 , 11 ]. Titanium exhibits allotropic transformation from a hexagonal close-packed α-phase at room temperature to a body-centred cubic β-phase above approximately 885 ºC. The oxidation kinetics of titanium alloys typically follow parabolic growth laws at moderate temperatures, indicating diffusion-controlled oxide scale formation, but can transition to linear kinetics as oxide spallation or scale degradation occurs at elevated temperatures [ 13 ]. The formation of a stable titanium dioxide (TiO 2 ) layer, particularly in the rutile modification, is understood to play a protective role by reducing further oxygen ingress and oxidation progression. However, factors such as temperature, alloy composition, and microstructure influence the integrity and protectiveness of the oxide scales, affecting oxidation resistance [ 14 , 15 ]. Recent studies have provided more nuanced insights into the role of β-phase stabilizers like Nb and Mo on oxidation resistance. The addition of Nb has been shown to improve oxide scale adherence and corrosion resistance by promoting the formation of Nb-rich oxides, which enhances protective behavior [ 16 ]. Conversely, Mo also acts as a β-stabilizer, but its effects on oxide morphology and oxidation kinetics can be complex. While Mo can improve corrosion resistance and mechanical stability at elevated temperatures, it may also give rise to volatile molybdenum oxides (such as MoO 3 ) at temperatures above 700 ºC, which can cause porosity, crack formation, and localized degradation of oxide films [ 17 – 18 ]. These contradictory influences necessitate a detailed, alloy-specific investigation into the high-temperature oxidation behavior of Ti-Nb-Mo systems. Despite advances in oxidation research on various titanium alloy systems, the specific oxidation behavior of metastable β-titanium alloys like Ti-35Nb-6Mo under prolonged high-temperature exposure, especially approaching 800 ºC, and their oxide scale evolution remain insufficiently explored. Most existing research focuses predominantly on commercially pure titanium or α-β alloys such as Ti-6Al-4V, where the interplay between phases significantly differs from predominantly β-alloys [ 8 , 19 ]. Furthermore, comprehensive surface characterization using synergistic techniques such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and focused ion beam (FIB) milling for depth-resolved analysis of oxide thickness and compositional gradients has not been systematically applied to Ti-35Nb-6Mo. This research aims to address these gaps by investigating in detail the oxidation kinetics, microstructural evolution, and surface characteristics of Ti-35Nb-6Mo subjected to oxidation at 600 ºC, 700 ºC, and 800 ºC for exposure times from 0.5 to 72 hours. These chosen temperatures simulate critical service conditions encountered in aerospace turbine engines, chemical reactors, and sterilization processes for biomedical implants [ 3 ]. The methodology combines gravimetric analysis to monitor oxidation mass gain, SEM and EDS for morphological and compositional surface evaluation, and FIB milling to provide cross-sectional insight into oxide layer thickness and structural integrity. Complementary surface roughness measurements via optical profilometry and wettability assessments using static contact angle measurements are integrated to correlate oxidation-induced surface modifications with functional properties relevant to corrosion resistance and implant biocompatibility. By explaining the oxidation mechanisms and surface evolution of Ti-35Nb-6Mo, this study will contribute essential knowledge towards the design and optimization of thermally stable β-titanium alloys for high-performance applications. Understanding the interplay between oxidation kinetics, oxide scale properties, and alloy composition will facilitate the development of improved alloys with enhanced lifetime and reliability in demanding oxidative environments, advancing technological applications in aerospace, chemical industries, and biomedical engineering. 2. Materials and Experimental Methods 2.1 Alloy Synthesis and Sample Preparation The Ti-35Nb-6Mo alloy was synthesized via powder metallurgy, consisting of 35 wt.% niobium, 6 wt.% molybdenum, and a titanium balance. The blended powders were sintered and compacted to yield homogeneous specimens. To ensure consistency during oxidation exposure, the alloy was cut into rectangular samples measuring approximately 12 mm × 6 mm × 2 mm using a high-precision Struers Secotom-15 cutter equipped with diamond-coated discs. The sample surfaces were mechanically polished using silicon carbide abrasive papers up to 1200 grit, followed by a final polishing step with an alumina slurry to achieve a mirror-like finish. Prior to heat treatment, residual contaminants were removed by ultrasonically cleaning the specimens in successive acetone and ethanol baths for 10 minutes each, followed by drying under ambient laboratory conditions. 2.2 Isothermal Oxidation and Kinetic Modeling High-temperature oxidation experiments were conducted in a controlled atmosphere muffle furnace. The specimens were subjected to isothermal oxidation in air at 600 ºC, 700 ºC, and 800 ºC for durations of 0.5, 1, 2, 4, 8, 24, 48, and 72 hours. Following each exposure interval, the samples were removed and allowed to cool naturally in ambient air. Oxidation kinetics were evaluated by recording the mass change before and after treatment using a Shimadzu AUW220D precision analytical balance with a readability of ± 0.01 mg. The total mass gain was normalized by the effective exposed surface area, which was determined using digital calipers with a 0.01 mm resolution. To identify the dominant oxidation mechanisms, the normalized mass gain data were fitted to linear, parabolic, and logarithmic kinetic models using regression analysis to extract the corresponding rate constants. All measurements were performed on at least three independently oxidized samples per condition to ensure statistical reliability. 2.3 Structural, Microstructural and Chemical Characterization Phase constitution of the as-prepared and oxidized samples was analyzed using a D/max-2550 VB/PC (Rigaku) X-ray diffractometer with Cu-Kα radiation. Scans were performed over a 2θ range of 20° to 80°. Surface morphology and oxide layer microstructures were examined using a Carl Zeiss Auriga Compact Field Emission Scanning Electron Microscope (FESEM) operating at 20 kV. Imaging was performed using both secondary electron (SE) detectors for high-resolution topographical contrast and backscattered electron (BSE) detectors for compositional contrast. Chemical characterization and elemental mapping were conducted using an integrated Oxford Instruments Energy Dispersive X-ray Spectroscopy (EDS) system to determine the atomic distribution of Ti, Nb, Mo, and O within the oxide layers. To assess oxide scale thickness and subsurface integrity, cross-sectional samples were prepared by milling perpendicular trenches into the oxidized specimens using a gallium ion beam coupled to the Zeiss Auriga FESEM. BSE imaging and EDS line scanning across the exposed interfaces allowed for elemental depth profiling and the observation of internal stratification and diffusion gradients. 2.4 Surface Topography and Wettability Assessments Oxidation-induced changes in surface roughness were quantified using a Profilm 3D (Filmetric, USA) non-contact optical profilometer. Measurements were acquired using a 0.75 mm cutoff length over 4 mm scan lengths. Three independent scans were performed at different locations on each sample to calculate the average roughness (S a ), root mean square roughness (S q ), and maximum peak height (S p ). On the other hand, surface wettability was evaluated via static water contact angle measurements using a Krüss DSA25 Drop Shape Analyzer. A 2 µL droplet of deionized water was deposited onto the oxidized surfaces, and the profile was imaged after a 10-second equilibration period. The final contact angle was recorded as the average of the left and right-side droplet profile measurements. 3. Results and Discussion 3.1 X-ray Diffraction (XRD) Analysis X-ray diffraction (XRD) was employed to examine the phase constitution of the Ti-35Nb-6Mo alloy in the as-prepared condition and to investigate the evolution of crystalline phases following oxidation at elevated temperatures for different exposure durations. The diffraction patterns provide direct evidence of β-phase stability, oxide formation, and temperature- and time-dependent crystallographic transformations occurring during thermal oxidation. The XRD pattern of the as-prepared Ti-35Nb-6Mo alloy is characterized predominantly by reflections corresponding to the β-Ti phase with a body-centered cubic (BCC) structure, confirming effective β-phase stabilization by Nb and Mo alloying additions [ 20 ]. Figure 1 illustrates the most intense diffraction peak observed at 2θ ≈ 38.6° is indexed to the β-Ti (110) plane. Additional peaks at approximately 55.7°, 69.8°, and 82.6° correspond to the β-Ti (200), β-Ti (211), and β-Ti (220) planes, respectively, in good agreement with standard β-Ti diffraction data (JCPDS card no. 44-1288) [ 21 ]. Weak diffraction peaks associated with α-Ti (hexagonal close-packed) are detected near 2θ ≈ 35.1° (100) and ≈ 40.2° (101) (JCPDS card no. 44-1294), indicating the presence of a small fraction of retained or stress-induced α-phase [ 21 ]. Importantly, no oxide-related peaks are observed in the as-prepared alloy, confirming the absence of detectable surface oxidation prior to heat treatment. After oxidation at 600°C for exposure times ranging from 8 to 72 hours (Fig. 2 (a)), the diffraction patterns remain dominated by β-Ti reflections, demonstrating that the bulk β-phase structure of the alloy is preserved during prolonged oxidation at this temperature. Weak diffraction peaks corresponding to rutile TiO 2 appear at 2θ ≈ 27.42°, indexed to the (110) plane, and at ≈ 36.01°, corresponding to the (101) plane (JCPDS card no. 21-1276), particularly after longer oxidation durations [ 22 ]. The low intensity and slow evolution of these rutile peaks indicate limited oxide crystallization, suggesting that the oxide layer formed at 600°C is thin and weakly crystalline. No reflections attributable to sub-stoichiometric titanium oxides such as Ti 2 O 3 or Ti 3 O 5 , nor any Nb- or Mo-based oxides, are detected, indicating minimal oxide complexity under these conditions. In contrast, oxidation at 700°C leads to a pronounced evolution of oxide phases with increasing exposure time, as clearly depicted in Fig. 2 (b). After 8 hours, rutile TiO 2 peaks begin to emerge, indicating the onset of crystalline oxide formation. With increasing oxidation duration to 24, 48, and 72 hours, the rutile TiO 2 reflections indexed to the (110), (101), and (211) planes become progressively sharper and more intense, reflecting enhanced crystallinity and increased oxide development. In addition to rutile TiO 2 , weak diffraction peaks attributed to Ti 3 O 5 are observed at 2θ ≈ 25.3°, 32.27°, 47.72°, 56.6°, 58.93°, and 67.12°, corresponding to monoclinic Ti 3 O 5 (JCPDS card no. 40–0806) [ 23 ]. The appearance of Ti 3 O 5 indicates the formation of sub-stoichiometric titanium oxides, likely arising from oxygen concentration gradients within the growing oxide layer [ 24 ]. Despite the increased presence of oxide phases, β-Ti peaks remain noticeable even after 72 hours, indicating partial attenuation rather than complete masking of the substrate signal. At 800°C, the XRD patterns (Fig. 2 (c)) are dominated by intense and well-defined rutile TiO 2 peaks, confirming that rutile is the thermodynamically stable and predominant oxide phase at elevated temperature [ 21 ]. Strong reflections indexed to the (110), (101), (211), and (220) planes of rutile TiO 2 are clearly observed (JCPDS card no. 21-1276). In addition, reflections associated with Ti 3 O 5 persist, suggesting continued formation of oxygen-deficient titanium oxides and non-uniform oxygen distribution within the oxide layer [ 25 ]. The β-Ti reflections are significantly weakened, indicating strong attenuation of the substrate signal by the thick and highly crystalline oxide scale. Notably, no diffraction peaks corresponding to Nb or Mo-based oxides are detected at any oxidation temperature, implying that Nb remains largely in solid solution and that Mo does not form stable crystalline oxides detectable by XRD under the present experimental conditions [ 26 ]. Figure 2 (d) provides a direct comparison of the diffraction patterns obtained after 24 h of oxidation at 600°C, 700°C, and 800°C. A clear progression in phase evolution is observed, with weak and broad rutile TiO 2 peaks at 600°C, moderately intense peaks with the emergence of Ti 3 O 5 at 700°C, and sharp, highly crystalline rutile peaks dominating at 800°C. Simultaneously, the gradual attenuation of β-Ti reflections with increasing temperature confirms the growth and thickening of the oxide scale. This comparative representation highlights the strong temperature dependence of oxide crystallinity and phase stability. Furthermore, the broadening of diffraction peaks provides important insight into the crystallite size and lattice strain within the oxide layers. In general, peak broadening arises from the combined effects of finite crystallite size and lattice microstrain, both of which contribute to variations in interplanar spacing and diffraction coherence length [ 23 , 24 ]. At 600°C, the observed broad and low-intensity peaks indicate the formation of a thin oxide layer with small crystallite size and a high degree of structural disorder, as shown in Fig. 2 (d). This behavior is characteristic of early-stage oxidation, where limited atomic mobility restricts grain growth and results in a poorly crystalline oxide structure [ 23 ]. The presence of defects such as vacancies and short-range disorder further contributes to peak broadening at this stage. With increasing temperature to 700°C, peak broadening decreases moderately, indicating an increase in crystallite size and partial relaxation of lattice strain. However, the persistence of some peak broadening suggests the presence of microstrain within the oxide scale, likely arising from oxygen concentration gradients and the coexistence of multiple oxide phases such as TiO 2 and Ti 3 O 5 [ 24 , 25 ]. The formation of sub-stoichiometric oxides introduces lattice distortions due to non-uniform oxygen distribution, which contributes to strain-induced peak broadening. At 800°C, the diffraction peaks become sharp and well-defined, indicating significant crystallite growth and improved crystallinity of the oxide scale (see Fig. 2 (d)). The reduction in peak broadening suggests a decrease in lattice strain due to enhanced atomic diffusion and structural reorganization at elevated temperature [ 25 ]. This transition reflects the formation of a highly crystalline rutile TiO 2 layer, which is thermodynamically stable and exhibits lower defect density compared to oxides formed at lower temperatures [ 22 ]. In addition to peak broadening, slight shifts in peak position were observed with increasing oxidation temperature and exposure time. According to Bragg’s law, peak position is directly related to interplanar spacing; therefore, any shift in diffraction angle reflects changes in lattice parameters due to thermal expansion, oxygen incorporation, or residual stress [ 24 , 25 ]. At lower temperatures (600°C), minor shifts toward lower 2θ values may occur due to lattice expansion associated with oxygen diffusion into the titanium matrix and the initial formation of oxide layers. This expansion corresponds to an increase in interplanar spacing and is commonly observed during early oxidation stages [ 23 ]. At intermediate temperature (700°C), peak shifts become more pronounced, reflecting lattice distortion caused by the formation of sub-stoichiometric oxides such as Ti 3 O 5 and the development of oxygen concentration gradients within the oxide scale [ 24 ]. These gradients generate localized tensile and compressive stresses, leading to variations in d-spacing and corresponding peak shifts. At higher temperature (800°C), peak positions tend to stabilize or exhibit slight shifts toward higher 2θ values, indicating partial lattice contraction and stress relaxation. This behavior is associated with the formation of a thick, highly crystalline rutile TiO 2 layer and the reduction of internal strain through grain growth and structural rearrangement [ 25 ]. Additionally, stress relaxation mechanisms such as crack formation and oxide scale delamination may contribute to the stabilization of lattice parameters at elevated temperature. Overall, the XRD results reveal a clear temperature- and time-dependent phase evolution during oxidation of the Ti-35Nb-6Mo alloy. At lower temperatures, oxidation is characterized by limited rutile formation and preservation of the β-Ti substrate signal. Intermediate temperatures promote the development of mixed oxide phases with increasing crystallinity, while high-temperature oxidation results in the dominance of highly crystalline rutile TiO 2 accompanied by sub-stoichiometric titanium oxides. These findings confirm that rutile TiO 2 is the principal oxidation product across all investigated conditions, with oxide crystallinity and phase complexity strongly governed by oxidation temperature and exposure duration. The combined analysis of peak broadening and peak shifting provides strong evidence for temperature-dependent evolution of crystallite size, lattice strain, and oxide phase stability in the Ti-35Nb-6Mo alloy during high-temperature oxidation. These findings are consistent with diffusion-controlled oxide growth mechanisms and the progressive transition from protective to non-protective oxidation regimes [ 25 ]. 3.2 Mass Gain and Oxidation Kinetics The oxidation behaviour of the Ti-35Nb-6Mo β-titanium alloy was quantitatively investigated by monitoring mass gain per unit surface area as a function of oxidation temperature and exposure time, with the numerical data summarized in Table 1 . The results reveal a pronounced temperature dependence of oxidation kinetics, reflecting distinct oxidation regimes that are directly governed by the evolution, crystallinity, and phase constitution of the oxide layers identified by XRD analysis. At 600 ºC, the alloy exhibits the slowest oxidation kinetics over the entire exposure duration of 72 h. The mass gain increases gradually from 0.149 mg cm − 2 at 0.5 h to 0.723 mg cm − 2 at 8 h, followed by a steady rise to 2.146 mg cm − 2 at 24 h and 4.585 mg cm − 2 at 48 h. A slight decrease to 3.759 mg cm − 2 at 72 h is observed, which is attributed to localized stabilization of the oxide layer rather than a fundamental change in the oxidation mechanism. Excluding this terminal deviation, the mass gain follows a near-linear dependence on time with an excellent linear fit, as shown in Fig. 3 . The oxidation behaviour at 600 ºC can therefore be expressed by the linear rate law [ 27 ]: Table 1 Specific mass gain (mg/cm 2 ) of Ti-35Nb-6Mo β-titanium alloy as a function of oxidation time at 600°C, 700°C, and 800°C. Time (h) Mass gain per unit area (mg/cm 2 ) 600ºC 700ºC 800ºC 0.5 0.149 0.375 1.925 1 0.086 0.611 2.306 2 0.323 1.260 3.267 4 0.447 2.404 4.360 8 0.723 3.964 5.773 24 2.146 5.251 10.749 48 4.585 5.679 19.021 72 3.759 9.511 25.553 $$\:\frac{{\Delta\:}m}{A}={k}_{l}t$$ 1 From the slope of the fitted curve, the apparent linear oxidation rate constant is estimated to be k l = 0.09–0.10 mg cm − 2 h − 1 . This low-rate constant indicates slow oxidation kinetics and is fully consistent with XRD observations, which show dominant β-Ti reflections with only weak and slowly emerging rutile TiO 2 peaks even after prolonged exposure [ 28 ]. The limited crystallinity and gradual development of rutile at this temperature suggest that the oxide layer remains thin and weakly crystalline, allowing oxidation to proceed under a surface-reaction-controlled regime with minimal acceleration over time [ 29 ]. The corresponding SEM micrographs included in Fig. 3 further confirm the formation of a smooth, compact, and continuous oxide layer at 600°C, supporting the observed protective oxidation behavior. When the oxidation temperature is increased to 700 ºC, the mass gain behaviour changes markedly, indicating a transition to a different oxidation mechanism. The mass gain rises rapidly during the initial stages, increasing from 0.375 mg cm − 2 at 0.5 h to 3.964 mg cm − 2 at 8 h, demonstrating accelerated oxide formation. Beyond this stage, the oxidation rate decreases significantly, with mass gain increasing only modestly to 5.251 mg cm − 2 at 24 h and 5.679 mg cm − 2 at 48 h. At 72 h, the mass gain increases again to 9.511 mg cm − 2 , suggesting partial destabilization or localized breakdown of the oxide layer at extended exposure times. The oxidation data at 700 ºC deviate from linear behaviour and are best described by a parabolic kinetic law (see Fig. 4 ), which represents diffusion-controlled growth. The oxidation behaviour can be expressed as [ 30 ]: $$\:{\left(\frac{{\Delta\:}m}{A}\right)}^{2}={k}_{p}t\:$$ 2 From the curvature of the fitted plot, the parabolic rate constant is estimated to be k p = 0.45–0.55 mg 2 cm − 4 h − 1 . The deceleration of the oxidation rate with time indicates that oxide growth increasingly limits ionic transport as the exposure proceeds. This behaviour correlates strongly with XRD results at 700 ºC, which show a progressive increase in rutile TiO 2 crystallinity along with the emergence of sub-stoichiometric titanium oxides (Ti 3 O 5 ) [ 31 ]. The co-existence of stoichiometric and sub-stoichiometric oxides implies the development of oxygen activity gradients within the oxide scale, leading to a chemically and structurally heterogeneous layer that slows further oxidation by restricting diffusion pathways [ 32 ]. The SEM images in Fig. 4 reveal a granular oxide morphology with increased surface coverage, consistent with diffusion-controlled oxide growth at 700°C. At 800 ºC, the oxidation kinetics undergo a further fundamental change, exhibiting the highest mass gain and a persistent linear trend over the entire exposure period. The mass gain increases sharply from 1.925 mg cm − 2 at 0.5 h to 5.773 mg cm − 2 at 8 h, followed by 10.749 mg cm − 2 at 24 h, 19.021 mg cm − 2 at 48 h, and reaching 25.553 mg cm − 2 at 72 h. The mass gain follows an almost perfectly linear relationship with time, with a high regression coefficient, as shown in Fig. 5 . The oxidation behaviour at this temperature is therefore well described by the linear rate law [ 27 ]: $$\:\frac{{\Delta\:}m}{A}={k}_{l}t$$ 3 From the slope of the fitted curve, the apparent linear oxidation rate constant at 800 ºC is estimated to be k l = 0.34–0.36 mg cm − 2 h − 1 , which is approximately three to four times higher than that observed at 600 ºC. This large increase in kₗ reflects a non-protective oxidation regime [ 50 ]. XRD analysis at 800 ºC shows intense and highly crystalline rutile TiO₂ peaks accompanied by persistent sub-stoichiometric oxides, indicating rapid oxide growth but limited long-term stability of the oxide layer. The sustained linear kinetics suggest that the oxide scale formed at this extreme temperature does not establish an effective diffusion barrier, allowing continuous oxygen ingress and oxidation throughout the exposure period [ 33 ]. The SEM micrographs associated with Fig. 5 show extensive cracking, porosity, and oxide fragmentation, confirming the non-protective nature of oxidation at 800°C. Taken together, the mass gain and oxidation kinetics data reveal a clear progression in oxidation behaviour with increasing temperature. Oxidation at 600 ºC is slow and controlled, characterized by low mass gain and near-linear kinetics associated with limited rutile crystallization. At 700 ºC, oxidation transitions to a diffusion-controlled regime, reflected by parabolic kinetics and linked to the development of more crystalline and compositionally complex oxide phases. At 800 ºC, oxidation becomes aggressive and non-protective, with high mass gain and sustained linear kinetics despite extensive rutile formation. These findings demonstrate that oxidation kinetics in Ti-35Nb-6Mo are governed not only by temperature but critically by the crystallographic nature and stability of the oxide phases formed during high-temperature exposure. 3.3 SEM Morphology of Oxide Layers Scanning Electron Microscopy (SEM) analysis of the Ti-35Nb-6Mo alloy after oxidation at different temperatures revealed distinct morphological characteristics corresponding to the degree of oxidation and structural integrity of the oxide layers formed, as shown in Figs. 6 and 7 . Figure 6 illustrates high-magnification SEM images of Ti-35Nb-6Mo alloy after 8 hours, highlighting fine-scale surface features, while Fig. 7 provides corresponding low-magnification views of Ti-35Nb-6Mo alloy after 48 hours, to illustrate overall surface morphology and large-scale oxide distribution. The visual evidence provided by the SEM micrographs closely parallels both the macroscopic gravimetric data and the crystallographic phase evolution established by the XRD analysis. At 600 ºC, both high-magnification (Fig. 6(a)) and low-magnification (Fig. 7 (a)) SEM images exhibited relatively smooth and featureless surfaces with a very thin oxide scale. This morphology indicates the formation of a compact and continuous oxide film, likely composed predominantly of TiO 2 , which serves as an effective barrier limiting further oxygen diffusion into the substrate [ 34 ]. The minimal surface roughness and absence of visible oxide crystals at this temperature support early-stage oxidation behavior, consistent with a protective oxide regime that impedes mass transport and restricts oxidation progression over time [ 35 ]. These microscopic observations corroborate the near-linear, slow mass gain kinetics. Furthermore, this aligns well with the XRD analysis, which demonstrated a largely amorphous or nanocrystalline oxide structure; the XRD patterns were heavily dominated by the β-Ti substrate, with only very weak rutile TiO 2 peaks emerging over the 72-hour period. The oxide formed at this temperature appears stable but thin, corroborating the small mass gain and limited roughness increase observed in profilometric studies. Upon increasing the temperature to 700 ºC, the SEM morphology (Figs. 6(b) and 7(b)) showed a clear transition to a granular and nodular surface composed of uniformly distributed oxide grains. This granular texture is a hallmark of grain boundary diffusion processes facilitating oxide grain nucleation and subsequent coalescence into a denser scale [ 36 ]. Such a morphology is indicative of the transition from early oxide formation to bulk oxide growth, reflecting enhanced diffusion kinetics at higher temperatures. The nature of the oxide grains at 700 ºC aligns with the formation of rutile TiO 2 , which is more thermodynamically stable and dense compared to anatase, though grain coalescence introduces microstructural heterogeneities such as microvoids at grain boundaries [ 37 ]. This microstructural shift directly explains the parabolic and logarithmic mass gain kinetics, where the growing, denser oxide scale acts as a partial diffusion barrier. The XRD data verifying the rapid crystallization of rutile peaks and the presence of intermediate Ti 3 O 5 suboxides further explains this complex, multilayered oxide development. However, these micro-void features may become preferential sites for oxygen ingress or stress localization, potentially compromising long-term scale integrity [ 38 ]. At 800 ºC, SEM analysis (Figs. 6(c) and 7(c)) revealed more pronounced structural changes characterized by thickened oxide layers displaying severe porosity, microcracks, and localized fracture or delamination of the scale. The oxide grains grew significantly coarser, and the scale showed unambiguous signs of mechanical degradation, suggestive of stress accumulation and mismatch in volumetric expansion between the oxide and substrate [ 39 ]. This mechanical instability is associated with thermal stresses induced by rapid oxide growth and the intrinsic brittleness of rutile TiO 2 at elevated temperatures [ 40 ]. The XRD patterns clearly validate this macroscopic breakdown, i.e., at 800 ºC, the massive crystallization of the rutile phase significantly attenuated the metallic substrate peaks, indicating the formation of a thick and rigid oxide layer incapable of accommodating thermal strains [ 41 ]. Furthermore, as suggested by the structural analyses, the probable volatilization of molybdenum as gaseous MoO 3 at this extreme temperature likely intensifies the formation of the observed microporosity by physically disrupting the scale from within as the gas escapes. Despite rutile’s thermodynamic stability in this temperature regime, its brittleness, combined with resulting growth stresses and potential volatile gas escape, drives crack formation and oxide spallation, thereby exposing fresh metal surfaces to oxidative environments. The structural discontinuity and porosity evident in SEM images at 800 ºC facilitate continuous oxygen ingress and accelerated oxidation kinetics, as reflected in the linear mass gain behavior observed experimentally. These morphological observations mark a distinct shift from protective to non-protective oxidation regimes with increasing temperature for the Ti-35Nb-6Mo alloy. The SEM morphology evolution observed herein aligns well with reported oxidation behavior of β-titanium alloys and titanium aluminides, in which the oxide scale transitions from smooth, protective films to fractured, porous, and less adherent oxide layers as temperature rises [ 42 ]. Overall, SEM characterization, when coupled with the gravimetric trends and crystallographic verification from XRD, provides a comprehensive understanding of the integrity and protective capacity of oxide scales developed on the alloy. 3.4 FIB Cross-sectional Analysis Figures 8 and 9 demonstrate the cross-sectional analysis of the Ti-35Nb-6Mo alloy using Focused Ion Beam (FIB) microscopy, providing critical quantitative insights into the oxide layer thickness and internal microstructural evolution resulting from high-temperature oxidation. At 600 ºC, FIB cross-sections revealed a relatively thin oxide layer (images not shown). The oxide layer exhibited a compact and uniform morphology with negligible porosity or delamination along the metal-oxide interface. This observation confirms strong adherence between the oxide film and the substrate, characteristic of low-temperature oxidation regimes where oxidation kinetics are limited and oxide growth is tightly controlled by surface reactions [ 43 ]. Crucially, this structural compactness directly correlates with the time-resolved XRD data, which confirmed the presence of a largely amorphous or nanocrystalline oxide film where the underlying β-Ti substrate signal remained dominant. Such a dense and continuous oxide scale corresponds perfectly with the highly restricted, near-linear oxidation mass gain kinetics established earlier, strongly supporting the protective role of this early-stage film. When the oxidation temperature increased to 700 ºC (Fig. 8 ), the oxide scale thickness grew notably. The cross-sectional microstructure displayed distinct multilayered features, indicative of a more complex oxidation process involving sequential oxide phase formation. The presence of these layered oxides, punctuated by occasional microvoids, strictly corroborates the XRD findings, which conclusively identified intermediate titanium suboxides (Ti 3 O 5 ) coexisting with the thermodynamically stable rutile TiO 2 phase. This stratified oxide formation aligns with thermodynamic models in which intermediate oxide phases act as metastable precursors near the metal interface before progressively transforming into stable rutile TiO 2 layers closer to the oxygen-rich surface [ 44 ]. The microvoids observed in Fig. 8 likely represent emerging stress-relief features arising from volume and structural mismatches during this complex, multiphase growth, signaling the onset of mechanical relaxation mechanisms within the scale [ 45 ]. At the highest oxidation temperature investigated, 800 ºC (Fig. 9 ), FIB cross-sections revealed substantially thicker oxide layers. These scales were structurally heterogeneous, exhibiting frequent cracking, delamination zones, and extensive void formation both within the oxide bulk and at the metal-oxide interface. This macroscopic breakdown is a direct consequence of the massive and rapid crystallization of the rutile TiO 2 phase confirmed by the XRD spectra. The thick, rigid, and highly crystalline rutile layer cannot effectively accommodate the combined effects of thermal stress, volumetric expansion mismatch between the oxide and substrate, and interfacial incompatibility during rapid growth [ 46 ]. Furthermore, the presence of these severe subsurface voids is driven by two concurrent degradation mechanisms. First, the Kirkendall effect plays a significant role, where differential diffusion rates of outward-migrating metal cations and inward-diffusing oxygen anions generate vacancies that coalesce into voids beneath the oxide layer [ 47 ]. Second, as established by the high-temperature XRD and EDS analyses, the localized volatilization of molybdenum into gaseous MoO 3 physically disrupts the scale architecture from within, leaving behind extensive microporosity as the gas escapes. Such porous and fractured oxide scales are highly detrimental to protective properties, explicitly explaining the experimentally observed linear and accelerated oxidation kinetics at this extreme temperature. This compromised scale integrity facilitates continuous oxygen ingress, thereby exacerbating material degradation. Collectively, the FIB cross-sectional analyses quantitatively and qualitatively validate the trends inferred from mass gain measurements, SEM surface observations, and XRD phase identifications. These structural insights reveal a clear correlation between oxidation temperature, phase evolution, oxide thickness, and overall scale integrity. Such findings are imperative for comprehensively understanding oxidation mechanisms in β-titanium alloys and for guiding the design of materials capable of sustaining stability and protection in demanding high-temperature environments. Furthermore, to correlate these severe structural changes with chemical stratification, an EDS line scan was performed across the FIB cross-section at 800 ºC (Fig. 9 (C) and (D)). The depth profile clearly illustrates a steep decline in oxygen concentration from the surface inward, accompanied by the depletion of Mo and Nb in the outer oxide layer and their subsequent retention in the subsurface matrix. A more detailed quantitative discussion of this elemental partitioning is provided in the subsequent EDS analysis. 3.5 EDS Elemental Analysis Energy Dispersive X-ray Spectroscopy (EDS) provided comprehensive elemental analysis of the oxidized surfaces of the Ti-35Nb-6Mo alloy subjected to varying temperatures, revealing critical insights into the chemical composition and spatial distribution of oxygen and alloying elements within the oxide layers, as depicted in Table 2 and 3 . At 600 ºC, the EDS spectra of the sample surfaces exhibited comparatively low oxygen levels, with dominant signals corresponding to titanium (Ti), niobium (Nb), and molybdenum (Mo). This indicates that the oxide film formed under these conditions was relatively thin and partially developed, allowing electron beam emissions to readily detect the underlying metallic substrate with minimal attenuation. This elemental observation flawlessly corroborates the time-resolved and temperature-dependent XRD diffractograms, which were heavily dominated by the β-Ti substrate peaks and showed only nascent, low-intensity rutile TiO 2 formation. These combined findings confirm the early-stage oxidation dynamics characterized by the formation of a protective yet thin oxide layer, consistent with the limited mass gain and minimal surface roughness changes observed at this temperature. Table 2 EDS point and area compositional analysis (wt.% and at.%) of the oxide cross-sections corresponding to the spectra locations marked in Fig. 8 (700°C). Spectrum No. Oxidation Time In Weight Atomic (%) Ti Nb O Mo 3 0.5 85Ti 12Nb 3Mo 89.90 6.49 2.37 1.24 4 57Ti 35Nb 8Mo 69.9 22.13 3.40 4.57 5 61Ti 31Nb 2O 6Mo 71.37 18.71 6.54 3.38 6 2 75Ti 20Nb 2O 3Mo 80.54 11.61 6.02 1.83 7 56Ti 36Nb 2O 6Mo 68.94 22.95 4.34 3.77 8 41Ti 33Nb 21O 5Mo 32.86 13.48 51.77 1.89 9 4 65Ti 19Nb 13O 3Mo 56.99 8.60 33.12 1.29 10 55Ti 30Nb 10O 5Mo 52.84 15.35 29.59 2.22 11 55Ti 12Nb 32O 1Mo 35.40 4.06 60.20 0.34 12 54Ti 30Nb 13O 3Mo 49.89 13.88 34.76 1.47 13 24 50Ti 21Nb 28O 1Mo 34.39 7.38 57.66 0.57 14 46Ti 38Nb 14O 2Mo 42.14 18.06 38.57 1.23 15 59Ti 26Nb 11O 4Mo 54.01 12.36 32.00 1.63 16 74Ti 11Nb 13O 2Mo 61.99 4.75 32.63 0.63 17 55Ti 32Nb 5O 8Mo 62.10 18.13 15.24 4.53 18 78Ti 16Nb 2O 4Mo 82.69 8.86 6.49 1.96 19 48 65Ti 22Nb 10O 3Mo 59.96 10.76 27.90 1.38 20 54Ti 17Nb 27O 2Mo 38.37 5.90 55.16 0.57 21 56Ti 29NB 12O 3Mo 51.30 13.74 33.77 1.19 Table 3 EDS point and area compositional analysis (wt.% and at.%) of the oxide cross-sections corresponding to the spectra locations marked in Fig. 9 (800°C). Spectrum No. Oxidation Time In Weight Atomic (%) Ti Nb O Mo 1 1hr 57Ti 15Nb 26O 2Mo 40.09 5.43 53.7 0.78 2 52Ti 38Nb 3O 7Mo 62.76 23.85 8.89 4.50 3 68Ti 24Nb 3O 5Mo 75.34 13.82 8.29 2.55 4 77Ti 18Nb 2O 3 Mo 82.06 9.77 6.26 1.91 5 56Ti 37Nb 1O 6Mo 67.60 23.69 4.88 3.83 6 54Ti 37Nb 1O 8Mo 66.20 24.18 4.46 5.16 7 2hr 56Ti 29Nb 13O 2Mo 50.22 13.55 35.37 0.86 8 65Ti 22Nb 11O 2Mo 58.42 10.37 30.26 0.95 9 57Ti 29Nb 10O 4Mo 54.94 14.06 29.28 1.72 10 71Ti 16Nb 11O 2Mo 62.72 7.06 29.49 0.73 11 63Ti 20Nb 16O 1Mo 53.42 8.64 37.41 0.53 12 54Ti 35Nb 8O 3Mo 55.15 18.60 24.87 1.38 When the oxidation temperature was elevated to 700 ºC, an intensification of oxygen peaks was documented in the EDS spectra, accompanied by a relative diminution in the intensity of Nb and Mo signals, particularly in near-surface regions. This shift reflects the formation of a more continuous titanium dioxide (TiO 2 ) layer, with the Ti/O atomic ratio approximating values between 0.6 and 0.8 across numerous surface points. Crucially, this measured ratio provides profound chemical validation for the crystallographic data; a Ti/O ratio of 0.6 perfectly matches the stoichiometry of the intermediate titanium suboxide Ti 3 O 5 , which was explicitly identified in the 700 ºC XRD spectra. This confirms the simultaneous presence of these intermediate suboxides coexisting with the thermodynamically stable rutile TiO 2 phase. This elemental and crystallographic synthesis robustly proves the multi-layered oxide growth mechanism, where metastable Ti 3 O 5 acts as a precursor bridging the metal substrate to the outer rutile layer. The corresponding decrease in Nb and Mo spectral intensity at the surface is interpreted as the result of limited outward diffusion of these refractory alloying elements toward the oxide front or their segregation beneath the oxide layer, a behavior in line with the β-phase stability of the substrate and reported diffusion kinetics in Ti-Nb-Mo systems. As shown in Table 2 , the oxygen concentration increases progressively with oxidation time at 700°C, confirming the formation of a thicker and more developed oxide scale. The Ti/O ratios further support the coexistence of TiO 2 and sub-stoichiometric Ti 3 O 5 phases. For samples oxidized at 800 ºC, the EDS elemental maps and point analyses revealed oxygen as the predominant element at the outer oxide surfaces, with the Ti/O ratio closely matching the stoichiometric ratio of rutile TiO 2 (approximately 1:2). This elemental quantification flawlessly supports the XRD results at 800 ºC, which showed diffractograms completely overwhelmed by highly crystalline rutile peaks and devoid of substrate signals. Notably, spectral peaks corresponding to Nb and Mo were largely absent from the oxide surface regions, implying these elements reside preferentially within the metallic substrate or potentially sub-oxide zones. The absence of molybdenum signals near the surface is particularly significant, suggesting volatilization of Mo as molybdenum trioxide (MoO 3 ) under high-temperature oxidative conditions, a phenomenon documented in prior studies on Mo-containing alloys, which involves Mo sublimation leading to oxide scale porosity and mechanical destabilization. Additionally, compositional heterogeneity detected in localized surface areas at 800 ºC likely arises from phase separation or anisotropic oxide growth, further complicating the oxide microstructure. Table 3 demonstrates the dominance of oxygen at 800°C, consistent with the formation of a fully developed rutile TiO 2 layer. The reduced presence of molybdenum at the surface further supports its volatilization as MoO 3 at elevated temperatures. The EDS elemental distribution maps conclusively demonstrated a steep oxygen concentration gradient near the sample surface, with a concurrent retention of Ti β-phase stabilizing elements Nb and Mo within the subsurface matrix, as shown in Fig. 9 (D). This compositional stratification underscores diffusion-controlled oxidation mechanisms where oxygen permeates inward, reacting with Ti to form progressively thicker, heavily stratified oxide layers, while refractory elements remain relatively immobile within the metallic phase. These findings elucidate the thermally driven evolution of oxide scale chemical architecture, providing essential insights into the formation, protective capacity, and possible degradation modes of the oxide films formed on Ti-35Nb-6Mo alloy under operational thermal stresses. In summary, the EDS analysis validated the progressive oxidation of the alloy, demonstrating a temperature-correlated enrichment of TiO 2 in the oxide scale and revealing the spatial elemental partitioning that governs oxidation kinetics and scale integrity. Such detailed elemental knowledge is critical for predicting the long-term oxidation resistance and failure mechanisms, thereby informing the alloy's application in high-temperature environments requiring superior oxidation stability. 3.6 Surface Roughness Measurements Surface roughness evolution in the Ti-35Nb-6Mo alloy after oxidation at increasing temperatures reveals a clear dependence on both thermal regimen and exposure time. To comprehensively evaluate these topographical modifications, the average roughness ( S a ), root mean square roughness ( S q ), maximum peak height ( S p ), and total peak-to-valley height ( S t ) were systematically analyzed. This multi-parameter approach provides critical insight into the relationship between crystallographic microstructure, oxide morphology, and final material performance. At 600 ºC, the topographical parameters remained consistently low over the entire 72-hour oxidation period, as illustrated in Fig. 10 . The S a values ranged between 0.35 and 0.77 µm. Correspondingly, S q tracked very closely to S a , while S p and S t remained minimal. The close proximity of S a and S q , combined with low maximum peak and valley heights, mathematically confirms the absence of deep pits or high protrusions. This limited roughening is primarily attributable to the formation of a thin, compact oxide layer. As corroborated by the XRD findings, the oxide formed at this temperature is largely nanocrystalline or amorphous. Because the oxide lacks significant grain coalescence, macroscopic crystallinity, or porosity, it introduces minimal topographical complexity to the surface. Both SEM and profilometric analyses indicate limited surface texturing, reinforcing the conclusion that early-stage, protective oxidation regimes tend to preserve bulk material smoothness and structural integrity. As illustrated in Fig. 10 , all roughness parameters remain relatively low and stable, confirming minimal surface modification at this temperature. When the oxidation temperature was raised to 700 ºC, more pronounced changes emerged in both surface chemistry and topography, demonstrated in Fig. 11 . The S a values increased substantially, reaching 11.87 µm after 72 h. Concurrently, S p and S t exhibited marked increases, reflecting the granular surface morphologies, as confirmed by SEM micrographs. The rise in maximum peak height ( S p ) represents a macroscopic manifestation of accelerated outward nodular growth and oxide crystallization events confirmed by XRD, specifically, the sharp emergence of crystalline rutile TiO 2 peaks and the intermediate suboxide Ti 3 O 5 [ 48 ]. Meanwhile, the increase in total profile height ( S t ) captures the onset of localized microvoids occurring between these coalescing crystals (peaks) [ 49 ]. Such topographical heterogeneity aligns well with the observed parabolic oxidation kinetics. Notably, these roughness parameters exhibit a gradual upward trend during the initial 8 hours, followed by more pronounced increases at longer exposure times, highlighting the complex coupling between oxidation time, scale evolution, and surface roughening. Figure 11 clearly shows a significant increase in roughness parameters, corresponding to oxide grain growth and surface texturing. At 800 ºC, the surface roughness behaviour becomes distinctly more erratic and severe (Fig. 12 ). The S a parameter surges to an extreme peak value of 20.37 µm after only 8 h of oxidation. This intense topographical change is accompanied by massive spikes in S q , S p , and S t . The large divergence between S q (which is highly sensitive to topographical extremes) and S a mathematically confirms a severely defective surface. The extreme S p values reflect the massive outward volumetric expansion of large, faceted rutile TiO 2 grains. More critically, the extraordinary total profile height ( S t ) perfectly captures the deep surface cracking, extensive porosity, and extreme valleys generated by growth stresses and the localized volatilization of MoO 3 gas [ 50 , 51 ]. Figure 12 highlights the abrupt variation in roughness parameters, particularly the sharp decrease at 24 h, which confirms oxide scale spallation followed by re-oxidation. Crucially, following this 8-hour peak, all roughness parameters precipitously drop at 24 h ( S a falls to 2.30 µm). This dramatic, simultaneous reduction in average roughness, peak height ( S p ), and total profile height ( S t ) is an unambiguous indicator of a massive scale spallation event. The highly stressed, rough outer oxide layer, comprising the tallest protruding crystals and the deepest superficial cracks, likely delaminated and flaked off entirely, exposing the relatively smoother, newly oxidizing metallic interface beneath it. Subsequent exposure (48 h to 72 h) shows all parameters building back up as newly formed oxide crystallites grow and the scale once again thickens and fractures. This non-monotonic roughness evolution reflects repeated, cyclic oxide growth, fracture, and re-oxidation processes [ 52 ]. The observed roughness evolution is tightly linked to the underlying crystallographic phenomena. At 600 ºC, minimal oxide crystallization results in low surface roughness across all measured parameters. At 700 ºC and particularly at 800 ºC, rapid nucleation of stable rutile TiO 2 drives outward peak growth ( S p ), while mechanical instability and volatile gas escape carve out deep valleys ( S t ), strongly influencing the mechanical behavior of the oxide scale [ 53 ]. Overall, the synergistic evolution of S a , S q , S p , and S t in the oxidized Ti-35Nb-6Mo alloy provides a comprehensive topographical map of oxide microstructural development. Smooth surfaces formed under low-temperature oxidation progressively evolve into highly crystalline, deeply cracked, and mechanically unstable topographies at higher temperatures. A rigorous interpretation of these combined profilometry parameters underscores the pivotal role of oxidation kinetics and oxide scale stability in governing surface degradation and functional performance. 3.7 Contact Angle Analysis The multidimensional roughness transitions have important implications for surface-related properties such as wettability; increased macro- and micro-roughness at elevated temperatures correlates with reduced contact angles and enhanced hydrophilicity [ 54 ]. While such surface characteristics may be beneficial in biomedical contexts, it can be detrimental for aerospace applications due to increased susceptibility to corrosion and fatigue [ 49 ]. Therefore, contact angle measurements conducted on the Ti-35Nb-6Mo alloy, which revealed a distinct decrease in water contact angle with increasing oxidation temperature, as depicted in Fig. 13 . Initially, the as-prepared alloy surfaces exhibited contact angles close to 90°, suggesting a hydrophobic or minimally wettable character [ 55 ]. Following oxidation at 600 ºC, the contact angle decreased to approximately 65°, representing a moderate increase in surface hydrophilicity. Further oxidation at 700 ºC and 800 ºC led to more pronounced reductions in contact angle to approximately 50° and 40°, respectively, indicating substantial improvements in surface energy and wettability [ 56 ]. This overarching trend is fundamentally driven by synergistic changes in both surface chemistry and surface morphology induced by high-temperature oxidation, as corroborated by XRD, SEM, and profilometric analyses. Chemically, the formation of titanium dioxide (TiO 2 ), particularly in its thermodynamically stable rutile phase, is well known to enhance surface hydrophilicity due to its polar nature and high surface energy, which promote strong hydrogen bonding interactions with water molecules [ 57 ]. As explicitly confirmed by XRD analysis, the oxide scale evolves from a thin, largely amorphous or nanocrystalline layer at 600 ºC to a highly crystalline, rutile-dominated scale at 700 ºC and 800 ºC. As the rutile TiO 2 phase thickens and becomes the dominant surface constituent, the surface’s affinity for water naturally increases [ 58 ]. In parallel, surface topographical features evolve significantly with oxidation temperature, as evidenced by surface roughness measurements and SEM observations. The XRD-confirmed rapid nucleation and aggressive crystalline growth of rutile TiO 2 and intermediate Ti 3 O 5 phases at higher temperatures contribute to a markedly roughened surface [ 59 ]. Classical wetting models proposed by Wenzel describe how increased surface roughness amplifies the intrinsic wetting behavior dictated by surface chemistry [ 60 ]: $$\:\text{c}\text{o}\text{s}{\theta\:}^{*}=r\text{c}\text{o}\text{s}\theta\:$$ 4 where \(\:{\theta\:}^{*}\:\) is the apparent contact angle on a rough surface, \(\:\theta\:\:\) is the intrinsic contact angle on a smooth surface, and \(\:r\:\) is the roughness factor defined as the ratio of actual surface area to projected area. According to this model, surface roughness amplifies the intrinsic wetting behavior, i.e., hydrophilic surfaces ( \(\:\theta\:<{90}^{\circ\:}\) ) become more hydrophilic with increasing roughness, while hydrophobic surfaces become more hydrophobic [ 60 ]. In the present case, the rough, highly crystalline TiO 2 -dominated surface promotes capillary effects and increases the real solid-liquid contact area, thereby enhancing water spreading and further reducing the apparent contact angle beyond that achievable by chemical modification alone [ 61 ]. Crucially, the time-resolved wettability data at 800 ºC perfectly mirrors the cyclic mechanical instability identified in the profilometry analysis. At 800 ºC, the contact angle reaches a minimum of 27.66° after 8 hours of exposure, aligning exactly with the extreme peak in surface roughness ( S a = 20.37 µm). However, at 24 hours, the contact angle abruptly rises to 48.62°. This rebound corroborates the massive scale spallation event discussed previously; as the highly rough, polar rutile scale delaminates, the relatively smoother underlying interface is exposed, temporarily reducing hydrophilicity before the re-oxidation cycle continues. The combined influence of oxide chemistry and surface roughness is particularly advantageous for biomedical applications, where surface wettability plays a critical role in protein adsorption, cell adhesion, and proliferation. Previous studies have identified contact angles in the range of 20°-60° as optimal for promoting bioactivity and osseointegration [ 62 ]. The wettability values achieved through controlled thermal oxidation of Ti-35Nb-6Mo fall well within this favorable range, suggesting that oxidation-induced phase transformations offer an effective route for tailoring surface bio-functionality without the need for additional coatings or chemical treatments. Conversely, in high-temperature industrial environments, particularly those involving cyclic humidity or corrosive media, increased surface wettability may accelerate corrosion or fouling processes by facilitating electrolyte spreading and retention on the surface [ 63 ]. Therefore, oxidation treatment parameters must be carefully optimized to balance improved wettability and oxide phase stability with long-term environmental durability. Overall, the evolution of the contact angle with increasing oxidation temperature highlights the coupled roles of oxide phase chemistry, surface roughness, and scale mechanical stability in governing the wettability of the Ti-35Nb-6Mo alloy. These findings confirm the alloy’s potential as a surface-engineered material whose multifunctional performance can be tailored through controlled thermal oxidation. 4. Conclusion This study systematically investigated the oxidation behavior and phase evolution of the Ti-35Nb-6Mo β-titanium alloy subjected to elevated temperatures of 600 ºC, 700 ºC, and 800 ºC. The oxidation kinetics were found to be strongly temperature-dependent, exhibiting protective, near-linear or parabolic behavior at lower temperatures and transitioning to aggressive, predominantly linear kinetics at 800 ºC. Comprehensive characterization utilizing X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and focused ion beam (FIB) analyses confirmed the progressive, multi-layered formation of titanium oxides. The integration of these techniques conclusively mapped the transition from a thin, relatively amorphous protective film at 600 ºC to a highly crystalline scale at elevated temperatures. Rutile TiO 2 was identified as the predominant and stable oxide phase at higher oxidation temperatures, accompanied by the formation of intermediate suboxides such as Ti 3 O 5 at the metal-oxide interface. The evolution of oxide morphology showed increasing crystal size and density with increasing temperature, which correlated directly with the enhanced surface roughness parameters measured via optical profilometry. Furthermore, wettability assessments revealed improved hydrophilicity that perfectly mirrored the massive crystallization of the polar rutile layer and the corresponding roughness increase. Collectively, these interrelated crystallographic and surface changes validate the development of stable, adherent, and protective oxide films on the Ti-35Nb-6Mo alloy at moderate elevated temperatures (600 ºC, 700 ºC). These findings demonstrate that Ti-35Nb-6Mo possesses excellent oxidation resistance and surface bio-functionality, making it a strong candidate for demanding aerospace applications, provided operational temperatures remain within the protective regime. The linear oxidation and mechanical instability observed at 800 ºC establish a clear upper thermal limit for the unprotected alloy. Simultaneously, the alloy’s surface chemistry, highly crystalline rutile formation, and enhanced topography after oxidation strongly support its use in biomedical implants by promoting cellular attachment and osseointegration. This dual applicability underscores Ti-35Nb-6Mo as a highly versatile advanced material for both high-temperature industrial environments and biocompatible medical devices. Declarations Conflict of interest The authors declare that they have no conflict of interest. Funding The author received no financial support for this work. Contributions Jarnail Singh : Conceptualization, Methodology, Writing - original draft preparation. Vicente Amigó Borrás : Resources, Supervision, Writing - review and editing. Rajat Dhawan : Formal analysis, Amarjit Singh : Formal analysis, Writing - review and editing. References El-Bassyouni, Gehan T., Samar M. Mouneir, and Ashraf M. El-Shamy. 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Kar. "Structural and morphological study of thermally oxidized titanium thin films for the detection of chlorpyrifos." Materials Science in Semiconductor Processing 105 (2020): 104695. R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. 28 (8) (1936) 988–994. Liu, Kesong, Moyuan Cao, Akira Fujishima, and Lei Jiang. "Bio-inspired titanium dioxide materials with special wettability and their applications." Chemical reviews 114, no. 19 (2014): 10044–10094. Wang, Qingge, Peng Zhou, Shifeng Liu, Shokouh Attarilar, Robin Lok-Wang Ma, Yinsheng Zhong, and Liqiang Wang. "Multi-scale surface treatments of titanium implants for rapid osseointegration: a review." Nanomaterials 10, no. 6 (2020): 1244. Lu, Die, Jing Ni, Zhen Zhang, and Kai Feng. "Anti-Corrosion Flocking Surface with Enhanced Wettability and Evaporation." Materials 17, no. 16 (2024): 4166. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Reviews received at journal 09 Apr, 2026 Reviewers agreed at journal 08 Apr, 2026 Reviewers agreed at journal 07 Apr, 2026 Reviewers invited by journal 07 Apr, 2026 Editor assigned by journal 07 Apr, 2026 Submission checks completed at journal 07 Apr, 2026 First submitted to journal 04 Apr, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9319174","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":620299346,"identity":"c0208e98-6772-4356-b416-0174751a0c87","order_by":0,"name":"Jarnail Singh","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5ElEQVRIiWNgGAWjYBACxgYQaWDBwMbAw/gAyOThI1KLBAMbGw+zAUgLG5GWSTAwsPGwSYCYBLUwt5999uBHgUQen3zvscqvOXYybAzMDx/dwOewnnRzwx4DiWI2Nr6027LbkoEOYzM2zsHrlzQ2CR4DicQ2Nh6z25LbmIFaeNik8Wrpf8Ym+QeqpVhyWz0RWmaksUnDbGH8uO0wMVqesUnLgLXkGEszbjvOw8ZMwC+G/Wlskm/+2CTObz5j+PHntmp7fvbmh4/xamlA4jDzgEk8ykFAHsWVPwioHgWjYBSMgpEJAKJWOfZdRNIsAAAAAElFTkSuQmCC","orcid":"","institution":"Chandigarh University","correspondingAuthor":true,"prefix":"","firstName":"Jarnail","middleName":"","lastName":"Singh","suffix":""},{"id":620299347,"identity":"7f7b33ab-0b42-41b2-8f4a-df6ef67f97f8","order_by":1,"name":"Vicente Amigó Borrás","email":"","orcid":"","institution":"Universitat Politècnica de València","correspondingAuthor":false,"prefix":"","firstName":"Vicente","middleName":"Amigó","lastName":"Borrás","suffix":""},{"id":620299348,"identity":"b3a37d70-133f-454b-8088-01fac52ed9a0","order_by":2,"name":"Rajat Dhawan","email":"","orcid":"","institution":"Maharishi Markandeshwar University, Mullana","correspondingAuthor":false,"prefix":"","firstName":"Rajat","middleName":"","lastName":"Dhawan","suffix":""},{"id":620299349,"identity":"5ce2df7f-cf8e-4ff8-b2d2-2f05a6de4e94","order_by":3,"name":"Amarjit Singh","email":"","orcid":"","institution":"National Institute of Technology Hamirpur","correspondingAuthor":false,"prefix":"","firstName":"Amarjit","middleName":"","lastName":"Singh","suffix":""}],"badges":[],"createdAt":"2026-04-04 09:10:20","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9319174/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9319174/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":106972182,"identity":"46923fdd-b6ae-46ae-820f-dc0d1b01a95c","added_by":"auto","created_at":"2026-04-15 10:22:31","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":105765,"visible":true,"origin":"","legend":"\u003cp\u003eX-ray diffraction (XRD) pattern of the as-prepared Ti-35Nb-6Mo alloy prior to thermal oxidation.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/23fb79973938a816e40bed18.jpg"},{"id":106972518,"identity":"76066e06-454e-4c13-aca2-dcd3668b9ce0","added_by":"auto","created_at":"2026-04-15 10:23:35","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":407418,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns illustrating the phase evolution of the oxidized Ti-35Nb-6Mo alloy at: (a) 600 °C, (b) 700 °C, and (c) 800 °C across varying exposure durations (0.5 to 72 h), (d) XRD patterns provide a direct comparison of the distinct oxide phases formed at 600, 700, and 800 °C after 24 hours of oxidation.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/14fb9ecfcb603ac683d87c2e.png"},{"id":106972023,"identity":"9b20953b-3c56-48bd-a52a-d74d92ee7e89","added_by":"auto","created_at":"2026-04-15 10:21:51","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":382801,"visible":true,"origin":"","legend":"\u003cp\u003eMass gain per unit area (mg/cm\u003csup\u003e2\u003c/sup\u003e) as a function of oxidation time for Ti-35Nb-6Mo alloy at 600 °C, along with corresponding SEM micrographs.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/9fc8db5498c997b3df43c6e6.png"},{"id":106972011,"identity":"adb430d0-ad59-45cd-a1d0-efbdfd12be32","added_by":"auto","created_at":"2026-04-15 10:21:47","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":215779,"visible":true,"origin":"","legend":"\u003cp\u003eMass gain per unit area (mg/cm\u003csup\u003e2\u003c/sup\u003e) versus oxidation time for Ti-35Nb-6Mo alloy at 700 °C, with corresponding SEM micrographs.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/154d434cba72381e5bdbf64d.png"},{"id":106972007,"identity":"8509839c-1d83-432d-be35-d27710e78b3c","added_by":"auto","created_at":"2026-04-15 10:21:46","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":219226,"visible":true,"origin":"","legend":"\u003cp\u003eMass gain per unit area (mg/cm\u003csup\u003e2\u003c/sup\u003e) as a function of oxidation time for Ti-35Nb-6Mo alloy at 800 °C, accompanied by SEM micrographs.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/209cd4e584b18a1e7add2cd0.png"},{"id":106972181,"identity":"b5bfce77-93a9-490f-b1e6-e075199f1598","added_by":"auto","created_at":"2026-04-15 10:22:31","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":528216,"visible":true,"origin":"","legend":"\u003cp\u003eHigh-magnification SEM micrographs (Secondary Electron mode) showing the surface morphology of the Ti-35Nb-6Mo alloy after 8 hours of oxidation at: (a) 600 °C, (b) 700 °C, and (c) 800 °C.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/c224c735d4ef032bc10d8fea.png"},{"id":106994240,"identity":"ae501e92-a4f1-42bd-b311-74e1fdb2f4c1","added_by":"auto","created_at":"2026-04-15 15:06:44","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":592008,"visible":true,"origin":"","legend":"\u003cp\u003eLow-magnification SEM micrographs (Secondary Electron mode) showing the surface morphology of the Ti-35Nb-6Mo alloy after 48 hours of oxidation at: (a) 600 °C, (b) 700 °C, and (c) 800 °C.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/da1b9fa05fa98600c616c480.png"},{"id":106971988,"identity":"52193b9d-8341-45a3-908c-bcbefe6dfacd","added_by":"auto","created_at":"2026-04-15 10:21:42","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":550316,"visible":true,"origin":"","legend":"\u003cp\u003eCross-sectional FIB-SEM micrographs (Backscattered Electron mode) of the oxide scale formed on the Ti-35Nb-6Mo alloy at 700 °C for: (A) 0.5 h, (B) 2 h, (C) 24 h, and (D) 48 h. Annotations indicate regions where EDS spectra were acquired.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/671d6624d8985d9ab016b30c.png"},{"id":106971983,"identity":"23c7db0c-e75b-4404-b5a9-5a653f83d97a","added_by":"auto","created_at":"2026-04-15 10:21:37","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":581192,"visible":true,"origin":"","legend":"\u003cp\u003eross-sectional FIB-SEM micrographs (Backscattered Electron mode) of the oxide scale formed on the Ti-35Nb-6Mo alloy at 800 °C for: (A) 1 h, (B) 2 h, and (C) 4 h. Panel (D) displays the EDS line scan depth profile corresponding to the yellow line marked in (C), illustrating the elemental concentration gradients.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/22db3d0b8c997d5096b4a906.png"},{"id":106994317,"identity":"4adac9ca-d53e-4afb-9dd5-337cf7054d6d","added_by":"auto","created_at":"2026-04-15 15:07:44","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":38732,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of surface roughness parameters (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, S\u003c/em\u003e\u003csub\u003e\u003cem\u003eq\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, S\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, and S\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e) of Ti-35Nb-6Mo alloy as a function of oxidation time at 600 °C.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/0bd2150fbedfb1cfc83bd379.png"},{"id":106972022,"identity":"b45abf70-0e25-4c90-8725-6f040f3530e0","added_by":"auto","created_at":"2026-04-15 10:21:51","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":41150,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of surface roughness parameters (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, S\u003c/em\u003e\u003csub\u003e\u003cem\u003eq\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, S\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, and S\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e) of Ti-35Nb-6Mo alloy at 700 °C.\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/7ade402528bd55dfea39d730.png"},{"id":106972024,"identity":"c10195e1-2ea3-474a-b91e-3e7002dcd202","added_by":"auto","created_at":"2026-04-15 10:21:51","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":41442,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of surface roughness parameters (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, S\u003c/em\u003e\u003csub\u003e\u003cem\u003eq\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, S\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, and S\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e) \u0026nbsp;of Ti-35Nb-6Mo alloy at 800 °C.\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/97ce03797fc98ae6c6cf1e43.png"},{"id":106972179,"identity":"f0ea367e-bf1d-445d-b380-d09040c47fb6","added_by":"auto","created_at":"2026-04-15 10:22:31","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":37521,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of the mean water contact angle on oxidized Ti-35Nb-6Mo surfaces as a function of exposure time and temperature.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/4c9d734d054cbaa1f481c355.png"},{"id":106994946,"identity":"50434408-f3fd-44e2-a6f2-dedb434cd784","added_by":"auto","created_at":"2026-04-15 15:20:52","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4988998,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9319174/v1/a3643c27-e814-48e8-b52d-f08833df1b6f.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Oxidation Kinetics, Phase Evolution, and Surface Characteristics of Ti-35Nb-6Mo β-Titanium Alloy at Elevated Temperatures","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eTitanium and its alloys have garnered significant attention in various advanced engineering fields, including aerospace, biomedical, chemical processing, and energy industries, primarily due to their exceptional strength-to-weight ratio, outstanding corrosion resistance, and biocompatibility [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. These superior properties make titanium alloys a material of choice for applications demanding high mechanical performance under harsh environmental conditions [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Among the different classes of titanium alloys, β-titanium alloys have emerged as particularly promising due to their enhanced plasticity, lower modulus of elasticity, and excellent corrosion resistance, which result mainly from the stabilization of the β-phase by alloying elements such as niobium (Nb) and molybdenum (Mo) [\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The Ti-35Nb-6Mo alloy specifically attracts attention for combining excellent mechanical properties with superior biocompatibility, making it highly suitable for orthopaedic implants and other biomedical devices requiring long-term durability and minimal adverse reactions in the human body [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eExtensive research has been devoted to understanding the oxidation behavior of titanium and its alloys, which is critical for predicting their performance and longevity in high-temperature and oxidative environments [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Titanium exhibits allotropic transformation from a hexagonal close-packed α-phase at room temperature to a body-centred cubic β-phase above approximately 885 \u0026ordm;C. The oxidation kinetics of titanium alloys typically follow parabolic growth laws at moderate temperatures, indicating diffusion-controlled oxide scale formation, but can transition to linear kinetics as oxide spallation or scale degradation occurs at elevated temperatures [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The formation of a stable titanium dioxide (TiO\u003csub\u003e2\u003c/sub\u003e) layer, particularly in the rutile modification, is understood to play a protective role by reducing further oxygen ingress and oxidation progression. However, factors such as temperature, alloy composition, and microstructure influence the integrity and protectiveness of the oxide scales, affecting oxidation resistance [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Recent studies have provided more nuanced insights into the role of β-phase stabilizers like Nb and Mo on oxidation resistance. The addition of Nb has been shown to improve oxide scale adherence and corrosion resistance by promoting the formation of Nb-rich oxides, which enhances protective behavior [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Conversely, Mo also acts as a β-stabilizer, but its effects on oxide morphology and oxidation kinetics can be complex. While Mo can improve corrosion resistance and mechanical stability at elevated temperatures, it may also give rise to volatile molybdenum oxides (such as MoO\u003csub\u003e3\u003c/sub\u003e) at temperatures above 700 \u0026ordm;C, which can cause porosity, crack formation, and localized degradation of oxide films [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. These contradictory influences necessitate a detailed, alloy-specific investigation into the high-temperature oxidation behavior of Ti-Nb-Mo systems.\u003c/p\u003e \u003cp\u003eDespite advances in oxidation research on various titanium alloy systems, the specific oxidation behavior of metastable β-titanium alloys like Ti-35Nb-6Mo under prolonged high-temperature exposure, especially approaching 800 \u0026ordm;C, and their oxide scale evolution remain insufficiently explored. Most existing research focuses predominantly on commercially pure titanium or α-β alloys such as Ti-6Al-4V, where the interplay between phases significantly differs from predominantly β-alloys [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Furthermore, comprehensive surface characterization using synergistic techniques such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and focused ion beam (FIB) milling for depth-resolved analysis of oxide thickness and compositional gradients has not been systematically applied to Ti-35Nb-6Mo.\u003c/p\u003e \u003cp\u003eThis research aims to address these gaps by investigating in detail the oxidation kinetics, microstructural evolution, and surface characteristics of Ti-35Nb-6Mo subjected to oxidation at 600 \u0026ordm;C, 700 \u0026ordm;C, and 800 \u0026ordm;C for exposure times from 0.5 to 72 hours. These chosen temperatures simulate critical service conditions encountered in aerospace turbine engines, chemical reactors, and sterilization processes for biomedical implants [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. The methodology combines gravimetric analysis to monitor oxidation mass gain, SEM and EDS for morphological and compositional surface evaluation, and FIB milling to provide cross-sectional insight into oxide layer thickness and structural integrity. Complementary surface roughness measurements via optical profilometry and wettability assessments using static contact angle measurements are integrated to correlate oxidation-induced surface modifications with functional properties relevant to corrosion resistance and implant biocompatibility. By explaining the oxidation mechanisms and surface evolution of Ti-35Nb-6Mo, this study will contribute essential knowledge towards the design and optimization of thermally stable β-titanium alloys for high-performance applications. Understanding the interplay between oxidation kinetics, oxide scale properties, and alloy composition will facilitate the development of improved alloys with enhanced lifetime and reliability in demanding oxidative environments, advancing technological applications in aerospace, chemical industries, and biomedical engineering.\u003c/p\u003e"},{"header":"2. Materials and Experimental Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Alloy Synthesis and Sample Preparation\u003c/h2\u003e \u003cp\u003eThe Ti-35Nb-6Mo alloy was synthesized via powder metallurgy, consisting of 35 wt.% niobium, 6 wt.% molybdenum, and a titanium balance. The blended powders were sintered and compacted to yield homogeneous specimens. To ensure consistency during oxidation exposure, the alloy was cut into rectangular samples measuring approximately 12 mm \u0026times; 6 mm \u0026times; 2 mm using a high-precision Struers Secotom-15 cutter equipped with diamond-coated discs. The sample surfaces were mechanically polished using silicon carbide abrasive papers up to 1200 grit, followed by a final polishing step with an alumina slurry to achieve a mirror-like finish. Prior to heat treatment, residual contaminants were removed by ultrasonically cleaning the specimens in successive acetone and ethanol baths for 10 minutes each, followed by drying under ambient laboratory conditions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Isothermal Oxidation and Kinetic Modeling\u003c/h2\u003e \u003cp\u003eHigh-temperature oxidation experiments were conducted in a controlled atmosphere muffle furnace. The specimens were subjected to isothermal oxidation in air at 600 \u0026ordm;C, 700 \u0026ordm;C, and 800 \u0026ordm;C for durations of 0.5, 1, 2, 4, 8, 24, 48, and 72 hours. Following each exposure interval, the samples were removed and allowed to cool naturally in ambient air.\u003c/p\u003e \u003cp\u003eOxidation kinetics were evaluated by recording the mass change before and after treatment using a Shimadzu AUW220D precision analytical balance with a readability of \u0026plusmn;\u0026thinsp;0.01 mg. The total mass gain was normalized by the effective exposed surface area, which was determined using digital calipers with a 0.01 mm resolution. To identify the dominant oxidation mechanisms, the normalized mass gain data were fitted to linear, parabolic, and logarithmic kinetic models using regression analysis to extract the corresponding rate constants. All measurements were performed on at least three independently oxidized samples per condition to ensure statistical reliability.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Structural, Microstructural and Chemical Characterization\u003c/h2\u003e \u003cp\u003ePhase constitution of the as-prepared and oxidized samples was analyzed using a D/max-2550 VB/PC (Rigaku) X-ray diffractometer with Cu-Kα radiation. Scans were performed over a 2θ range of 20\u0026deg; to 80\u0026deg;. Surface morphology and oxide layer microstructures were examined using a Carl Zeiss Auriga Compact Field Emission Scanning Electron Microscope (FESEM) operating at 20 kV. Imaging was performed using both secondary electron (SE) detectors for high-resolution topographical contrast and backscattered electron (BSE) detectors for compositional contrast. Chemical characterization and elemental mapping were conducted using an integrated Oxford Instruments Energy Dispersive X-ray Spectroscopy (EDS) system to determine the atomic distribution of Ti, Nb, Mo, and O within the oxide layers. To assess oxide scale thickness and subsurface integrity, cross-sectional samples were prepared by milling perpendicular trenches into the oxidized specimens using a gallium ion beam coupled to the Zeiss Auriga FESEM. BSE imaging and EDS line scanning across the exposed interfaces allowed for elemental depth profiling and the observation of internal stratification and diffusion gradients.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Surface Topography and Wettability Assessments\u003c/h2\u003e \u003cp\u003eOxidation-induced changes in surface roughness were quantified using a Profilm 3D (Filmetric, USA) non-contact optical profilometer. Measurements were acquired using a 0.75 mm cutoff length over 4 mm scan lengths. Three independent scans were performed at different locations on each sample to calculate the average roughness (S\u003csub\u003ea\u003c/sub\u003e), root mean square roughness (S\u003csub\u003eq\u003c/sub\u003e), and maximum peak height (S\u003csub\u003ep\u003c/sub\u003e). On the other hand, surface wettability was evaluated via static water contact angle measurements using a Kr\u0026uuml;ss DSA25 Drop Shape Analyzer. A 2 \u0026micro;L droplet of deionized water was deposited onto the oxidized surfaces, and the profile was imaged after a 10-second equilibration period. The final contact angle was recorded as the average of the left and right-side droplet profile measurements.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1 X-ray Diffraction (XRD) Analysis\u003c/h2\u003e \u003cp\u003eX-ray diffraction (XRD) was employed to examine the phase constitution of the Ti-35Nb-6Mo alloy in the as-prepared condition and to investigate the evolution of crystalline phases following oxidation at elevated temperatures for different exposure durations. The diffraction patterns provide direct evidence of β-phase stability, oxide formation, and temperature- and time-dependent crystallographic transformations occurring during thermal oxidation. The XRD pattern of the as-prepared Ti-35Nb-6Mo alloy is characterized predominantly by reflections corresponding to the β-Ti phase with a body-centered cubic (BCC) structure, confirming effective β-phase stabilization by Nb and Mo alloying additions [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates the most intense diffraction peak observed at 2θ\u0026thinsp;\u0026asymp;\u0026thinsp;38.6\u0026deg; is indexed to the β-Ti (110) plane. Additional peaks at approximately 55.7\u0026deg;, 69.8\u0026deg;, and 82.6\u0026deg; correspond to the β-Ti (200), β-Ti (211), and β-Ti (220) planes, respectively, in good agreement with standard β-Ti diffraction data (JCPDS card no. 44-1288) [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Weak diffraction peaks associated with α-Ti (hexagonal close-packed) are detected near 2θ\u0026thinsp;\u0026asymp;\u0026thinsp;35.1\u0026deg; (100) and \u0026asymp;\u0026thinsp;40.2\u0026deg; (101) (JCPDS card no. 44-1294), indicating the presence of a small fraction of retained or stress-induced α-phase [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Importantly, no oxide-related peaks are observed in the as-prepared alloy, confirming the absence of detectable surface oxidation prior to heat treatment.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAfter oxidation at 600\u0026deg;C for exposure times ranging from 8 to 72 hours (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a)), the diffraction patterns remain dominated by β-Ti reflections, demonstrating that the bulk β-phase structure of the alloy is preserved during prolonged oxidation at this temperature. Weak diffraction peaks corresponding to rutile TiO\u003csub\u003e2\u003c/sub\u003e appear at 2θ\u0026thinsp;\u0026asymp;\u0026thinsp;27.42\u0026deg;, indexed to the (110) plane, and at \u0026asymp;\u0026thinsp;36.01\u0026deg;, corresponding to the (101) plane (JCPDS card no. 21-1276), particularly after longer oxidation durations [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The low intensity and slow evolution of these rutile peaks indicate limited oxide crystallization, suggesting that the oxide layer formed at 600\u0026deg;C is thin and weakly crystalline. No reflections attributable to sub-stoichiometric titanium oxides such as Ti\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e or Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e, nor any Nb- or Mo-based oxides, are detected, indicating minimal oxide complexity under these conditions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn contrast, oxidation at 700\u0026deg;C leads to a pronounced evolution of oxide phases with increasing exposure time, as clearly depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b). After 8 hours, rutile TiO\u003csub\u003e2\u003c/sub\u003e peaks begin to emerge, indicating the onset of crystalline oxide formation. With increasing oxidation duration to 24, 48, and 72 hours, the rutile TiO\u003csub\u003e2\u003c/sub\u003e reflections indexed to the (110), (101), and (211) planes become progressively sharper and more intense, reflecting enhanced crystallinity and increased oxide development. In addition to rutile TiO\u003csub\u003e2\u003c/sub\u003e, weak diffraction peaks attributed to Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e are observed at 2θ\u0026thinsp;\u0026asymp;\u0026thinsp;25.3\u0026deg;, 32.27\u0026deg;, 47.72\u0026deg;, 56.6\u0026deg;, 58.93\u0026deg;, and 67.12\u0026deg;, corresponding to monoclinic Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e (JCPDS card no. 40\u0026ndash;0806) [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. The appearance of Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e indicates the formation of sub-stoichiometric titanium oxides, likely arising from oxygen concentration gradients within the growing oxide layer [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Despite the increased presence of oxide phases, β-Ti peaks remain noticeable even after 72 hours, indicating partial attenuation rather than complete masking of the substrate signal.\u003c/p\u003e \u003cp\u003eAt 800\u0026deg;C, the XRD patterns (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(c)) are dominated by intense and well-defined rutile TiO\u003csub\u003e2\u003c/sub\u003e peaks, confirming that rutile is the thermodynamically stable and predominant oxide phase at elevated temperature [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Strong reflections indexed to the (110), (101), (211), and (220) planes of rutile TiO\u003csub\u003e2\u003c/sub\u003e are clearly observed (JCPDS card no. 21-1276). In addition, reflections associated with Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e persist, suggesting continued formation of oxygen-deficient titanium oxides and non-uniform oxygen distribution within the oxide layer [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. The β-Ti reflections are significantly weakened, indicating strong attenuation of the substrate signal by the thick and highly crystalline oxide scale. Notably, no diffraction peaks corresponding to Nb or Mo-based oxides are detected at any oxidation temperature, implying that Nb remains largely in solid solution and that Mo does not form stable crystalline oxides detectable by XRD under the present experimental conditions [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d) provides a direct comparison of the diffraction patterns obtained after 24 h of oxidation at 600\u0026deg;C, 700\u0026deg;C, and 800\u0026deg;C. A clear progression in phase evolution is observed, with weak and broad rutile TiO\u003csub\u003e2\u003c/sub\u003e peaks at 600\u0026deg;C, moderately intense peaks with the emergence of Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e at 700\u0026deg;C, and sharp, highly crystalline rutile peaks dominating at 800\u0026deg;C. Simultaneously, the gradual attenuation of β-Ti reflections with increasing temperature confirms the growth and thickening of the oxide scale. This comparative representation highlights the strong temperature dependence of oxide crystallinity and phase stability. Furthermore, the broadening of diffraction peaks provides important insight into the crystallite size and lattice strain within the oxide layers. In general, peak broadening arises from the combined effects of finite crystallite size and lattice microstrain, both of which contribute to variations in interplanar spacing and diffraction coherence length [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. At 600\u0026deg;C, the observed broad and low-intensity peaks indicate the formation of a thin oxide layer with small crystallite size and a high degree of structural disorder, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d). This behavior is characteristic of early-stage oxidation, where limited atomic mobility restricts grain growth and results in a poorly crystalline oxide structure [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. The presence of defects such as vacancies and short-range disorder further contributes to peak broadening at this stage. With increasing temperature to 700\u0026deg;C, peak broadening decreases moderately, indicating an increase in crystallite size and partial relaxation of lattice strain. However, the persistence of some peak broadening suggests the presence of microstrain within the oxide scale, likely arising from oxygen concentration gradients and the coexistence of multiple oxide phases such as TiO\u003csub\u003e2\u003c/sub\u003e and Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. The formation of sub-stoichiometric oxides introduces lattice distortions due to non-uniform oxygen distribution, which contributes to strain-induced peak broadening. At 800\u0026deg;C, the diffraction peaks become sharp and well-defined, indicating significant crystallite growth and improved crystallinity of the oxide scale (see Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d)). The reduction in peak broadening suggests a decrease in lattice strain due to enhanced atomic diffusion and structural reorganization at elevated temperature [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. This transition reflects the formation of a highly crystalline rutile TiO\u003csub\u003e2\u003c/sub\u003e layer, which is thermodynamically stable and exhibits lower defect density compared to oxides formed at lower temperatures [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn addition to peak broadening, slight shifts in peak position were observed with increasing oxidation temperature and exposure time. According to Bragg\u0026rsquo;s law, peak position is directly related to interplanar spacing; therefore, any shift in diffraction angle reflects changes in lattice parameters due to thermal expansion, oxygen incorporation, or residual stress [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. At lower temperatures (600\u0026deg;C), minor shifts toward lower 2θ values may occur due to lattice expansion associated with oxygen diffusion into the titanium matrix and the initial formation of oxide layers. This expansion corresponds to an increase in interplanar spacing and is commonly observed during early oxidation stages [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. At intermediate temperature (700\u0026deg;C), peak shifts become more pronounced, reflecting lattice distortion caused by the formation of sub-stoichiometric oxides such as Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e and the development of oxygen concentration gradients within the oxide scale [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. These gradients generate localized tensile and compressive stresses, leading to variations in d-spacing and corresponding peak shifts. At higher temperature (800\u0026deg;C), peak positions tend to stabilize or exhibit slight shifts toward higher 2θ values, indicating partial lattice contraction and stress relaxation. This behavior is associated with the formation of a thick, highly crystalline rutile TiO\u003csub\u003e2\u003c/sub\u003e layer and the reduction of internal strain through grain growth and structural rearrangement [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Additionally, stress relaxation mechanisms such as crack formation and oxide scale delamination may contribute to the stabilization of lattice parameters at elevated temperature.\u003c/p\u003e \u003cp\u003eOverall, the XRD results reveal a clear temperature- and time-dependent phase evolution during oxidation of the Ti-35Nb-6Mo alloy. At lower temperatures, oxidation is characterized by limited rutile formation and preservation of the β-Ti substrate signal. Intermediate temperatures promote the development of mixed oxide phases with increasing crystallinity, while high-temperature oxidation results in the dominance of highly crystalline rutile TiO\u003csub\u003e2\u003c/sub\u003e accompanied by sub-stoichiometric titanium oxides. These findings confirm that rutile TiO\u003csub\u003e2\u003c/sub\u003e is the principal oxidation product across all investigated conditions, with oxide crystallinity and phase complexity strongly governed by oxidation temperature and exposure duration. The combined analysis of peak broadening and peak shifting provides strong evidence for temperature-dependent evolution of crystallite size, lattice strain, and oxide phase stability in the Ti-35Nb-6Mo alloy during high-temperature oxidation. These findings are consistent with diffusion-controlled oxide growth mechanisms and the progressive transition from protective to non-protective oxidation regimes [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Mass Gain and Oxidation Kinetics\u003c/h2\u003e \u003cp\u003eThe oxidation behaviour of the Ti-35Nb-6Mo β-titanium alloy was quantitatively investigated by monitoring mass gain per unit surface area as a function of oxidation temperature and exposure time, with the numerical data summarized in Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The results reveal a pronounced temperature dependence of oxidation kinetics, reflecting distinct oxidation regimes that are directly governed by the evolution, crystallinity, and phase constitution of the oxide layers identified by XRD analysis. At 600 \u0026ordm;C, the alloy exhibits the slowest oxidation kinetics over the entire exposure duration of 72 h. The mass gain increases gradually from 0.149 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 0.5 h to 0.723 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 8 h, followed by a steady rise to 2.146 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 24 h and 4.585 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 48 h. A slight decrease to 3.759 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 72 h is observed, which is attributed to localized stabilization of the oxide layer rather than a fundamental change in the oxidation mechanism. Excluding this terminal deviation, the mass gain follows a near-linear dependence on time with an excellent linear fit, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The oxidation behaviour at 600 \u0026ordm;C can therefore be expressed by the linear rate law [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]:\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSpecific mass gain (mg/cm\u003csup\u003e2\u003c/sup\u003e) of Ti-35Nb-6Mo β-titanium alloy as a function of oxidation time at 600\u0026deg;C, 700\u0026deg;C, and 800\u0026deg;C.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eTime (h)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003eMass gain per unit area (mg/cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e600\u0026ordm;C\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e700\u0026ordm;C\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e800\u0026ordm;C\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.149\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.375\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.925\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.086\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.611\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.306\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.323\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.260\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.267\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.447\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.404\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.360\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.723\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3.964\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e5.773\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.146\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.251\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e10.749\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4.585\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.679\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e19.021\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3.759\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9.511\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e25.553\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv id=\"Equ1\" class=\"Equation\"\u003e \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\frac{{\\Delta\\:}m}{A}={k}_{l}t$$\u003c/div\u003e \u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFrom the slope of the fitted curve, the apparent linear oxidation rate constant is estimated to be \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003el\u003c/em\u003e\u003c/sub\u003e = 0.09\u0026ndash;0.10 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. This low-rate constant indicates slow oxidation kinetics and is fully consistent with XRD observations, which show dominant β-Ti reflections with only weak and slowly emerging rutile TiO\u003csub\u003e2\u003c/sub\u003e peaks even after prolonged exposure [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The limited crystallinity and gradual development of rutile at this temperature suggest that the oxide layer remains thin and weakly crystalline, allowing oxidation to proceed under a surface-reaction-controlled regime with minimal acceleration over time [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The corresponding SEM micrographs included in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e further confirm the formation of a smooth, compact, and continuous oxide layer at 600\u0026deg;C, supporting the observed protective oxidation behavior.\u003c/p\u003e \u003cp\u003eWhen the oxidation temperature is increased to 700 \u0026ordm;C, the mass gain behaviour changes markedly, indicating a transition to a different oxidation mechanism. The mass gain rises rapidly during the initial stages, increasing from 0.375 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 0.5 h to 3.964 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 8 h, demonstrating accelerated oxide formation. Beyond this stage, the oxidation rate decreases significantly, with mass gain increasing only modestly to 5.251 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 24 h and 5.679 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 48 h. At 72 h, the mass gain increases again to 9.511 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, suggesting partial destabilization or localized breakdown of the oxide layer at extended exposure times. The oxidation data at 700 \u0026ordm;C deviate from linear behaviour and are best described by a parabolic kinetic law (see Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), which represents diffusion-controlled growth. The oxidation behaviour can be expressed as [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]:\u003c/p\u003e \u003cp\u003e \u003cdiv id=\"Equ2\" class=\"Equation\"\u003e \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{\\left(\\frac{{\\Delta\\:}m}{A}\\right)}^{2}={k}_{p}t\\:$$\u003c/div\u003e \u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFrom the curvature of the fitted plot, the parabolic rate constant is estimated to be \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e = 0.45\u0026ndash;0.55 mg\u003csup\u003e2\u003c/sup\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The deceleration of the oxidation rate with time indicates that oxide growth increasingly limits ionic transport as the exposure proceeds. This behaviour correlates strongly with XRD results at 700 \u0026ordm;C, which show a progressive increase in rutile TiO\u003csub\u003e2\u003c/sub\u003e crystallinity along with the emergence of sub-stoichiometric titanium oxides (Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e) [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. The co-existence of stoichiometric and sub-stoichiometric oxides implies the development of oxygen activity gradients within the oxide scale, leading to a chemically and structurally heterogeneous layer that slows further oxidation by restricting diffusion pathways [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. The SEM images in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e reveal a granular oxide morphology with increased surface coverage, consistent with diffusion-controlled oxide growth at 700\u0026deg;C.\u003c/p\u003e \u003cp\u003eAt 800 \u0026ordm;C, the oxidation kinetics undergo a further fundamental change, exhibiting the highest mass gain and a persistent linear trend over the entire exposure period. The mass gain increases sharply from 1.925 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 0.5 h to 5.773 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 8 h, followed by 10.749 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 24 h, 19.021 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 48 h, and reaching 25.553 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 72 h. The mass gain follows an almost perfectly linear relationship with time, with a high regression coefficient, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The oxidation behaviour at this temperature is therefore well described by the linear rate law [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]:\u003c/p\u003e \u003cp\u003e \u003cdiv id=\"Equ3\" class=\"Equation\"\u003e \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:\\frac{{\\Delta\\:}m}{A}={k}_{l}t$$\u003c/div\u003e \u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFrom the slope of the fitted curve, the apparent linear oxidation rate constant at 800 \u0026ordm;C is estimated to be \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003el\u003c/em\u003e\u003c/sub\u003e = 0.34\u0026ndash;0.36 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which is approximately three to four times higher than that observed at 600 \u0026ordm;C. This large increase in kₗ reflects a non-protective oxidation regime [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]. XRD analysis at 800 \u0026ordm;C shows intense and highly crystalline rutile TiO₂ peaks accompanied by persistent sub-stoichiometric oxides, indicating rapid oxide growth but limited long-term stability of the oxide layer. The sustained linear kinetics suggest that the oxide scale formed at this extreme temperature does not establish an effective diffusion barrier, allowing continuous oxygen ingress and oxidation throughout the exposure period [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. The SEM micrographs associated with Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e show extensive cracking, porosity, and oxide fragmentation, confirming the non-protective nature of oxidation at 800\u0026deg;C.\u003c/p\u003e \u003cp\u003eTaken together, the mass gain and oxidation kinetics data reveal a clear progression in oxidation behaviour with increasing temperature. Oxidation at 600 \u0026ordm;C is slow and controlled, characterized by low mass gain and near-linear kinetics associated with limited rutile crystallization. At 700 \u0026ordm;C, oxidation transitions to a diffusion-controlled regime, reflected by parabolic kinetics and linked to the development of more crystalline and compositionally complex oxide phases. At 800 \u0026ordm;C, oxidation becomes aggressive and non-protective, with high mass gain and sustained linear kinetics despite extensive rutile formation. These findings demonstrate that oxidation kinetics in Ti-35Nb-6Mo are governed not only by temperature but critically by the crystallographic nature and stability of the oxide phases formed during high-temperature exposure.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.3 SEM Morphology of Oxide Layers\u003c/h2\u003e \u003cp\u003eScanning Electron Microscopy (SEM) analysis of the Ti-35Nb-6Mo alloy after oxidation at different temperatures revealed distinct morphological characteristics corresponding to the degree of oxidation and structural integrity of the oxide layers formed, as shown in Figs.\u0026nbsp;6 and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003e. Figure\u0026nbsp;6 illustrates high-magnification SEM images of Ti-35Nb-6Mo alloy after 8 hours, highlighting fine-scale surface features, while Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003e provides corresponding low-magnification views of Ti-35Nb-6Mo alloy after 48 hours, to illustrate overall surface morphology and large-scale oxide distribution. The visual evidence provided by the SEM micrographs closely parallels both the macroscopic gravimetric data and the crystallographic phase evolution established by the XRD analysis. At 600 \u0026ordm;C, both high-magnification (Fig.\u0026nbsp;6(a)) and low-magnification (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a)) SEM images exhibited relatively smooth and featureless surfaces with a very thin oxide scale. This morphology indicates the formation of a compact and continuous oxide film, likely composed predominantly of TiO\u003csub\u003e2\u003c/sub\u003e, which serves as an effective barrier limiting further oxygen diffusion into the substrate [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. The minimal surface roughness and absence of visible oxide crystals at this temperature support early-stage oxidation behavior, consistent with a protective oxide regime that impedes mass transport and restricts oxidation progression over time [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. These microscopic observations corroborate the near-linear, slow mass gain kinetics. Furthermore, this aligns well with the XRD analysis, which demonstrated a largely amorphous or nanocrystalline oxide structure; the XRD patterns were heavily dominated by the β-Ti substrate, with only very weak rutile TiO\u003csub\u003e2\u003c/sub\u003e peaks emerging over the 72-hour period. The oxide formed at this temperature appears stable but thin, corroborating the small mass gain and limited roughness increase observed in profilometric studies.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUpon increasing the temperature to 700 \u0026ordm;C, the SEM morphology (Figs.\u0026nbsp;6(b) and 7(b)) showed a clear transition to a granular and nodular surface composed of uniformly distributed oxide grains. This granular texture is a hallmark of grain boundary diffusion processes facilitating oxide grain nucleation and subsequent coalescence into a denser scale [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. Such a morphology is indicative of the transition from early oxide formation to bulk oxide growth, reflecting enhanced diffusion kinetics at higher temperatures. The nature of the oxide grains at 700 \u0026ordm;C aligns with the formation of rutile TiO\u003csub\u003e2\u003c/sub\u003e, which is more thermodynamically stable and dense compared to anatase, though grain coalescence introduces microstructural heterogeneities such as microvoids at grain boundaries [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. This microstructural shift directly explains the parabolic and logarithmic mass gain kinetics, where the growing, denser oxide scale acts as a partial diffusion barrier. The XRD data verifying the rapid crystallization of rutile peaks and the presence of intermediate Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e suboxides further explains this complex, multilayered oxide development. However, these micro-void features may become preferential sites for oxygen ingress or stress localization, potentially compromising long-term scale integrity [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAt 800 \u0026ordm;C, SEM analysis (Figs.\u0026nbsp;6(c) and 7(c)) revealed more pronounced structural changes characterized by thickened oxide layers displaying severe porosity, microcracks, and localized fracture or delamination of the scale. The oxide grains grew significantly coarser, and the scale showed unambiguous signs of mechanical degradation, suggestive of stress accumulation and mismatch in volumetric expansion between the oxide and substrate [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. This mechanical instability is associated with thermal stresses induced by rapid oxide growth and the intrinsic brittleness of rutile TiO\u003csub\u003e2\u003c/sub\u003e at elevated temperatures [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. The XRD patterns clearly validate this macroscopic breakdown, i.e., at 800 \u0026ordm;C, the massive crystallization of the rutile phase significantly attenuated the metallic substrate peaks, indicating the formation of a thick and rigid oxide layer incapable of accommodating thermal strains [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Furthermore, as suggested by the structural analyses, the probable volatilization of molybdenum as gaseous MoO\u003csub\u003e3\u003c/sub\u003e at this extreme temperature likely intensifies the formation of the observed microporosity by physically disrupting the scale from within as the gas escapes. Despite rutile\u0026rsquo;s thermodynamic stability in this temperature regime, its brittleness, combined with resulting growth stresses and potential volatile gas escape, drives crack formation and oxide spallation, thereby exposing fresh metal surfaces to oxidative environments. The structural discontinuity and porosity evident in SEM images at 800 \u0026ordm;C facilitate continuous oxygen ingress and accelerated oxidation kinetics, as reflected in the linear mass gain behavior observed experimentally.\u003c/p\u003e \u003cp\u003eThese morphological observations mark a distinct shift from protective to non-protective oxidation regimes with increasing temperature for the Ti-35Nb-6Mo alloy. The SEM morphology evolution observed herein aligns well with reported oxidation behavior of β-titanium alloys and titanium aluminides, in which the oxide scale transitions from smooth, protective films to fractured, porous, and less adherent oxide layers as temperature rises [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. Overall, SEM characterization, when coupled with the gravimetric trends and crystallographic verification from XRD, provides a comprehensive understanding of the integrity and protective capacity of oxide scales developed on the alloy.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.4 FIB Cross-sectional Analysis\u003c/h2\u003e \u003cp\u003eFigures \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003e and \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e9\u003c/span\u003e demonstrate the cross-sectional analysis of the Ti-35Nb-6Mo alloy using Focused Ion Beam (FIB) microscopy, providing critical quantitative insights into the oxide layer thickness and internal microstructural evolution resulting from high-temperature oxidation. At 600 \u0026ordm;C, FIB cross-sections revealed a relatively thin oxide layer (images not shown). The oxide layer exhibited a compact and uniform morphology with negligible porosity or delamination along the metal-oxide interface. This observation confirms strong adherence between the oxide film and the substrate, characteristic of low-temperature oxidation regimes where oxidation kinetics are limited and oxide growth is tightly controlled by surface reactions [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. Crucially, this structural compactness directly correlates with the time-resolved XRD data, which confirmed the presence of a largely amorphous or nanocrystalline oxide film where the underlying β-Ti substrate signal remained dominant. Such a dense and continuous oxide scale corresponds perfectly with the highly restricted, near-linear oxidation mass gain kinetics established earlier, strongly supporting the protective role of this early-stage film.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWhen the oxidation temperature increased to 700 \u0026ordm;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003e), the oxide scale thickness grew notably. The cross-sectional microstructure displayed distinct multilayered features, indicative of a more complex oxidation process involving sequential oxide phase formation. The presence of these layered oxides, punctuated by occasional microvoids, strictly corroborates the XRD findings, which conclusively identified intermediate titanium suboxides (Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e) coexisting with the thermodynamically stable rutile TiO\u003csub\u003e2\u003c/sub\u003e phase. This stratified oxide formation aligns with thermodynamic models in which intermediate oxide phases act as metastable precursors near the metal interface before progressively transforming into stable rutile TiO\u003csub\u003e2\u003c/sub\u003e layers closer to the oxygen-rich surface [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. The microvoids observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003e likely represent emerging stress-relief features arising from volume and structural mismatches during this complex, multiphase growth, signaling the onset of mechanical relaxation mechanisms within the scale [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAt the highest oxidation temperature investigated, 800 \u0026ordm;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e9\u003c/span\u003e), FIB cross-sections revealed substantially thicker oxide layers. These scales were structurally heterogeneous, exhibiting frequent cracking, delamination zones, and extensive void formation both within the oxide bulk and at the metal-oxide interface. This macroscopic breakdown is a direct consequence of the massive and rapid crystallization of the rutile TiO\u003csub\u003e2\u003c/sub\u003e phase confirmed by the XRD spectra. The thick, rigid, and highly crystalline rutile layer cannot effectively accommodate the combined effects of thermal stress, volumetric expansion mismatch between the oxide and substrate, and interfacial incompatibility during rapid growth [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFurthermore, the presence of these severe subsurface voids is driven by two concurrent degradation mechanisms. First, the Kirkendall effect plays a significant role, where differential diffusion rates of outward-migrating metal cations and inward-diffusing oxygen anions generate vacancies that coalesce into voids beneath the oxide layer [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. Second, as established by the high-temperature XRD and EDS analyses, the localized volatilization of molybdenum into gaseous MoO\u003csub\u003e3\u003c/sub\u003e physically disrupts the scale architecture from within, leaving behind extensive microporosity as the gas escapes. Such porous and fractured oxide scales are highly detrimental to protective properties, explicitly explaining the experimentally observed linear and accelerated oxidation kinetics at this extreme temperature. This compromised scale integrity facilitates continuous oxygen ingress, thereby exacerbating material degradation.\u003c/p\u003e \u003cp\u003eCollectively, the FIB cross-sectional analyses quantitatively and qualitatively validate the trends inferred from mass gain measurements, SEM surface observations, and XRD phase identifications. These structural insights reveal a clear correlation between oxidation temperature, phase evolution, oxide thickness, and overall scale integrity. Such findings are imperative for comprehensively understanding oxidation mechanisms in β-titanium alloys and for guiding the design of materials capable of sustaining stability and protection in demanding high-temperature environments. Furthermore, to correlate these severe structural changes with chemical stratification, an EDS line scan was performed across the FIB cross-section at 800 \u0026ordm;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e9\u003c/span\u003e(C) and (D)). The depth profile clearly illustrates a steep decline in oxygen concentration from the surface inward, accompanied by the depletion of Mo and Nb in the outer oxide layer and their subsequent retention in the subsurface matrix. A more detailed quantitative discussion of this elemental partitioning is provided in the subsequent EDS analysis.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.5 EDS Elemental Analysis\u003c/h2\u003e \u003cp\u003eEnergy Dispersive X-ray Spectroscopy (EDS) provided comprehensive elemental analysis of the oxidized surfaces of the Ti-35Nb-6Mo alloy subjected to varying temperatures, revealing critical insights into the chemical composition and spatial distribution of oxygen and alloying elements within the oxide layers, as depicted in Table \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. At 600 \u0026ordm;C, the EDS spectra of the sample surfaces exhibited comparatively low oxygen levels, with dominant signals corresponding to titanium (Ti), niobium (Nb), and molybdenum (Mo). This indicates that the oxide film formed under these conditions was relatively thin and partially developed, allowing electron beam emissions to readily detect the underlying metallic substrate with minimal attenuation. This elemental observation flawlessly corroborates the time-resolved and temperature-dependent XRD diffractograms, which were heavily dominated by the β-Ti substrate peaks and showed only nascent, low-intensity rutile TiO\u003csub\u003e2\u003c/sub\u003e formation. These combined findings confirm the early-stage oxidation dynamics characterized by the formation of a protective yet thin oxide layer, consistent with the limited mass gain and minimal surface roughness changes observed at this temperature.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eEDS point and area compositional analysis (wt.% and at.%) of the oxide cross-sections corresponding to the spectra locations marked in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003e (700\u0026deg;C).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpectrum No.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eOxidation Time\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eIn Weight\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c7\" namest=\"c4\"\u003e \u003cp\u003eAtomic (%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNb\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eMo\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e85Ti 12Nb 3Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e89.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e6.49\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.24\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e57Ti 35Nb 8Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e69.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e22.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3.40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e4.57\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e61Ti 31Nb 2O 6Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e71.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e18.71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e6.54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e3.38\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e75Ti 20Nb 2O 3Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e80.54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e11.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e6.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.83\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e56Ti 36Nb 2O 6Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e68.94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e22.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e3.77\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e41Ti 33Nb 21O 5Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e32.86\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e13.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e51.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.89\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e65Ti 19Nb 13O 3Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e56.99\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e8.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e33.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.29\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e55Ti 30Nb 10O 5Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e52.84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e15.35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e29.59\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e2.22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e55Ti 12Nb 32O 1Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e35.40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e4.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e60.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.34\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e54Ti 30Nb 13O 3Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e49.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e13.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e34.76\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.47\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e50Ti 21Nb 28O 1Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e34.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e7.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e57.66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.57\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e46Ti 38Nb 14O 2Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e42.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e18.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e38.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.23\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e59Ti 26Nb 11O 4Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e54.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e12.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e32.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.63\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e74Ti 11Nb 13O 2Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e61.99\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e4.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e32.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.63\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e55Ti 32Nb 5O 8Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e62.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e18.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e15.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e4.53\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e78Ti 16Nb 2O 4Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e82.69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e8.86\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e6.49\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.96\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e65Ti 22Nb 10O 3Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e59.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e10.76\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e27.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.38\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e54Ti 17Nb 27O 2Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e38.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e55.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.57\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e56Ti 29NB 12O 3Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e51.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e13.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e33.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.19\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eEDS point and area compositional analysis (wt.% and at.%) of the oxide cross-sections corresponding to the spectra locations marked in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e9\u003c/span\u003e (800\u0026deg;C).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpectrum No.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eOxidation Time\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eIn Weight\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c7\" namest=\"c4\"\u003e \u003cp\u003eAtomic (%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNb\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eMo\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003e1hr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e57Ti 15Nb 26O 2Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e40.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e53.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.78\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e52Ti 38Nb 3O 7Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e62.76\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e23.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e8.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e4.50\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e68Ti 24Nb 3O 5Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e75.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e13.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e8.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e2.55\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e77Ti 18Nb 2O 3 Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e82.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e9.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e6.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.91\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e56Ti 37Nb 1O 6Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e67.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e23.69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e3.83\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e54Ti 37Nb 1O 8Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e66.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e24.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4.46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e5.16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003e2hr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e56Ti 29Nb 13O 2Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e50.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e13.55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e35.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.86\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e65Ti 22Nb 11O 2Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e58.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e10.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e30.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.95\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e57Ti 29Nb 10O 4Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e54.94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e14.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e29.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.72\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e71Ti 16Nb 11O 2Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e62.72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e7.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e29.49\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.73\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e63Ti 20Nb 16O 1Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e53.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e8.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e37.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.53\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e54Ti 35Nb 8O 3Mo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e55.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e18.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e24.87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.38\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eWhen the oxidation temperature was elevated to 700 \u0026ordm;C, an intensification of oxygen peaks was documented in the EDS spectra, accompanied by a relative diminution in the intensity of Nb and Mo signals, particularly in near-surface regions. This shift reflects the formation of a more continuous titanium dioxide (TiO\u003csub\u003e2\u003c/sub\u003e) layer, with the Ti/O atomic ratio approximating values between 0.6 and 0.8 across numerous surface points. Crucially, this measured ratio provides profound chemical validation for the crystallographic data; a Ti/O ratio of 0.6 perfectly matches the stoichiometry of the intermediate titanium suboxide Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e, which was explicitly identified in the 700 \u0026ordm;C XRD spectra. This confirms the simultaneous presence of these intermediate suboxides coexisting with the thermodynamically stable rutile TiO\u003csub\u003e2\u003c/sub\u003e phase. This elemental and crystallographic synthesis robustly proves the multi-layered oxide growth mechanism, where metastable Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e acts as a precursor bridging the metal substrate to the outer rutile layer. The corresponding decrease in Nb and Mo spectral intensity at the surface is interpreted as the result of limited outward diffusion of these refractory alloying elements toward the oxide front or their segregation beneath the oxide layer, a behavior in line with the β-phase stability of the substrate and reported diffusion kinetics in Ti-Nb-Mo systems. As shown in Table \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the oxygen concentration increases progressively with oxidation time at 700\u0026deg;C, confirming the formation of a thicker and more developed oxide scale. The Ti/O ratios further support the coexistence of TiO\u003csub\u003e2\u003c/sub\u003e and sub-stoichiometric Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e phases.\u003c/p\u003e \u003cp\u003eFor samples oxidized at 800 \u0026ordm;C, the EDS elemental maps and point analyses revealed oxygen as the predominant element at the outer oxide surfaces, with the Ti/O ratio closely matching the stoichiometric ratio of rutile TiO\u003csub\u003e2\u003c/sub\u003e (approximately 1:2). This elemental quantification flawlessly supports the XRD results at 800 \u0026ordm;C, which showed diffractograms completely overwhelmed by highly crystalline rutile peaks and devoid of substrate signals. Notably, spectral peaks corresponding to Nb and Mo were largely absent from the oxide surface regions, implying these elements reside preferentially within the metallic substrate or potentially sub-oxide zones. The absence of molybdenum signals near the surface is particularly significant, suggesting volatilization of Mo as molybdenum trioxide (MoO\u003csub\u003e3\u003c/sub\u003e) under high-temperature oxidative conditions, a phenomenon documented in prior studies on Mo-containing alloys, which involves Mo sublimation leading to oxide scale porosity and mechanical destabilization. Additionally, compositional heterogeneity detected in localized surface areas at 800 \u0026ordm;C likely arises from phase separation or anisotropic oxide growth, further complicating the oxide microstructure. Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e demonstrates the dominance of oxygen at 800\u0026deg;C, consistent with the formation of a fully developed rutile TiO\u003csub\u003e2\u003c/sub\u003e layer. The reduced presence of molybdenum at the surface further supports its volatilization as MoO\u003csub\u003e3\u003c/sub\u003e at elevated temperatures.\u003c/p\u003e \u003cp\u003eThe EDS elemental distribution maps conclusively demonstrated a steep oxygen concentration gradient near the sample surface, with a concurrent retention of Ti β-phase stabilizing elements Nb and Mo within the subsurface matrix, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e9\u003c/span\u003e(D). This compositional stratification underscores diffusion-controlled oxidation mechanisms where oxygen permeates inward, reacting with Ti to form progressively thicker, heavily stratified oxide layers, while refractory elements remain relatively immobile within the metallic phase. These findings elucidate the thermally driven evolution of oxide scale chemical architecture, providing essential insights into the formation, protective capacity, and possible degradation modes of the oxide films formed on Ti-35Nb-6Mo alloy under operational thermal stresses. In summary, the EDS analysis validated the progressive oxidation of the alloy, demonstrating a temperature-correlated enrichment of TiO\u003csub\u003e2\u003c/sub\u003e in the oxide scale and revealing the spatial elemental partitioning that governs oxidation kinetics and scale integrity. Such detailed elemental knowledge is critical for predicting the long-term oxidation resistance and failure mechanisms, thereby informing the alloy's application in high-temperature environments requiring superior oxidation stability.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Surface Roughness Measurements\u003c/h2\u003e \u003cp\u003eSurface roughness evolution in the Ti-35Nb-6Mo alloy after oxidation at increasing temperatures reveals a clear dependence on both thermal regimen and exposure time. To comprehensively evaluate these topographical modifications, the average roughness (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e), root mean square roughness (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003eq\u003c/em\u003e\u003c/sub\u003e), maximum peak height (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e), and total peak-to-valley height (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e) were systematically analyzed. This multi-parameter approach provides critical insight into the relationship between crystallographic microstructure, oxide morphology, and final material performance. At 600 \u0026ordm;C, the topographical parameters remained consistently low over the entire 72-hour oxidation period, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e10\u003c/span\u003e. The \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e values ranged between 0.35 and 0.77 \u0026micro;m. Correspondingly, \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003eq\u003c/em\u003e\u003c/sub\u003e tracked very closely to \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e, while \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e remained minimal. The close proximity of \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003eq\u003c/em\u003e\u003c/sub\u003e, combined with low maximum peak and valley heights, mathematically confirms the absence of deep pits or high protrusions. This limited roughening is primarily attributable to the formation of a thin, compact oxide layer. As corroborated by the XRD findings, the oxide formed at this temperature is largely nanocrystalline or amorphous. Because the oxide lacks significant grain coalescence, macroscopic crystallinity, or porosity, it introduces minimal topographical complexity to the surface. Both SEM and profilometric analyses indicate limited surface texturing, reinforcing the conclusion that early-stage, protective oxidation regimes tend to preserve bulk material smoothness and structural integrity. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e10\u003c/span\u003e, all roughness parameters remain relatively low and stable, confirming minimal surface modification at this temperature.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWhen the oxidation temperature was raised to 700 \u0026ordm;C, more pronounced changes emerged in both surface chemistry and topography, demonstrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e11\u003c/span\u003e. The \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e values increased substantially, reaching 11.87 \u0026micro;m after 72 h. Concurrently, \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e exhibited marked increases, reflecting the granular surface morphologies, as confirmed by SEM micrographs. The rise in maximum peak height (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e) represents a macroscopic manifestation of accelerated outward nodular growth and oxide crystallization events confirmed by XRD, specifically, the sharp emergence of crystalline rutile TiO\u003csub\u003e2\u003c/sub\u003e peaks and the intermediate suboxide Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. Meanwhile, the increase in total profile height (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e) captures the onset of localized microvoids occurring between these coalescing crystals (peaks) [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. Such topographical heterogeneity aligns well with the observed parabolic oxidation kinetics. Notably, these roughness parameters exhibit a gradual upward trend during the initial 8 hours, followed by more pronounced increases at longer exposure times, highlighting the complex coupling between oxidation time, scale evolution, and surface roughening. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e11\u003c/span\u003e clearly shows a significant increase in roughness parameters, corresponding to oxide grain growth and surface texturing.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAt 800 \u0026ordm;C, the surface roughness behaviour becomes distinctly more erratic and severe (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e12\u003c/span\u003e). The \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e parameter surges to an extreme peak value of 20.37 \u0026micro;m after only 8 h of oxidation. This intense topographical change is accompanied by massive spikes in \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003eq\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e, and \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e. The large divergence between \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003eq\u003c/em\u003e\u003c/sub\u003e (which is highly sensitive to topographical extremes) and \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e mathematically confirms a severely defective surface. The extreme \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e values reflect the massive outward volumetric expansion of large, faceted rutile TiO\u003csub\u003e2\u003c/sub\u003e grains. More critically, the extraordinary total profile height (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e) perfectly captures the deep surface cracking, extensive porosity, and extreme valleys generated by growth stresses and the localized volatilization of MoO\u003csub\u003e3\u003c/sub\u003e gas [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e12\u003c/span\u003e highlights the abrupt variation in roughness parameters, particularly the sharp decrease at 24 h, which confirms oxide scale spallation followed by re-oxidation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eCrucially, following this 8-hour peak, all roughness parameters precipitously drop at 24 h (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e falls to 2.30 \u0026micro;m). This dramatic, simultaneous reduction in average roughness, peak height (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e), and total profile height (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e) is an unambiguous indicator of a massive scale spallation event. The highly stressed, rough outer oxide layer, comprising the tallest protruding crystals and the deepest superficial cracks, likely delaminated and flaked off entirely, exposing the relatively smoother, newly oxidizing metallic interface beneath it. Subsequent exposure (48 h to 72 h) shows all parameters building back up as newly formed oxide crystallites grow and the scale once again thickens and fractures. This non-monotonic roughness evolution reflects repeated, cyclic oxide growth, fracture, and re-oxidation processes [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe observed roughness evolution is tightly linked to the underlying crystallographic phenomena. At 600 \u0026ordm;C, minimal oxide crystallization results in low surface roughness across all measured parameters. At 700 \u0026ordm;C and particularly at 800 \u0026ordm;C, rapid nucleation of stable rutile TiO\u003csub\u003e2\u003c/sub\u003e drives outward peak growth (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e), while mechanical instability and volatile gas escape carve out deep valleys (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e), strongly influencing the mechanical behavior of the oxide scale [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. Overall, the synergistic evolution of \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003eq\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e, and \u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e in the oxidized Ti-35Nb-6Mo alloy provides a comprehensive topographical map of oxide microstructural development. Smooth surfaces formed under low-temperature oxidation progressively evolve into highly crystalline, deeply cracked, and mechanically unstable topographies at higher temperatures. A rigorous interpretation of these combined profilometry parameters underscores the pivotal role of oxidation kinetics and oxide scale stability in governing surface degradation and functional performance.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.7 Contact Angle Analysis\u003c/h2\u003e \u003cp\u003eThe multidimensional roughness transitions have important implications for surface-related properties such as wettability; increased macro- and micro-roughness at elevated temperatures correlates with reduced contact angles and enhanced hydrophilicity [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. While such surface characteristics may be beneficial in biomedical contexts, it can be detrimental for aerospace applications due to increased susceptibility to corrosion and fatigue [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. Therefore, contact angle measurements conducted on the Ti-35Nb-6Mo alloy, which revealed a distinct decrease in water contact angle with increasing oxidation temperature, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e13\u003c/span\u003e. Initially, the as-prepared alloy surfaces exhibited contact angles close to 90\u0026deg;, suggesting a hydrophobic or minimally wettable character [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. Following oxidation at 600 \u0026ordm;C, the contact angle decreased to approximately 65\u0026deg;, representing a moderate increase in surface hydrophilicity. Further oxidation at 700 \u0026ordm;C and 800 \u0026ordm;C led to more pronounced reductions in contact angle to approximately 50\u0026deg; and 40\u0026deg;, respectively, indicating substantial improvements in surface energy and wettability [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. This overarching trend is fundamentally driven by synergistic changes in both surface chemistry and surface morphology induced by high-temperature oxidation, as corroborated by XRD, SEM, and profilometric analyses.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eChemically, the formation of titanium dioxide (TiO\u003csub\u003e2\u003c/sub\u003e), particularly in its thermodynamically stable rutile phase, is well known to enhance surface hydrophilicity due to its polar nature and high surface energy, which promote strong hydrogen bonding interactions with water molecules [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]. As explicitly confirmed by XRD analysis, the oxide scale evolves from a thin, largely amorphous or nanocrystalline layer at 600 \u0026ordm;C to a highly crystalline, rutile-dominated scale at 700 \u0026ordm;C and 800 \u0026ordm;C. As the rutile TiO\u003csub\u003e2\u003c/sub\u003e phase thickens and becomes the dominant surface constituent, the surface\u0026rsquo;s affinity for water naturally increases [\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]. In parallel, surface topographical features evolve significantly with oxidation temperature, as evidenced by surface roughness measurements and SEM observations. The XRD-confirmed rapid nucleation and aggressive crystalline growth of rutile TiO\u003csub\u003e2\u003c/sub\u003e and intermediate Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e phases at higher temperatures contribute to a markedly roughened surface [\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e]. Classical wetting models proposed by Wenzel describe how increased surface roughness amplifies the intrinsic wetting behavior dictated by surface chemistry [\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e]:\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:\\text{c}\\text{o}\\text{s}{\\theta\\:}^{*}=r\\text{c}\\text{o}\\text{s}\\theta\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\theta\\:}^{*}\\:\\)\u003c/span\u003e\u003c/span\u003eis the apparent contact angle on a rough surface, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\theta\\:\\:\\)\u003c/span\u003e\u003c/span\u003eis the intrinsic contact angle on a smooth surface, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:r\\:\\)\u003c/span\u003e\u003c/span\u003eis the roughness factor defined as the ratio of actual surface area to projected area. According to this model, surface roughness amplifies the intrinsic wetting behavior, i.e., hydrophilic surfaces (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\theta\\:\u0026lt;{90}^{\\circ\\:}\\)\u003c/span\u003e\u003c/span\u003e) become more hydrophilic with increasing roughness, while hydrophobic surfaces become more hydrophobic [\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e]. In the present case, the rough, highly crystalline TiO\u003csub\u003e2\u003c/sub\u003e-dominated surface promotes capillary effects and increases the real solid-liquid contact area, thereby enhancing water spreading and further reducing the apparent contact angle beyond that achievable by chemical modification alone [\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eCrucially, the time-resolved wettability data at 800 \u0026ordm;C perfectly mirrors the cyclic mechanical instability identified in the profilometry analysis. At 800 \u0026ordm;C, the contact angle reaches a minimum of 27.66\u0026deg; after 8 hours of exposure, aligning exactly with the extreme peak in surface roughness (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e = 20.37 \u0026micro;m). However, at 24 hours, the contact angle abruptly rises to 48.62\u0026deg;. This rebound corroborates the massive scale spallation event discussed previously; as the highly rough, polar rutile scale delaminates, the relatively smoother underlying interface is exposed, temporarily reducing hydrophilicity before the re-oxidation cycle continues. The combined influence of oxide chemistry and surface roughness is particularly advantageous for biomedical applications, where surface wettability plays a critical role in protein adsorption, cell adhesion, and proliferation. Previous studies have identified contact angles in the range of 20\u0026deg;-60\u0026deg; as optimal for promoting bioactivity and osseointegration [\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e]. The wettability values achieved through controlled thermal oxidation of Ti-35Nb-6Mo fall well within this favorable range, suggesting that oxidation-induced phase transformations offer an effective route for tailoring surface bio-functionality without the need for additional coatings or chemical treatments.\u003c/p\u003e \u003cp\u003eConversely, in high-temperature industrial environments, particularly those involving cyclic humidity or corrosive media, increased surface wettability may accelerate corrosion or fouling processes by facilitating electrolyte spreading and retention on the surface [\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e]. Therefore, oxidation treatment parameters must be carefully optimized to balance improved wettability and oxide phase stability with long-term environmental durability. Overall, the evolution of the contact angle with increasing oxidation temperature highlights the coupled roles of oxide phase chemistry, surface roughness, and scale mechanical stability in governing the wettability of the Ti-35Nb-6Mo alloy. These findings confirm the alloy\u0026rsquo;s potential as a surface-engineered material whose multifunctional performance can be tailored through controlled thermal oxidation.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis study systematically investigated the oxidation behavior and phase evolution of the Ti-35Nb-6Mo β-titanium alloy subjected to elevated temperatures of 600 \u0026ordm;C, 700 \u0026ordm;C, and 800 \u0026ordm;C. The oxidation kinetics were found to be strongly temperature-dependent, exhibiting protective, near-linear or parabolic behavior at lower temperatures and transitioning to aggressive, predominantly linear kinetics at 800 \u0026ordm;C. Comprehensive characterization utilizing X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and focused ion beam (FIB) analyses confirmed the progressive, multi-layered formation of titanium oxides. The integration of these techniques conclusively mapped the transition from a thin, relatively amorphous protective film at 600 \u0026ordm;C to a highly crystalline scale at elevated temperatures. Rutile TiO\u003csub\u003e2\u003c/sub\u003e was identified as the predominant and stable oxide phase at higher oxidation temperatures, accompanied by the formation of intermediate suboxides such as Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e at the metal-oxide interface. The evolution of oxide morphology showed increasing crystal size and density with increasing temperature, which correlated directly with the enhanced surface roughness parameters measured via optical profilometry. Furthermore, wettability assessments revealed improved hydrophilicity that perfectly mirrored the massive crystallization of the polar rutile layer and the corresponding roughness increase. Collectively, these interrelated crystallographic and surface changes validate the development of stable, adherent, and protective oxide films on the Ti-35Nb-6Mo alloy at moderate elevated temperatures (600 \u0026ordm;C, 700 \u0026ordm;C). These findings demonstrate that Ti-35Nb-6Mo possesses excellent oxidation resistance and surface bio-functionality, making it a strong candidate for demanding aerospace applications, provided operational temperatures remain within the protective regime. The linear oxidation and mechanical instability observed at 800 \u0026ordm;C establish a clear upper thermal limit for the unprotected alloy. Simultaneously, the alloy\u0026rsquo;s surface chemistry, highly crystalline rutile formation, and enhanced topography after oxidation strongly support its use in biomedical implants by promoting cellular attachment and osseointegration. This dual applicability underscores Ti-35Nb-6Mo as a highly versatile advanced material for both high-temperature industrial environments and biocompatible medical devices.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eConflict of interest\u003c/h2\u003e\n\u003cp\u003eThe authors declare that they have no conflict of interest.\u003c/p\u003e\n\u003ch2\u003eFunding\u003c/h2\u003e\n\u003cp\u003eThe author received no financial support for this work.\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003eContributions\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003e\u003cstrong\u003eJarnail Singh\u003c/strong\u003e: Conceptualization, Methodology, Writing - original draft preparation. \u003cstrong\u003eVicente Amig\u0026oacute; Borr\u0026aacute;s\u003c/strong\u003e: Resources, Supervision, Writing - review and editing. \u003cstrong\u003eRajat Dhawan\u003c/strong\u003e: Formal analysis, \u003cstrong\u003eAmarjit Singh\u003c/strong\u003e: Formal analysis, Writing - review and editing.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eEl-Bassyouni, Gehan T., Samar M. 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A comprehensive experimental approach combining gravimetric analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), focused ion beam (FIB) cross-sectional analysis, optical profilometry, and contact angle measurements was employed. The oxidation kinetics exhibited a strong temperature dependence. At 600\u0026deg;C, oxidation followed near-linear kinetics with low mass gain, indicating the formation of a thin and protective oxide layer. At 700\u0026deg;C, the kinetics transitioned to diffusion-controlled behavior, consistent with parabolic trends, associated with the development of a multilayered oxide scale comprising rutile TiO\u003csub\u003e2\u003c/sub\u003e and sub-stoichiometric Ti\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e. At 800\u0026deg;C, oxidation became non-protective, showing sustained linear kinetics and significantly higher mass gain due to oxide scale cracking, porosity, and spallation. Microstructural analysis revealed progressive oxide thickening and increased crystallinity with temperature, with rutile TiO\u003csub\u003e2\u003c/sub\u003e dominating at higher temperatures. Surface roughness increased markedly with oxidation severity, while contact angle measurements indicated enhanced hydrophilicity due to combined effects of oxide chemistry and surface topography. The results demonstrate that Ti-35Nb-6Mo exhibits stable oxidation resistance up to 700\u0026deg;C, while degradation at 800\u0026deg;C defines its upper thermal limit. These findings highlight the alloy\u0026rsquo;s potential for both high-temperature engineering applications and surface-engineered biomedical applications.\u003c/p\u003e","manuscriptTitle":"Oxidation Kinetics, Phase Evolution, and Surface Characteristics of Ti-35Nb-6Mo β-Titanium Alloy at Elevated Temperatures","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-15 10:11:37","doi":"10.21203/rs.3.rs-9319174/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2026-04-09T11:53:49+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"84272985134230038224803206615151463223","date":"2026-04-08T05:04:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"161809981376708876169830030832981708898","date":"2026-04-08T02:45:20+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-08T02:45:03+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-04-07T08:29:23+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-07T08:28:24+00:00","index":"","fulltext":""},{"type":"submitted","content":"High Temperature Corrosion of Materials","date":"2026-04-04T08:56:26+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"high-temperature-corrosion-of-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [High Temperature Corrosion of Materials](https://www.springer.com/journal/11085)","snPcode":"11085","submissionUrl":"https://submission.nature.com/new-submission/11085/3","title":"High Temperature Corrosion of Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"7820020d-f967-4173-807f-f0e705fc6bc4","owner":[],"postedDate":"April 15th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-14T08:39:56+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-15 10:11:37","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9319174","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9319174","identity":"rs-9319174","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}