Effects of Annealing on Phase Transformation and Corrosion Mechanism of Severely Deformed Al 0.3 CoCrFeNi High-Entropy Alloy

Effects of Annealing on Phase Transformation and Corrosion Mechanism of Severely Deformed Al 0.3 CoCrFeNi High-Entropy Alloy | 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 Article Effects of Annealing on Phase Transformation and Corrosion Mechanism of Severely Deformed Al 0.3 CoCrFeNi High-Entropy Alloy Fu-Yun Tsai, Alice Pandaleon, Sean O'Brien, Andrew Martin, Alana Pauls, and 10 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8223266/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 10 You are reading this latest preprint version Abstract The effects of transitional phase states on corrosion behavior in the equiatomic-derived high-entropy alloy Al₀.₃CoCrFeNi were investigated following 90% cold-rolling and subsequent isothermal annealing at 620°C for durations ranging from 1 to 50 hours. Electron and X-ray diffraction analysis reveal a progressive transformation from single-phase FCC to an FCC + B2 + σ trinity. Nano-scale B2 and σ nuclei emerge within partially recrystallized grains after 1–2 h, lowering the cold rolling micro-strain from 0.26 to 0.13. This heterogeneous state has the maximum hardness (552 VHN) while enlarging the chloride-passive window by > 200 mV relative to the solutionized material. Prolonged annealing (> 10 h) coarsens the intermetallics and exhausts Al from the matrix, thereby reducing pitting resistance due to the loss of protective Al₂O₃ film. Characterization by electron microscopy and atom probe tomography reveals that Al-rich B2 nuclei are the primary Al sink that compromises Al 2 O 3 formation at long times. The results demonstrate that a narrowly defined heat-treatment window balances strengthening with passivation in Al 0.3 CoCrFeNi, providing a mechanistic blueprint for designing corrosion-tolerant high-entropy alloys (HEAs). Physical sciences/Chemistry Physical sciences/Engineering Physical sciences/Materials science Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Introduction The fabrication and design of alloys in metallurgy have led to significant technological advancements across various industries (infrastructure, aerospace, automotive, etc.) [ 1 – 4 ], due to the ability to enhance and tailor specific properties of metals for particular applications. Whilst traditional alloys are typically designed with one or two primary elements with minor additions of others, increasing the proportion of additional elements can result in the formation of brittle intermetallic compounds, which can reduce mechanical performance. The introduction of HEAs overcomes the limitations of traditional alloys by introducing many interacting elements in the system, which increases tunability and minimizes the detrimental influence of any single element within a system. HEAs typically incorporate five or more elements in near-equiatomic proportions (5–35 atomic percent each) [ 5 , 6 ]. Recent research in HEAs has shown significant congruent improvements in strength, ductility corrosion resistance, and high-temperature performance [ 3 , 4 , 7 – 10 ]. The Al₀.₃CoCrFeNi alloy was developed as a derivative of the equiatomic CoCrFeNi system to enhance strength and corrosion resistance through the addition of aluminum. This system typically exhibits a multi-phase microstructure, including FCC, B2, and occasionally σ phases, which contribute to tunable mechanical properties [ 7 , 11 ]. Prior work [ 12 ] demonstrated that its tensile yield strength can be tailored from 160 MPa to 1800 MPa through thermomechanical processing alone, without altering composition. The corrosion resistance of Al₀.₃CoCrFeNi also shows promise in aggressive environments. Li et al. [ 13 ] reported that Al additions promote the formation of protective Al₂O₃ scales, while Firouzdor et al. [ 14 ] and Shi et al. [ 11 ] found that small Al additions (e.g., 0.3 at%) enhance passivation. However, excessive Al can promote BCC and B2 phase formation, which may reduce corrosion resistance and pitting stability [ 9 ]. Furthermore, Shi [ 9 ] noted that B2 phase presence diminishes pitting resistance, and studies by Zhang et al. [ 15 ] and Qiu et al. [ 10 ] have shown that annealed conditions generally offer improved corrosion resistance compared to solutionized or remelted states. These findings highlight Al₀.₃CoCrFeNi as a promising alloy system for corrosion-resistant applications, particularly when phase composition and thermal history are carefully controlled. Although several studies have examined corrosion mechanisms and passivity of HEAs in chloride environments [ 10 , 13 , 15 – 19 ], systematic investigations specifically linking annealing time, microstructural evolution, and corrosion resistance remain limited. In particular, few studies have explored how phase transformations affect oxide film formation during corrosion, or how the availability of key oxide-forming elements changes following these transformations. Moreover, the differences in oxide formation pathways and kinetics between highly deformed and fully or partially annealed HEAs are still not well understood. While many investigations focus on the effects of annealing temperature [ 11 ], the influence of annealing duration phase transformations and subsequent passivation behavior has received comparatively little attention [ 20 ]. This study aims to address these gaps. Figure 1 illustrates the dynamic interplay between microstructural evolution and environmental degradation in HEAs. Initially, the alloy exhibits a single-phase FCC structure formed by solutionizing, which transforms under cold rolling into a heavily deformed microstructure with elongated grains and high dislocation density. Annealing then promotes the formation of fine-grained multiphase structures with nanoscale B2 and σ precipitates. In corrosive environments such as 3.5 wt% (≈ 0.6 M) NaCl solution at near-neutral pH, ongoing microstructural changes—including recrystallization and precipitation—interact with corrosive processes, which may preferentially attack phase boundaries or precipitate-decorated grain boundaries. The schematic emphasizes how processing history dictates phase transformation pathways (highlighted in orange), while feedback loops from corrosion (indicated by black arrows) influence phase stability and degradation mechanisms. This schematic underscores the need to better understand the coupling between microstructure and corrosion to engineer HEAs with optimized performance under service conditions. Hypothetically, the addition of Al into a multi-element mixture such as CoCrFeNi would alter the relaxation pathway of the alloy due to the influence and interaction of Al towards each of the other elements. Previously, Preferential Interactivity Parameter (PIP[ 21 ]) have shown that the oxidation affinity, size, standard reduction potential (E 0 ) and cohesive energy density (CED) controls the speciation and surface formation of metal alloys and HEAs [ 22 ]. Thermodynamically, the addition of Al to the alloy increases the overall chemical driving force ( \(\:⟨\widehat{T}⟩\) ), particularly due to Al’s strong affinity for certain elements. Even a minor addition (e.g., 0.3 at%) promotes interactions between Al and thermodynamically compatible elements to reduce system energy and approach pseudo-equilibrium under non-equilibrium conditions. Elements such as Ni and Cr are relatively close to Al on the PIP scale, making them more susceptible to Al-induced changes. These interactions can influence phase stability and relaxation behavior of Ni and Cr during thermal or corrosion processes. In this study, Al₀.₃CoCrFeNi alloy were subjected to high-temperature solid solution treatment, followed by 90% cold rolling and annealing at 620°C for durations ranging from 1 to 50 hours. With increasing annealing time, the microstructure evolved from a single-phase FCC structure to a multiphase mixture containing FCC, B2, and sigma phases. While prior studies have associated such phase mixtures with improved strength and hardness [ 12 ], they may also compromise corrosion resistance [ 14 , 23 ]. Our findings reveal that short-term annealing (1–2 h) significantly enhances corrosion resistance, whereas extended annealing reduces it slightly, yet still maintains better performance than conventional stainless steel. This evolution in corrosion behavior is closely linked to the increasing presence of B2 and sigma phases and concurrent microstructural stress relief, as indicated by decreasing micro-strain. Notably, hardness increases up to 4 h of annealing before marginally declining, suggesting a trade-off between strengthening and corrosion resistance mechanisms. Through this research, we demonstrate that appropriate heat treatment can achieve a desirable balance between strength and corrosion resistance in HEAs. The findings also provide insights into the transformation pathways influenced by prior sample conditions. This approach could be effectively applied to other HEAs and alloy systems. Experimental materials and methods 2.1 Alloy fabrication and Thermomechanical processing The Al₀.₃CoCrFeNi alloy was synthesized using the traditional arc melting method with an Arcast arc melter (Arc 200) [ 24 ]. The buttons were inverted five times and remelted to ensure complete homogeneity. Figure 2 summarizes all subsequent thermomechanical processing after alloy synthesis. First a solutionizing treatment at 1150°C for 10 hours was performed in a vacuum furnace to homogenize the material and dissolve any pre-existing precipitates, resulting in a FCC single-phase microstructure. Next, the material was subjected to 50% cold rolling, introducing significant strain, increasing dislocation density. A second solutionizing step at 1150°C for 0.5 hour was then performed after encapsulating the samples in titanium foils with a tantalum getter, followed by water quenching to facilitate recrystallization. Further deformation through 90% cold rolling increased dislocation density and enhanced hardness. Finally, the samples were annealed at 620°C for periods ranging from 1 to 50 hours. 2.2 Microstructural characterization Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) maps were prepared using a FEI Nova NanoSEM equipped with an EBSD detector. X-ray diffraction (XRD) studies were conducted at room temperature using a Rigaku SmartLab X-ray Diffractometer with Cu Kα radiation (λ = 1.54 Å, 45 kV, 44 mA) to determine the phase composition. Specific samples for transmission electron microscopy (TEM) were prepared using a FEI Nova 200 dual-beam focused ion beam (FIB) system and analyzed in a ThermoFisher Talos F200X microscope operating at 200 kV. Atom probe tomography (APT) specimens were prepared using an FEI Helios dual-beam FIB-SEM, with a Pt capping layer deposited to protect against Ga ion damage. Needle-shaped tips were fabricated from the uncrystallized region of the HEA alloy under 90% cold rolling (CR90%) followed by annealing at 620°C for 1 hour. APT analysis was performed on a CAMECA LEAP 4000X HR system using a 355 nm UV laser (200 pJ, 200 kHz) at a specimen base temperature of 45 K, with a detection rate of 0.003 ions/pulse. The chamber pressure was maintained below 2 × 10⁻¹¹ Torr. Data were reconstructed and analyzed using IVAS 3.8.2, with a detector efficiency of ~ 36%. The Atom Probe Tomography (APT) specimen was prepared using a site-specific focused ion beam (FIB) lift-out technique (Thermo Fisher Scientific Quanta 200 FIB-SEM). Final needle sharpening involved annular milling with progressively reduced ion beam currents (30 nA down to 10 pA) and low-kV (2–5 kV) cleaning steps, resulting in tips with apex radii of ~ 50–100 nm and minimized Ga ion implantation. APT analyses were then performed on a CAMECA LEAP system (3000XR) at the temperature range (40–50 K), using a target evaporation rate of 0.5% and a pulse fraction set to 20% of the steady-state DC voltage. Data reconstruction and comprehensive quantitative analysis, including 3D compositional mapping, 1D concentration profiles, proximity histograms, and cluster analysis, were subsequently conducted using CAMECA's IVAS 3.6.10 software. X-ray Absorption Near Edge Structure (XANES) data were analyzed using SIXPACK software, with the photoelectron energy origin (E₀) determined by the first inflection point of the absorption edge jump. For calibration, the main peak of the spectra was aligned with literature values for the respective transition metals. Normalization of the spectra was performed by adjusting the pre-edge area to 0 and the post-edge area to 1, ensuring consistency and comparability between different spectra. X-ray Photoelectron Spectroscopy (XPS) characterization was carried out using a spherical capacitance analyzer with advanced detector technology and a high-performance Al X-ray monochromator source. All survey spectra scans were taken at a pass energy of 58.7 eV. The data were acquired overnight and analyzed using Perkin-Elmer XPS systems equipped with SCAs and Omni FOCUS™ lenses. 2.4 Mechanical testing Microhardness testing was performed using a standard Vickers microhardness tester with a 500 g load applied for 10 seconds. An average of 10 indents per condition was considered to ensure accuracy. 2.5 Electrochemical measurements Electrochemical tests were conducted using a Biologic VMP-300 potentiostat controlled by EC-lab software in a conventional three-electrode cell. The sample served as the working electrode with an exposure area of approximately 0.079 cm², a platinum mesh was used as the counter electrode, and a saturated calomel electrode (SCE) served as the reference electrode. Prior to polarization tests, the samples were immersed in a 3.5 wt% NaCl solution for 3 hours to stabilize the open circuit potentia. Results and discussion 3.1 Microstructural evolution during 620°C annealing Figure S1 , Fig.S2, Fig. 3 and Fig. 4 collectively illustrate the microstructural evolution of the Al₀.₃CoCrFeNi alloy after 90% cold rolling and subsequent annealing at 620°C for varying durations (0, 10, and 50 hours). The unannealed sample (0h, Fig. S1 ) exhibits a heavily deformed structure with elongated grains, high dislocation density, and no additional phase formation, indicative of severe plastic deformation. Upon annealing for 10 hours (Fig. 3 ), the material shows the onset of recrystallization with more pronounced grain boundary contrast and the initial precipitation of secondary phases. EBSD analysis confirms the microstructure remains predominantly FCC (90%), but minor fractions of sigma (8%) and Al-Ni-rich (B2) (2%) phases are now present as shown in Fig.S2. Further annealing to 50 hours leads to a fully developed, coarser grain structure with well-defined precipitates located both at grain boundaries and within grains. Quantitatively, a dramatic phase transformation occurs: the FCC phase reduces significantly to 41%, while the sigma phase becomes dominant at 45%, and Al-Ni (B2) notably increases to 15% as shown in Fig.S2. Over estimation of sigma and B2 can be expected due to a lower confidence index in EBSD of these phases. However, the dramatic increase in phases fraction of these intermetallic is confirmed by additional TEM analysis. This indicates substantial growth and segregation of these secondary phases at the expense of the FCC matrix during prolonged annealing. 3.2 Early-stage precipitation revealed by correlative TEM–APT (1 h) Figure 4 presents TEM and APT images of the Al₀.₃CoCrFeNi alloy under 90% cold rolling (CR90%) followed by annealing at 620°C for 1 hour. As shown in Fig. 4 (a), TEM images reveal that the alloy exhibits regions of partial recrystallization, with some areas remaining unrecrystallized. Within the recrystallized regions, B2 (Ni/Al rich regions)and sigma phases (Cr rich region) are observed in (b-1) –(b-5). The APT images (shown in Fig. 4 (c)-(f)) provide detailed 3D structural information in the unrecrystallized region indicated as the region of interest (ROI) in (a). Isosurface are used to delineate Al rich (red in Fig. 4 (d) and Cr rich (green in Fig. 4 (d)) regions. Although B2 and sigma precipitates were not observed in the unrecrystallized region through TEM, APT results indicate the presence of domains rich in Ni and Al. These domains appear to act as nuclei for the formation of B2 phases, even though the precipitates are not visible in the TEM images. The line scan further confirms the presence of nuclei for both B2 and sigma phases. 3.3 Hardness and lattice micro-strain Figure 5 (a) shows the hardness variation with annealing time. The hardness increases initially, peaking at 4 hours due to partial recrystallization and the formation of nanoscale B2 and sigma precipitates, which hinder dislocation motion. Beyond 4 hours, hardness decreases as the microstructure becomes more homogeneous and precipitates coarsen, reducing their effectiveness in strengthening. Figure 5 (b) illustrates microstrain measurements obtained via the Williamson-Hall method. The cold-rolled sample exhibits high microstrain due to plastic deformation. Annealing at 620°C for 1 hour significantly reduces microstrain, indicating partial recrystallization and phase transformation. Further annealing results in minimal changes in microstrain, suggesting stress relief and continued phase transformations. 3.4 Electrochemical behavior in 3.5 wt.% NaCl The potentiodynamic polarization (PDP) curves for the Al₀.₃CoCrFeNi alloy, tested in a 3.5 wt.% NaCl solution, are shown in Fig. 5 (c). For comparison with the annealed specimens, both the solutionized HEA and SS 304L were also tested. Among the alloys, the solutionized HEA exhibited the least noble corrosion potential. However, its pitting potential exceeded that of SS 304L by more than 100 mV. When comparing the spontaneous passivity of all tested alloys, the passive corrosion current densities were found to be within a similar range. Following annealing, the corrosion potential of the HEA became more noble. The 1-hour, 2-hour, and 50-hour annealed specimens all exhibited similarly higher corrosion potentials, while the 4-hour and 16-hour samples showed slightly less noble values. The pitting potential also significantly increased after annealing, with the 1-hour and 2-hour specimens showing similar pitting potentials near 750 mVSCE. However, with extended annealing duration, the pitting potential progressively decreased, with the 50-hour annealed sample approaching 550 mVSCE, indicating an increased susceptibility to pitting corrosion. Overall, the 1-hour and 2-hour annealing treatments resulted in the widest passive windows and the highest pitting potentials, thus offering the greatest resistance to localized corrosion. Metastable pitting was observed in both the SS 304L and the 16-hour annealed HEA specimen. Table.1 shows the corrosion and pitting potentials of the HEA alloys annealed for different durations compared to conventional stainless steel. The results indicate that the HEA alloy has significantly better corrosion resistance after annealing, regardless of the duration. Additionally, whether annealed or not, the corrosion resistance of the HEA alloy surpasses that of conventional stainless steel. Table.1 Corrosion parameters for Al 0.3 CoCrFeNi alloys and conventional stainless steel Sample E corr (mV) E_pit (mV) 620-50h -125 ± 4 640 ± 125 620-16h -131 ± 9 637 ± 111 620-4h -131 ± 30 685 ± 19 620-2h -125 ± 4 759 ± 20 620-1h -125 ± 8 778 ± 28 Base -147 ± 26 492 ± 87 SS304L -105 ± 15 279 ± 22 Figure 5 (d) illustrates the relationship between microstrain and corrosion resistance in Al 0.3 CoCrFeNi HEA alloys with varying annealing times. The data in Fig. 5 (b) shows that increased microstrain correlates with decreased corrosion resistance. Microstrain can introduce lattice defects and dislocations, which may serve as nucleation sites for corrosion initiation. These defects can accelerate corrosion by compromising the integrity of the passive layer. 3.5 Near-surface oxidation states after polarization Figure 6 XANES spectra of the Al₀.₃CoCrFeNi alloy after PDP testing in 3.5 wt% NaCl solution, comparing the effects of different heat treatments on corrosion behavior. Since the phonon energy of the main peak is similar across all three heat-treated samples before corrosion, only the data after corrosion is shown. XANES results include total electron yield (TEY) and total fluorescence yield (TFY) data, with TEY (probe depth: 5–10 nm) being surface-sensitive and TFY (probe depth: ~150 nm) being bulk-sensitive. As the TFY data shows minimal differences between corroded and uncorroded regions, only the TEY data is presented in Fig. 6 . The TEY spectrum in Fig. 6 a for Cr shows a sharp peak in the base condition (0 h), indicating a significant surface oxide layer (e.g., Cr₂O₃) formed during corrosion. [ 25 ] This high intensity reflects Cr’s high reactivity, establishing a protective oxide within the 5–10 nm surface region. After 2 h annealing at 620°C, the peak intensity decreases slightly, suggesting an initial thinning or restructuring of the surface oxide, possibly due to microstructural changes or partial oxide dissolution. At 50 h, the intensity drops more noticeably, indicating a significant reduction in surface oxide concentration. This decreasing trend suggests that prolonged annealing weakens Cr’s surface oxide stability, potentially due to diffusion of corrosive species into deeper layers or oxide breakdown, while the + 3 oxidation state remains stable as no peak shifts are observed. The TEY spectrum in Fig. 6 b for Fe exhibits a prominent peak in the base condition (0 h), indicating surface oxidation (e.g., Fe₂O₃ or Fe₃O 4 ). [ 26 ] After 2 h annealing at 620°C, the peak intensity increases slightly, suggesting an enhancement in surface oxide formation or thickening, possibly due to improved Fe diffusion or oxide stabilization with initial heat treatment. At 50 h, the intensity rises further, reflecting a significant increase in surface oxide concentration. This increasing trend indicates that prolonged annealing strengthens Fe’s surface oxide layer, potentially due to sustained reaction with the corrosive NaCl environment, with the + 2/+3 oxidation state remaining stable as no edge shifts occur. Based on the L-XAS TEY data in Fig. 6 c, with peaks at 853.5 eV (L₃-edge) and 873 eV (L₂-edge) and a slight intensity increase at 50 h, the Ni is likely predominantly in an oxide state (e.g., Ni²⁺ as NiO) at the surface (5–10 nm), rather than metallic Ni⁰. The stable intensity from 0 h to 2 h suggests an initial oxide layer formed during corrosion, and the slight increase at 50 h indicates minor oxide growth or thickening due to annealing. The edge positions (853.5 eV and 873 eV) support Ni²⁺, and the intensity trend aligns with surface oxidation. The lack of a significant shift reflects a consistent oxidation state, with the 50 h increase driving limited oxide enhancement. Based on the L-XAS TEY data in Fig. 6 d, with peaks at 779 eV (L₃-edge) and 795 eV (L₂-edge) and an intensity increase at 50 h, the Co is likely predominantly in an oxide state (e.g., Co²⁺ as CoO, possibly Co³⁺ as Co₂O₃) at the surface (5–10 nm), rather than metallic Co⁰. The stable intensity from 0 h to 2 h suggests an initial oxide layer formed during corrosion, and the increase at 50 h indicates enhanced oxide formation or thickening due to annealing. The L₃-edge at 779 eV aligns with Co²⁺, and the intensity rise supports oxide growth, though a minor metallic contribution is possible if the oxide is thin. The lack of a significant shift may reflect a consistent oxidation state, with the 50 h increase driving further oxidation. Although XANES is effective for detecting oxidation states and shifts in Fe, Ni, and Co, it is less sensitive to Al signals due to the lower X-ray absorption cross-section of Al in the relevant energy ranges, particularly in HEAs. XPS and TEM, however, are more effective at detecting Al, especially when Al oxides like Al₂O₃ form during corrosion, as they show clear changes in peak positions corresponding to different oxidation states. While XANES reveals oxidation and reduction processes through peak shifts, XPS mainly provides chemical state information and may not detect such changes unless large oxidation state variations occur. Therefore, combining multiple techniques offers a more comprehensive understanding of the corrosion behavior in HEAs. 3.6 Corrosion product morphology through microstructure characterization Figure 7 shows the TEM images of the different heat-treated Al 0.3 CoCrFeNi HEA alloys after corrosion. For short annealing, two distinct layers are observed on the surface of the CR90-620C/2h sample as shown in Fig. 7 (a). Note that the specimens were subjected to anodic polarization up to the pitting potential, resulting in localized breakdown of the passive film and initiation of stable pitting corrosion. Consequently, the protective oxide layers originally present on the surface have been extensively damaged or dissolved in the pitted regions. The observed surface features (e.g., corrosion product aggregates) are characteristic of post-pitting degradation and do not represent the intact passive film formed under open-circuit or early-stage polarization conditions. These images serve to illustrate the morphology and subsurface damage induced by pitting attack rather than the structure of the protective film. The outermost layer is rich in Cr (60 at%), while the second layer consists of Al₂O₃, as confirmed by characteristic Al 2p and O 1s binding energies in XPS spectra and composition from EDS analysis in Table.2. In contrast, the CR90-620°C/16h sample does not exhibit the presence of Al₂O₃ as shown in Fig. 7 (b). Instead, a Cr-rich oxide layer forms on the surface. Furthermore, the matrix of the 16-hour annealed sample contains B2 phases, with B2 phases forming along specific grain boundaries—phases that are not observed in the 2-hour annealed sample. For the 2-hour annealed sample, the rapid formation of Al₂O₃ suggests a kinetic pathway where aluminum preferentially reacts with oxygen rather than diffusing to form the B2 phase. This preference indicates that the alloy may not reach the equilibrium state required to stabilize the B2 phase during the shorter annealing time. Li et al. [ 13 ] proposed that in Al 1.0 CoCrFeNi and Al 1.3 CoCrFeNi alloys, Al reacts with NaCl to form AlCl₃, which subsequently leads to the formation of a protective Al₂O₃ layer on the surface. This mechanism effectively protects the alloy from corrosion. In the current study, the presence of Al₂O₃ in the CR90-620C/2h sample suggests that it also plays a protective role, contributing to better corrosion resistance in shorter annealing times (1–2 hours) compared to longer times (16 hours or more). The presence of a Cr-rich layer above the Al₂O₃ has also been reported in previous studies. Gwalani et al. [ 22 ] used a combination of in situ atom probe tomography and thermo-kinetic treatment applying preferential interactivity parameter approach to investigate the oxidation mechanism/sequence. They reported that the early stages of oxidation can be interpreted through a multi-pronged coupling of thermodynamic parameters such as atomic size, cohesive energy, electronegative and so on. An aluminum oxide (Al₂O₃) layer tends to form first on HEAs due to the higher affinity of aluminum for oxygen, leading to its preferential oxidation over elements like Cr, Fe, Co, and Ni. According to the Ellingham diagram, aluminum has a significantly lower Gibbs free energy of oxidation than these elements, making the formation of Al₂O₃ thermodynamically more favorable. This behavior is particularly advantageous in HEAs, as the rapid formation of a protective Al₂O₃ layer can enhance the oxidation resistance of the material. Table.2 Atomic ratio of metals in Al 0.3 CoCrFeNi HEA alloy (a) annealing at 620℃ for 2h and (b) 16 h after corroding in 3.5wt% NaCl solution atomic ratio for metal Al Cr Fe Co Ni (a) 620C-2h (Al 2 O 3 ) 9.7 2.9 1.0 1.2 1.0 620C-2h Cr-rich oxide 0.6 3.4 1.0 0.7 0.3 (b) 620C-16h Cr-rich oxide 0.6 2.3 1.0 1.0 1.2 assuming iron as 1, and other components are the ratio to iron. Figure 8 shows the Cr and Al signals in the differently heat-treated HEAs after corrosion. The data indicate that protective oxide layers containing Cr₂O₃ and Al₂O₃ formed after corrosion. All raw XPS spectra are provided in the supplementary information (Fig.sS5 and S6) for comprehensive detail. It appears that Al₂O₃ reaches its maximum intensity in the HEA after cold rolling and annealing at 620°C for2 hours. However, the intensity of Cr₂O₃ decreases after corrosion in the HEA subjected to longer annealing. Change in Corrosion Mechanisms as a Function of Microstructural Change Figure 9 shows the schematic illustration of the effect of annealing time on corrosion behaviors of Al 0.3 CoCrFeNi HEA alloy. Before corrosion, annealing at 620°C for 2 hours results in partial recrystallization, especially in areas that experienced significant strain during cold rolling. These recrystallized regions exhibit equiaxed grains with low dislocation density. However, most regions remain un-recrystallized due to the high degree of plastic deformation caused by cold rolling, retaining elongated grains and a higher dislocation density. Moreover, Ni/Al-rich domains act as nuclei for B2 phase formation. After corrosion, NaCl solution dissolves the metal matrix, leading to the formation of a protective Al₂O₃ oxide layer due to its thermodynamic stability. The Cr-rich phase forms on top of Al₂O₃ because chromium also has lower redox potential compared to other metal elements (Fe, Co and Ni) [ 22 ] and higher diffusion coefficients in aluminum oxide [ 27 ]. Roy et al. [ 27 ] studied cationic and anionic diffusion through stable aluminum oxides in a model HEA using atomistic simulations.. They found that metal ions such as Cr, Fe, Co, and Ni have higher diffusion coefficients than Al or O ions, indicating that these metal ions move more easily or rapidly than Al or O ions. This finding aligns with our observations, where we identified a Cr-rich phase on top of the Al₂O₃ layer. For longer annealing time at 16 h, more regions were recrystallized compared to the 2-hour annealing condition, primarily exhibiting FCC phases. In addition, B2 and sigma phases formed in some regions due to their thermodynamic stability. After corrosion, due to the formation of the B2 phase, aluminum preferentially formed B2 in the matrix rather than Al₂O₃ on the surface, compared to the 2-hour condition. Cr₂O₃ and FeCr₂O₄ spinel oxides were formed on the surface due to their thermodynamic stability [ 28 ]. The previous work on stainless steels and nickel-based alloys in simulated pressurized water reactor conditions has consistently shown the presence of these stable oxide phases contributing to corrosion performance. [ 29 – 31 ] Conclusion This study investigates the impact of various annealing times on the corrosion resistance and mechanical properties of the Al 0.3 CoCrFeNi alloy. Annealing at 620°C transformed the cold-rolled Al 0.3 CoCrFeNi alloy from a deformed structure into a heterogeneous microstructure featuring both recrystallized and un-recrystallized regions. Initially (1–4h), the recrystallized areas contained a mix of FCC and B2 phases, while un-recrystallized regions retained high dislocation density and nano-sized Ni-Al-rich domains. This annealing-induced microstructure optimizes the balance between mechanical strength and corrosion resistance. By 50h, B2 and sigma phases dominated, with Al favoring B2 formation over Al₂O₃, which significantly impacted both mechanical and corrosion properties. Short-term annealing (1–2h) notably improved corrosion resistance due to the formation of Al-rich oxide films. However, longer annealing times increased the fractions of B2 and sigma phases, consequently reducing the Al available for protective oxide formation. While this led to lower corrosion resistance compared to short-term annealing, the HEA still outperformed stainless steel. Ultimately, this study highlights that optimizing microstructure through thermal treatments is crucial for enhancing the Al 0.3 CoCrFeNi HEA's performance in corrosive environments and improving its mechanical strength. Declarations Competing interest The authors declare no competing interests. Funding The research was supported (in part) by ONR under Grant N00014-23-1-2758. Instrumentation at the AIF was acquired with support from the National Science Foundation (DMR-1726294). The AIF is supported by the National Science Foundation (award number ECCS-1542015). Alice Pandaleon was supported by the NSF-funded MAT-DAT REU Site (DMR-2150360). Author Contribution F.Y.T., A.P., S.O., A.M., A.Pa., F.I., M.L., Z.Z., and M.J.U. performed experiments, data collection, and preliminary analysis.J.G., M.M.T., R.G., R.B., and E.K. contributed to the interpretation of results and methodology development.F.Y.T., A.P., S.O., A.M., A.Pa., F.I., and M.L. assisted with data curation, visualization, and figure preparation.B.G. led project conceptualization, experimental design, and overall supervision.F.Y.T. and B.G. prepared the initial manuscript draft.All authors (F.Y.T., A.P., S.O., A.M., A.Pa., F.I., M.L., M.J.U., Z.Z., J.G., M.M.T., R.G., R.B., E.K., B.G.) reviewed and approved the final manuscript. Acknowledgement The TEM characterization work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina. The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI). The authors thank the AIF staff for their assistance. Data Availability All data were available in the main text or the supplementary materials. References Miracle, D.B., Critical Assessment 14: High entropy alloys and their development as structural materials . Materials Science and Technology, 2024. 31(10): p. 1142–1147. Stepanov, N.D., et al., Effect of Al on structure and mechanical properties of AlxNbTiVZr (x = 0, 0.5, 1, 1.5) high entropy alloys . Materials Science and Technology, 2015. 31(10): p. 1184–1193. Kumar, A. and M. Gupta, An Insight into Evolution of Light Weight High Entropy Alloys: A Review . Metals, 2016. 6(9): p. 199. Sonal, S. and J. Lee, Recent Advances in Additive Manufacturing of High Entropy Alloys and Their Nuclear and Wear-Resistant Applications. Metals, 2021. 11(12): p. 1980. Yeh, J.W., et al., Nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes . Advanced Engineering Materials, 2004. 6(5): p. 299–303. Cantor, B., et al., Microstructural development in equiatomic multicomponent alloys . Materials Science and Engineering: A, 2004. 375–377: p. 213–218. Gwalani, B., et al., Tensile yield strength of a single bulk Al0.3CoCrFeNi high entropy alloy can be tuned from 160 MPa to 1800 MPa . Scripta Materialia, 2019. 162: p. 18–23. Miracle, D.B. and O.N. Senkov, A critical review of high entropy alloys and related concepts . Acta Materialia, 2017. 122: p. 448–511. Shi, Z.J., et al., Effect of thermally induced B2 phase on the corrosion behavior of an Al0.3CoCrFeNi high entropy alloy . Journal of Alloys and Compounds, 2022. 903: p. 163886. Qiu, Y., et al., Corrosion of high entropy alloys . npj Materials Degradation, 2017. 1(1). Shi, Y., et al., Corrosion of Al CoCrFeNi high-entropy alloys: Al-content and potential scan-rate dependent pitting behavior . Corrosion Science, 2017. 119: p. 33–45. Gwalani, B., et al., Compositionally graded high entropy alloy with a strong front and ductile back . Materials Today Communications, 2019. 20: p. 100602. Li, L., et al., Al CoCrFeNi high entropy alloys with superior hot corrosion resistance to Na2SO4 + 25% NaCl at 900°C . Corrosion Science, 2021. 187: p. 109479. Firouzdor, V., et al., Corrosion of a stainless steel and nickel-based alloys in high temperature supercritical carbon dioxide environment . Corrosion Science, 2013. 69: p. 281–291. Zhang, X., et al., Influence of remelting and annealing treatment on corrosion resistance of AlFeNiCoCuCr high entropy alloy in 3.5% NaCl solution . Journal of Alloys and Compounds, 2019. 775: p. 565–570. Lu, P., et al., Computational materials design of a corrosion resistant high entropy alloy for harsh environments . Scripta Materialia, 2018. 153: p. 19–22. Liang, J., et al., Corrosion resistance and mechanism of high-entropy alloys: A review . Materials and Corrosion, 2024. 75(4): p. 424–432. Chen, M., et al., Phase, Microstructure and Corrosion Behaviour of Al(0.3)FeCoNiCr(x) High-Entropy Alloys via Cr Addition . Materials (Basel), 2024. 17(21). Sahu, S., et al., Localized Corrosion Behavior of Non-Equiatomic NiFeCrMnCo Multi-Principal Element Alloys . Electrochimica Acta, 2020. 354: p. 136749. Liu, Z., et al., Microstructure, mechanical properties and corrosion resistance of direct energy deposited AlCu0.25CoCrFeNi2.1 high entropy alloy: Tailored by annealing heat treatment . Materials Characterization, 2024. 213: p. 114035. Martin, A. and M. Thuo, Predicting Emergence of Nanoscale Order in Surface Oxides through Preferential Interactivity Parameter . ACS Nano, 2024. 18(9): p. 6740–6747. Gwalani, B., et al., Mechanistic understanding of speciated oxide growth in high entropy alloys . Nat Commun, 2024. 15(1): p. 5026. Lee, C.P., et al., Enhancing pitting corrosion resistance of AlxCrFe1.5MnNi0.5 high-entropy alloys by anodic treatment in sulfuric acid . Thin Solid Films, 2008. 517(3): p. 1301–1305. Gwalani, B., et al., Modifying transformation pathways in high entropy alloys or complex concentrated alloys via thermo-mechanical processing . Acta Materialia, 2018. 153: p. 169–185. Seong, S., et al., Experimental evidence for mixed-valent Cr ions in half-metallic CrO2: Temperature-dependent XMCD study . Journal of Magnetism and Magnetic Materials, 2018. 452: p. 447–450. Fonin, M., et al., Surface electronic structure of theFe3O4(100): Evidence of a half-metal to metal transition . Physical Review B, 2005. 72(10). Roy, I., P.K. Ray, and G. Balasubramanian, Diffusion of multi-principal elements through stable Cr2O3 and Al2O3 scales . Materialia, 2022. 24: p. 101497. Puigdomenech, B.B.a.I., Pourbaix diagrams for the ternary system of Fe-Cr-Ni . Corrosion. 55(11): p. 1077–1087. Tsai, F.Y., Corrosion Sensitivity of Stainless Steels in Pressurized Water Reactor Water Chemistry: Can KOH replace LiOH in PWRs. Doctoral dissertation, North Carolina State University https:// repository.lib.ncsu.edu/items/937cd4c 1-72a3-4d73-b5d8-8def6b7670ac, 2022. Tsai, F.-Y., et al., Corrosion sensitivity of nickel-based Alloy Inconel 600 in pressurized water reactor water chemistry: Can KOH replace LiOH? Corrosion Science, 2025. 255: p. 113052. Tsai, F.-Y., et al., Effect of irradiation on the corrosion of 304 stainless steel in pressurized water reactor (PWR) simulated water chemistry . Corrosion Science, 2024. 240: p. 112454. Additional Declarations No competing interests reported. Supplementary Files SupplementaryfilesofEffectsofAnnealingonPhaseTransformationandCorrosionMechanismofSeverelyDeformedHighEntropyAlloy.pdf Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 30 Mar, 2026 Reviews received at journal 29 Mar, 2026 Reviewers agreed at journal 17 Mar, 2026 Reviewers agreed at journal 11 Jan, 2026 Reviews received at journal 08 Jan, 2026 Reviewers agreed at journal 17 Dec, 2025 Reviewers invited by journal 02 Dec, 2025 Editor assigned by journal 02 Dec, 2025 Submission checks completed at journal 02 Dec, 2025 First submitted to journal 27 Nov, 2025 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-8223266","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":554790333,"identity":"f8bdcd4d-6e7e-496e-a2c9-f1021033b55e","order_by":0,"name":"Fu-Yun Tsai","email":"","orcid":"","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"Fu-Yun","middleName":"","lastName":"Tsai","suffix":""},{"id":554790334,"identity":"c8c4b80a-21cf-46a1-9216-1fba1e6082fd","order_by":1,"name":"Alice Pandaleon","email":"","orcid":"","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"Alice","middleName":"","lastName":"Pandaleon","suffix":""},{"id":554790335,"identity":"1c37987c-4fab-42ef-add3-44916cba2dad","order_by":2,"name":"Sean O'Brien","email":"","orcid":"","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"Sean","middleName":"","lastName":"O'Brien","suffix":""},{"id":554790336,"identity":"c81bd83d-9c35-426f-83f8-7077dbd833f1","order_by":3,"name":"Andrew Martin","email":"","orcid":"","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"Andrew","middleName":"","lastName":"Martin","suffix":""},{"id":554790337,"identity":"8cc35dfd-8ca2-4235-9d54-dbf43747e5df","order_by":4,"name":"Alana Pauls","email":"","orcid":"","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"Alana","middleName":"","lastName":"Pauls","suffix":""},{"id":554790338,"identity":"8f560fc9-b1e8-46c4-8046-38a514e72d2b","order_by":5,"name":"Farhan Ishrak","email":"","orcid":"","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"Farhan","middleName":"","lastName":"Ishrak","suffix":""},{"id":554790339,"identity":"7dd8af0c-b817-44c2-a7a4-1857c4c1e576","order_by":6,"name":"Michael Lastovich","email":"","orcid":"","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"Michael","middleName":"","lastName":"Lastovich","suffix":""},{"id":554790340,"identity":"f087d073-31d9-4b65-a10a-32ea9f0d8e8c","order_by":7,"name":"Md Jasim Uddin","email":"","orcid":"","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"Md","middleName":"Jasim","lastName":"Uddin","suffix":""},{"id":554790341,"identity":"03841418-53c7-4a9c-aeb7-9789df450e84","order_by":8,"name":"Zengqing Zhuo","email":"","orcid":"","institution":"Lawrence Berkeley National Lab","correspondingAuthor":false,"prefix":"","firstName":"Zengqing","middleName":"","lastName":"Zhuo","suffix":""},{"id":554790342,"identity":"b23a637a-3daf-497e-9416-96761b93a7f9","order_by":9,"name":"Jinghua Guo","email":"","orcid":"","institution":"Lawrence Berkeley National Lab","correspondingAuthor":false,"prefix":"","firstName":"Jinghua","middleName":"","lastName":"Guo","suffix":""},{"id":554790343,"identity":"3efea7b3-a91b-43d6-9754-b8d385e920de","order_by":10,"name":"Martin Mwangi Thuo","email":"","orcid":"","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"Martin","middleName":"Mwangi","lastName":"Thuo","suffix":""},{"id":554790346,"identity":"2b3c06e6-0ffd-409e-8e0d-25d33686a1a7","order_by":11,"name":"Rajeev Gupta","email":"","orcid":"","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"Rajeev","middleName":"","lastName":"Gupta","suffix":""},{"id":554790347,"identity":"163c9ac4-2bc1-4355-8965-32937ba17803","order_by":12,"name":"Rajarshi Banerjee","email":"","orcid":"","institution":"University of North Texas","correspondingAuthor":false,"prefix":"","firstName":"Rajarshi","middleName":"","lastName":"Banerjee","suffix":""},{"id":554790348,"identity":"b9ffde5e-369c-4a70-8c2e-026ea06419e3","order_by":13,"name":"Elizabeth Kautz","email":"","orcid":"","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"Elizabeth","middleName":"","lastName":"Kautz","suffix":""},{"id":554790349,"identity":"947eafa6-3b2f-40ad-81a5-ba3bd7131d71","order_by":14,"name":"Bharat Gwalani","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAx0lEQVRIiWNgGAWjYBACAxDxgIFBDspnJlJLAgODMelaEhuI1mLOv8bwQ0LNnfTt7MefSTBUWMP04gaWM94YSyQce5a7syfHTILhTDphLQY3zm6QSGA7nLvhBg+bBGPbYaK0bP6R8O9wusEN9mcSjP+I0XK+d5tEYtvhBIMbDGYSjA1EaLGcwf/NIrHvsOGGMznGFgnH0o0JajHnP5Z848O3w/IGx48/vPGhxlqWoBYGiQQkTgIORaiA/wBRykbBKBgFo2AkAwAXI0PlzgNBlAAAAABJRU5ErkJggg==","orcid":"","institution":"North Carolina State University","correspondingAuthor":true,"prefix":"","firstName":"Bharat","middleName":"","lastName":"Gwalani","suffix":""}],"badges":[],"createdAt":"2025-11-27 15:08:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8223266/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8223266/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":97667817,"identity":"39646792-516b-4ad8-a7d8-f0dd72864494","added_by":"auto","created_at":"2025-12-08 09:24:19","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":6221868,"visible":true,"origin":"","legend":"","description":"","filename":"HEACorrosionAl0.311302025.docx","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/5b7c2b1c87e8a1658f1405bf.docx"},{"id":97667802,"identity":"9e24f9f6-07f5-4f18-b3ff-32aab5da0f39","added_by":"auto","created_at":"2025-12-08 09:24:17","extension":"json","order_by":1,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":14780,"visible":true,"origin":"","legend":"","description":"","filename":"a11139111e394a499b69bf60776beada.json","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/50f273c53ccf0ce533cc0560.json"},{"id":97448524,"identity":"3eb071fa-da2b-409b-8347-d407cb6ac1a0","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"pdf","order_by":2,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1200992,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryfilesofEffectsofAnnealingonPhaseTransformationandCorrosionMechanismofSeverelyDeformedHighEntropyAlloy.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/cffb6aaae01711630436025e.pdf"},{"id":97668962,"identity":"89f629c9-135d-4889-8a36-d3c744ead257","added_by":"auto","created_at":"2025-12-08 09:26:48","extension":"xml","order_by":3,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":96011,"visible":true,"origin":"","legend":"","description":"","filename":"a11139111e394a499b69bf60776beada1enriched.xml","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/faf62454110ce500db56b225.xml"},{"id":97667439,"identity":"dc18ddde-bcf1-4eef-b2fa-82e272e99749","added_by":"auto","created_at":"2025-12-08 09:23:32","extension":"png","order_by":4,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":929259,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/6b5a7d795794aca4373e130b.png"},{"id":97448527,"identity":"97ca69ed-7850-4e77-ae37-9f1bd2390dbd","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"png","order_by":5,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":384703,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/074601063ed70bcb9b6673d1.png"},{"id":97667489,"identity":"f4a270d5-6102-4a5a-9c1b-d5e22d1b4b04","added_by":"auto","created_at":"2025-12-08 09:23:39","extension":"png","order_by":6,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1640495,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/287e8ad5f9466388cc001c52.png"},{"id":97667803,"identity":"591176e8-441f-4f1c-a201-7c5da0c52849","added_by":"auto","created_at":"2025-12-08 09:24:17","extension":"png","order_by":7,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1115404,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/b3aa3963f768d720218d854f.png"},{"id":97448535,"identity":"7cff195a-8e03-4989-b417-2c477352cd31","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"png","order_by":8,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":254754,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/f1f9b633be22a64f887cda65.png"},{"id":97669002,"identity":"67aa1e2e-1b59-4ac4-a5b8-1f10edbbac28","added_by":"auto","created_at":"2025-12-08 09:26:56","extension":"png","order_by":9,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":269675,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/77418420a2310044ee357efe.png"},{"id":97448536,"identity":"bc65a554-2538-4bf4-8291-2561ae32548f","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"png","order_by":10,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1259257,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/674ea698545c98568b938ab4.png"},{"id":97667495,"identity":"585f4f0d-d19b-4c72-9448-55f69902f545","added_by":"auto","created_at":"2025-12-08 09:23:39","extension":"png","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":86388,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/2fef9f32b98290384635628b.png"},{"id":97668630,"identity":"0e98054a-9faf-4334-9b04-c82cb7ae6dbb","added_by":"auto","created_at":"2025-12-08 09:25:52","extension":"png","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":238888,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/dedbaeb69d5b459aa600f6cc.png"},{"id":97667130,"identity":"58969536-ba5b-4347-ae69-0b19a2fe023a","added_by":"auto","created_at":"2025-12-08 09:22:53","extension":"png","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":50528,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/4c144a3e11d6ad1ab0e1632a.png"},{"id":97667493,"identity":"7f95cd4f-db93-42e8-b0ad-4db8a44bbd1e","added_by":"auto","created_at":"2025-12-08 09:23:39","extension":"png","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":47168,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/8694e65aba3a0a69d8dd468d.png"},{"id":97448541,"identity":"b10760f1-1bac-449a-9870-15249c7fcbd9","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"png","order_by":15,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":301111,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/4ae282aed72c778570af125d.png"},{"id":97448548,"identity":"6bc7016a-631e-4520-9432-81f8d25beaec","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"png","order_by":16,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":197010,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/454670da0de676b358967e83.png"},{"id":97448550,"identity":"84247214-8b9a-4334-a1de-dcc8db583209","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"png","order_by":17,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":61395,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/c922d59659ced2b803323d46.png"},{"id":97668944,"identity":"31f0f807-79b4-4c48-98b4-eb5231bcba51","added_by":"auto","created_at":"2025-12-08 09:26:47","extension":"png","order_by":18,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":40127,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/2901926b6e54329d78e37dc4.png"},{"id":97667765,"identity":"ced60ddd-b82a-4865-81ae-c9d9e2f377eb","added_by":"auto","created_at":"2025-12-08 09:24:14","extension":"png","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":193800,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/4fcde3df26ec0a94a068a776.png"},{"id":97668981,"identity":"9c501158-7c1a-47ec-bc58-23f5979502c2","added_by":"auto","created_at":"2025-12-08 09:26:53","extension":"png","order_by":20,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":24341,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/470860284d7d18fb0377c52d.png"},{"id":97448546,"identity":"54d4cd13-def1-4ba4-a525-2045f0a9d1f8","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"png","order_by":21,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":34252,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/78b1cc443c02a4b64ba00b5a.png"},{"id":97448552,"identity":"244c075b-22d9-4e99-8806-791047d46ba8","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"xml","order_by":22,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":94686,"visible":true,"origin":"","legend":"","description":"","filename":"a11139111e394a499b69bf60776beada1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/6c737afb12e8c6af2a0bd6b2.xml"},{"id":97448549,"identity":"dd5dbf48-3965-48c2-a496-c3c0233223e6","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"html","order_by":23,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":104131,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/04e6186916fcf8e95f97f92d.html"},{"id":97448523,"identity":"03549b0a-c83e-461d-9fd0-ebfc8823703a","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":929259,"visible":true,"origin":"","legend":"\u003cp\u003eConceptual map highlighting the largely unexplored link between transient microstructures in Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi alloy and their coupled strength–corrosion responses.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/f87263814741e1ada417c1fc.png"},{"id":97448521,"identity":"b4fe232b-0a0b-4203-b53c-40b6a8ce6d43","added_by":"auto","created_at":"2025-12-04 13:12:09","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":379644,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Thermomechanical schedule used in this study: dual solutionizing, sequential 50 % and 90 % cold-rolling, followed by isothermal holds at 620 °C.\u003cbr\u003e\n(b) Secondary-electron image of the alloy in the re-solutionized base state, showing equiaxed FCC grains; (c) higher-magnification view of the boxed region in (b).\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/3daae894b7073b76551496e7.png"},{"id":97448522,"identity":"204f4aef-a7a0-444f-a2ce-0b513ff8301d","added_by":"auto","created_at":"2025-12-04 13:12:09","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1638236,"visible":true,"origin":"","legend":"\u003cp\u003eThe 90% Cold rolled and annealed Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi alloy at 620 ℃ for 10 h and 50 h condition: (a) low-magnification Inverse Pole Figure (IPF) map; Phase map (b) and (e), Cr map (c) and (f), Al map (d) and (g)\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/5fbd5c6493f29eedc7728205.png"},{"id":97448532,"identity":"51ff7aaa-9bc1-4269-881a-f76ccc3e04af","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1208151,"visible":true,"origin":"","legend":"\u003cp\u003eEarly precipitation after 1 h anneal at 620 °C.(a) Bright-field TEM overview identifying recrystallised (R) and unrecrystallised (U) areas.\u003cbr\u003e\n(b1–b5) STEM-EDS maps from the boxed recrystallised grain revealing nano-scale B2 (Ni-Al-rich) and σ (Cr-rich) particles.(c) Isocomposition surfaces from the unrecrystallised region highlighted in (a).(d) Three-dimensional APT reconstruction (Al in red, Cr in green).(e) Proxigram across a representative Ni-Al domain.(f) One-dimensional concentration profile confirming Ni/Al and Cr partitioning.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/5793fa9cbbb339a192442343.png"},{"id":97448529,"identity":"e6380230-3121-467b-8f27-de036cfaca5f","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":280400,"visible":true,"origin":"","legend":"\u003cp\u003eProperty evolution with annealing time. (a) Vickers hardness versus time at 620 °C.\u003cbr\u003e\n(b) Root-mean-square lattice micro-strain determined by Williamson–Hall analysis.\u003cbr\u003e\n(c) Potentiodynamic polarisation curves recorded in 3.5 wt.% NaCl solution; “Base” = CR50+1100C/1h in recrystallized condition. (d) Correlation between micro-strain and pitting potential\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/f194195cf8851bb67312e8f3.png"},{"id":97668227,"identity":"7242ce87-7862-440a-a4ba-fdd1165f66d1","added_by":"auto","created_at":"2025-12-08 09:25:06","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":353810,"visible":true,"origin":"","legend":"\u003cp\u003eXAENS spectra of Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi alloy: (a) Ni-L, (b) Fe-L, (c) Cr-L, and (d) Co-L, for corroded areas across 3 annealing conditions (base condition, CR90+620℃/2h, and CR95+620℃/50h)\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/d2b3dc0abee90caa400a8e88.png"},{"id":97667835,"identity":"e33cb123-d17a-4202-8219-c4375bab4570","added_by":"auto","created_at":"2025-12-08 09:24:21","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1259257,"visible":true,"origin":"","legend":"\u003cp\u003eTEM images of Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi alloy after corrosion. (a) 2 h anneal: bilayer structure consisting of an inner Al₂O₃ layer as shown in EDS line profile in (a-7) and an outer Cr-rich oxide. (b) 16 h anneal: single Cr-rich oxide; Aluminum preferentially forms the phase within the matrix, as indicated by the accompanying EDS line profile in (b-7). Elemental maps (1–6) correspond to Cr, Al, Co, Fe, O and Ni, respectively.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/90ac8b90821213f40dfc6a3a.png"},{"id":97448530,"identity":"9a029f81-737c-46de-89ef-ca40815b26ab","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":101381,"visible":true,"origin":"","legend":"\u003cp\u003eXPS spectra of Al 2p and Cr 2p₃/₂ peaks for the base alloy and specimens annealed 2 h and 50 h, post-corrosion. Al₂O₃ intensity maximizes after 2 h, whereas Cr₂O₃ dominates in the base condition\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/c86894f4d40ff50406a6e513.png"},{"id":97448537,"identity":"2ea70734-a66e-4fa5-8a73-a74ea457e609","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":317221,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic depicting the influence of annealing time on corrosion mechanisms in Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi: short-time annealing retains matrix, enabling an Al₂O₃-first passive film, whereas long-time annealing partitions Al into B2\u003c/p\u003e","description":"","filename":"image9.png","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/e68274d10120228f6f344185.png"},{"id":97677730,"identity":"35eaa0cc-07e1-48bc-9842-2bdb85ef6f76","added_by":"auto","created_at":"2025-12-08 09:54:17","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6649340,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/2b72a573-17ac-4117-85f4-9fc0b5533e9d.pdf"},{"id":97448534,"identity":"1b6904b3-ac81-4c4b-b2be-49d76545e113","added_by":"auto","created_at":"2025-12-04 13:12:10","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":1200992,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryfilesofEffectsofAnnealingonPhaseTransformationandCorrosionMechanismofSeverelyDeformedHighEntropyAlloy.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8223266/v1/3537af8c2984db85c1855ee3.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effects of Annealing on Phase Transformation and Corrosion Mechanism of Severely Deformed Al 0.3 CoCrFeNi High-Entropy Alloy","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe fabrication and design of alloys in metallurgy have led to significant technological advancements across various industries (infrastructure, aerospace, automotive, etc.) [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], due to the ability to enhance and tailor specific properties of metals for particular applications. Whilst traditional alloys are typically designed with one or two primary elements with minor additions of others, increasing the proportion of additional elements can result in the formation of brittle intermetallic compounds, which can reduce mechanical performance. The introduction of HEAs overcomes the limitations of traditional alloys by introducing many interacting elements in the system, which increases tunability and minimizes the detrimental influence of any single element within a system. HEAs typically incorporate five or more elements in near-equiatomic proportions (5\u0026ndash;35 atomic percent each) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Recent research in HEAs has shown significant congruent improvements in strength, ductility corrosion resistance, and high-temperature performance [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan additionalcitationids=\"CR8 CR9\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe Al₀.₃CoCrFeNi alloy was developed as a derivative of the equiatomic CoCrFeNi system to enhance strength and corrosion resistance through the addition of aluminum. This system typically exhibits a multi-phase microstructure, including FCC, B2, and occasionally σ phases, which contribute to tunable mechanical properties [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Prior work [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] demonstrated that its tensile yield strength can be tailored from 160 MPa to 1800 MPa through thermomechanical processing alone, without altering composition.\u003c/p\u003e\u003cp\u003eThe corrosion resistance of Al₀.₃CoCrFeNi also shows promise in aggressive environments. Li et al. [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] reported that Al additions promote the formation of protective Al₂O₃ scales, while Firouzdor et al. [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] and Shi et al. [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] found that small Al additions (e.g., 0.3 at%) enhance passivation. However, excessive Al can promote BCC and B2 phase formation, which may reduce corrosion resistance and pitting stability [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Furthermore, Shi [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] noted that B2 phase presence diminishes pitting resistance, and studies by Zhang et al. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] and Qiu et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] have shown that annealed conditions generally offer improved corrosion resistance compared to solutionized or remelted states.\u003c/p\u003e\u003cp\u003eThese findings highlight Al₀.₃CoCrFeNi as a promising alloy system for corrosion-resistant applications, particularly when phase composition and thermal history are carefully controlled.\u003c/p\u003e\u003cp\u003eAlthough several studies have examined corrosion mechanisms and passivity of HEAs in chloride environments [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan additionalcitationids=\"CR16 CR17 CR18\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], systematic investigations specifically linking annealing time, microstructural evolution, and corrosion resistance remain limited. In particular, few studies have explored how phase transformations affect oxide film formation during corrosion, or how the availability of key oxide-forming elements changes following these transformations. Moreover, the differences in oxide formation pathways and kinetics between highly deformed and fully or partially annealed HEAs are still not well understood. While many investigations focus on the effects of annealing temperature [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], the influence of annealing duration phase transformations and subsequent passivation behavior has received comparatively little attention [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. This study aims to address these gaps.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates the dynamic interplay between microstructural evolution and environmental degradation in HEAs. Initially, the alloy exhibits a single-phase FCC structure formed by solutionizing, which transforms under cold rolling into a heavily deformed microstructure with elongated grains and high dislocation density. Annealing then promotes the formation of fine-grained multiphase structures with nanoscale B2 and σ precipitates. In corrosive environments such as 3.5 wt% (\u0026asymp;\u0026thinsp;0.6 M) NaCl solution at near-neutral pH, ongoing microstructural changes\u0026mdash;including recrystallization and precipitation\u0026mdash;interact with corrosive processes, which may preferentially attack phase boundaries or precipitate-decorated grain boundaries. The schematic emphasizes how processing history dictates phase transformation pathways (highlighted in orange), while feedback loops from corrosion (indicated by black arrows) influence phase stability and degradation mechanisms. This schematic underscores the need to better understand the coupling between microstructure and corrosion to engineer HEAs with optimized performance under service conditions.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eHypothetically, the addition of Al into a multi-element mixture such as CoCrFeNi would alter the relaxation pathway of the alloy due to the influence and interaction of Al towards each of the other elements. Previously, Preferential Interactivity Parameter (PIP[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]) have shown that the oxidation affinity, size, standard reduction potential (E\u003csup\u003e0\u003c/sup\u003e) and cohesive energy density (CED) controls the speciation and surface formation of metal alloys and HEAs [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Thermodynamically, the addition of Al to the alloy increases the overall chemical driving force (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lang;\\widehat{T}\u0026rang;\\)\u003c/span\u003e\u003c/span\u003e), particularly due to Al\u0026rsquo;s strong affinity for certain elements. Even a minor addition (e.g., 0.3 at%) promotes interactions between Al and thermodynamically compatible elements to reduce system energy and approach pseudo-equilibrium under non-equilibrium conditions. Elements such as Ni and Cr are relatively close to Al on the PIP scale, making them more susceptible to Al-induced changes. These interactions can influence phase stability and relaxation behavior of Ni and Cr during thermal or corrosion processes.\u003c/p\u003e\u003cp\u003eIn this study, Al₀.₃CoCrFeNi alloy were subjected to high-temperature solid solution treatment, followed by 90% cold rolling and annealing at 620\u0026deg;C for durations ranging from 1 to 50 hours. With increasing annealing time, the microstructure evolved from a single-phase FCC structure to a multiphase mixture containing FCC, B2, and sigma phases. While prior studies have associated such phase mixtures with improved strength and hardness [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], they may also compromise corrosion resistance [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Our findings reveal that short-term annealing (1\u0026ndash;2 h) significantly enhances corrosion resistance, whereas extended annealing reduces it slightly, yet still maintains better performance than conventional stainless steel. This evolution in corrosion behavior is closely linked to the increasing presence of B2 and sigma phases and concurrent microstructural stress relief, as indicated by decreasing micro-strain. Notably, hardness increases up to 4 h of annealing before marginally declining, suggesting a trade-off between strengthening and corrosion resistance mechanisms.\u003c/p\u003e\u003cp\u003eThrough this research, we demonstrate that appropriate heat treatment can achieve a desirable balance between strength and corrosion resistance in HEAs. The findings also provide insights into the transformation pathways influenced by prior sample conditions. This approach could be effectively applied to other HEAs and alloy systems.\u003c/p\u003e"},{"header":"Experimental materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003ch2\u003e2.1 Alloy fabrication and Thermomechanical processing\u003c/h2\u003e\n\u003cp\u003eThe Al₀.₃CoCrFeNi alloy was synthesized using the traditional arc melting method with an Arcast arc melter (Arc 200) [\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e]. The buttons were inverted five times and remelted to ensure complete homogeneity. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e summarizes all subsequent thermomechanical processing after alloy synthesis. First a solutionizing treatment at 1150\u0026deg;C for 10 hours was performed in a vacuum furnace to homogenize the material and dissolve any pre-existing precipitates, resulting in a FCC single-phase microstructure. Next, the material was subjected to 50% cold rolling, introducing significant strain, increasing dislocation density. A second solutionizing step at 1150\u0026deg;C for 0.5 hour was then performed after encapsulating the samples in titanium foils with a tantalum getter, followed by water quenching to facilitate recrystallization. Further deformation through 90% cold rolling increased dislocation density and enhanced hardness. Finally, the samples were annealed at 620\u0026deg;C for periods ranging from 1 to 50 hours.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n\u003ch2\u003e2.2 Microstructural characterization\u003c/h2\u003e\n\u003cp\u003eScanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) maps were prepared using a FEI Nova NanoSEM equipped with an EBSD detector. X-ray diffraction (XRD) studies were conducted at room temperature using a Rigaku SmartLab X-ray Diffractometer with Cu K\u0026alpha; radiation (\u0026lambda;\u0026thinsp;=\u0026thinsp;1.54 \u0026Aring;, 45 kV, 44 mA) to determine the phase composition. Specific samples for transmission electron microscopy (TEM) were prepared using a FEI Nova 200 dual-beam focused ion beam (FIB) system and analyzed in a ThermoFisher Talos F200X microscope operating at 200 kV.\u003c/p\u003e\n\u003cp\u003eAtom probe tomography (APT) specimens were prepared using an FEI Helios dual-beam FIB-SEM, with a Pt capping layer deposited to protect against Ga ion damage. Needle-shaped tips were fabricated from the uncrystallized region of the HEA alloy under 90% cold rolling (CR90%) followed by annealing at 620\u0026deg;C for 1 hour.\u003c/p\u003e\n\u003cp\u003eAPT analysis was performed on a CAMECA LEAP 4000X HR system using a 355 nm UV laser (200 pJ, 200 kHz) at a specimen base temperature of 45 K, with a detection rate of 0.003 ions/pulse. The chamber pressure was maintained below 2 \u0026times; 10⁻\u0026sup1;\u0026sup1; Torr. Data were reconstructed and analyzed using IVAS 3.8.2, with a detector efficiency of ~\u0026thinsp;36%.\u003c/p\u003e\n\u003cp\u003eThe Atom Probe Tomography (APT) specimen was prepared using a site-specific focused ion beam (FIB) lift-out technique (Thermo Fisher Scientific Quanta 200 FIB-SEM). Final needle sharpening involved annular milling with progressively reduced ion beam currents (30 nA down to 10 pA) and low-kV (2\u0026ndash;5 kV) cleaning steps, resulting in tips with apex radii of ~\u0026thinsp;50\u0026ndash;100 nm and minimized Ga ion implantation. APT analyses were then performed on a CAMECA LEAP system (3000XR) at the temperature range (40\u0026ndash;50 K), using a target evaporation rate of 0.5% and a pulse fraction set to 20% of the steady-state DC voltage. Data reconstruction and comprehensive quantitative analysis, including 3D compositional mapping, 1D concentration profiles, proximity histograms, and cluster analysis, were subsequently conducted using CAMECA's IVAS 3.6.10 software.\u003c/p\u003e\n\u003cp\u003eX-ray Absorption Near Edge Structure (XANES) data were analyzed using SIXPACK software, with the photoelectron energy origin (E₀) determined by the first inflection point of the absorption edge jump. For calibration, the main peak of the spectra was aligned with literature values for the respective transition metals. Normalization of the spectra was performed by adjusting the pre-edge area to 0 and the post-edge area to 1, ensuring consistency and comparability between different spectra.\u003c/p\u003e\n\u003cp\u003eX-ray Photoelectron Spectroscopy (XPS) characterization was carried out using a spherical capacitance analyzer with advanced detector technology and a high-performance Al X-ray monochromator source. All survey spectra scans were taken at a pass energy of 58.7 eV. The data were acquired overnight and analyzed using Perkin-Elmer XPS systems equipped with SCAs and Omni FOCUS\u0026trade; lenses.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n\u003ch2\u003e2.4 Mechanical testing\u003c/h2\u003e\n\u003cp\u003eMicrohardness testing was performed using a standard Vickers microhardness tester with a 500 g load applied for 10 seconds. An average of 10 indents per condition was considered to ensure accuracy.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n\u003ch2\u003e2.5 Electrochemical measurements\u003c/h2\u003e\n\u003cp\u003eElectrochemical tests were conducted using a Biologic VMP-300 potentiostat controlled by EC-lab software in a conventional three-electrode cell. The sample served as the working electrode with an exposure area of approximately 0.079 cm\u0026sup2;, a platinum mesh was used as the counter electrode, and a saturated calomel electrode (SCE) served as the reference electrode. Prior to polarization tests, the samples were immersed in a 3.5 wt% NaCl solution for 3 hours to stabilize the open circuit potentia.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n\u003ch2\u003e3.1 Microstructural evolution during 620\u0026deg;C annealing\u003c/h2\u003e\n\u003cp\u003eFigure\u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e, Fig.S2, Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e collectively illustrate the microstructural evolution of the Al₀.₃CoCrFeNi alloy after 90% cold rolling and subsequent annealing at 620\u0026deg;C for varying durations (0, 10, and 50 hours).\u003c/p\u003e\n\u003cp\u003eThe unannealed sample (0h, Fig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e) exhibits a heavily deformed structure with elongated grains, high dislocation density, and no additional phase formation, indicative of severe plastic deformation. Upon annealing for 10 hours (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e), the material shows the onset of recrystallization with more pronounced grain boundary contrast and the initial precipitation of secondary phases. EBSD analysis confirms the microstructure remains predominantly FCC (90%), but minor fractions of sigma (8%) and Al-Ni-rich (B2) (2%) phases are now present as shown in Fig.S2.\u003c/p\u003e\n\u003cp\u003eFurther annealing to 50 hours leads to a fully developed, coarser grain structure with well-defined precipitates located both at grain boundaries and within grains. Quantitatively, a dramatic phase transformation occurs: the FCC phase reduces significantly to 41%, while the sigma phase becomes dominant at 45%, and Al-Ni (B2) notably increases to 15% as shown in Fig.S2. Over estimation of sigma and B2 can be expected due to a lower confidence index in EBSD of these phases. However, the dramatic increase in phases fraction of these intermetallic is confirmed by additional TEM analysis. This indicates substantial growth and segregation of these secondary phases at the expense of the FCC matrix during prolonged annealing.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n\u003ch2\u003e3.2 Early-stage precipitation revealed by correlative TEM\u0026ndash;APT (1 h)\u003c/h2\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e presents TEM and APT images of the Al₀.₃CoCrFeNi alloy under 90% cold rolling (CR90%) followed by annealing at 620\u0026deg;C for 1 hour. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e (a), TEM images reveal that the alloy exhibits regions of partial recrystallization, with some areas remaining unrecrystallized. Within the recrystallized regions, B2 (Ni/Al rich regions)and sigma phases (Cr rich region) are observed in (b-1) \u0026ndash;(b-5). The APT images (shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e (c)-(f)) provide detailed 3D structural information in the unrecrystallized region indicated as the region of interest (ROI) in (a). Isosurface are used to delineate Al rich (red in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e(d) and Cr rich (green in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e(d)) regions. Although B2 and sigma precipitates were not observed in the unrecrystallized region through TEM, APT results indicate the presence of domains rich in Ni and Al. These domains appear to act as nuclei for the formation of B2 phases, even though the precipitates are not visible in the TEM images. The line scan further confirms the presence of nuclei for both B2 and sigma phases.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n\u003ch2\u003e3.3 Hardness and lattice micro-strain\u003c/h2\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e(a) shows the hardness variation with annealing time. The hardness increases initially, peaking at 4 hours due to partial recrystallization and the formation of nanoscale B2 and sigma precipitates, which hinder dislocation motion. Beyond 4 hours, hardness decreases as the microstructure becomes more homogeneous and precipitates coarsen, reducing their effectiveness in strengthening.\u003c/p\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e(b) illustrates microstrain measurements obtained via the Williamson-Hall method. The cold-rolled sample exhibits high microstrain due to plastic deformation. Annealing at 620\u0026deg;C for 1 hour significantly reduces microstrain, indicating partial recrystallization and phase transformation. Further annealing results in minimal changes in microstrain, suggesting stress relief and continued phase transformations.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n\u003ch2\u003e3.4 Electrochemical behavior in 3.5 wt.% NaCl\u003c/h2\u003e\n\u003cp\u003eThe potentiodynamic polarization (PDP) curves for the Al₀.₃CoCrFeNi alloy, tested in a 3.5 wt.% NaCl solution, are shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e (c). For comparison with the annealed specimens, both the solutionized HEA and SS 304L were also tested. Among the alloys, the solutionized HEA exhibited the least noble corrosion potential. However, its pitting potential exceeded that of SS 304L by more than 100 mV. When comparing the spontaneous passivity of all tested alloys, the passive corrosion current densities were found to be within a similar range.\u003c/p\u003e\n\u003cp\u003eFollowing annealing, the corrosion potential of the HEA became more noble. The 1-hour, 2-hour, and 50-hour annealed specimens all exhibited similarly higher corrosion potentials, while the 4-hour and 16-hour samples showed slightly less noble values. The pitting potential also significantly increased after annealing, with the 1-hour and 2-hour specimens showing similar pitting potentials near 750 mVSCE. However, with extended annealing duration, the pitting potential progressively decreased, with the 50-hour annealed sample approaching 550 mVSCE, indicating an increased susceptibility to pitting corrosion. Overall, the 1-hour and 2-hour annealing treatments resulted in the widest passive windows and the highest pitting potentials, thus offering the greatest resistance to localized corrosion. Metastable pitting was observed in both the SS 304L and the 16-hour annealed HEA specimen.\u003c/p\u003e\n\u003cp\u003eTable.1 shows the corrosion and pitting potentials of the HEA alloys annealed for different durations compared to conventional stainless steel. The results indicate that the HEA alloy has significantly better corrosion resistance after annealing, regardless of the duration. Additionally, whether annealed or not, the corrosion resistance of the HEA alloy surpasses that of conventional stainless steel.\u003c/p\u003e\n\u003cp\u003eTable.1 Corrosion parameters for Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi alloys and conventional stainless steel\u0026nbsp;\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"char\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Taba\" border=\"1\"\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eSample\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eE\u003csub\u003ecorr\u003c/sub\u003e (mV)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eE_pit (mV)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e620-50h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e-125\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e640\u0026thinsp;\u0026plusmn;\u0026thinsp;125\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e620-16h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e-131\u0026thinsp;\u0026plusmn;\u0026thinsp;9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e637\u0026thinsp;\u0026plusmn;\u0026thinsp;111\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e620-4h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e-131\u0026thinsp;\u0026plusmn;\u0026thinsp;30\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e685\u0026thinsp;\u0026plusmn;\u0026thinsp;19\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e620-2h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e-125\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e759\u0026thinsp;\u0026plusmn;\u0026thinsp;20\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e620-1h\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e-125\u0026thinsp;\u0026plusmn;\u0026thinsp;8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e778\u0026thinsp;\u0026plusmn;\u0026thinsp;28\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eBase\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e-147\u0026thinsp;\u0026plusmn;\u0026thinsp;26\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e492\u0026thinsp;\u0026plusmn;\u0026thinsp;87\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSS304L\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e-105\u0026thinsp;\u0026plusmn;\u0026thinsp;15\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\"\u0026plusmn;\"\u003e\n\u003cp\u003e279\u0026thinsp;\u0026plusmn;\u0026thinsp;22\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e (d) illustrates the relationship between microstrain and corrosion resistance in Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi HEA alloys with varying annealing times. The data in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e (b) shows that increased microstrain correlates with decreased corrosion resistance. Microstrain can introduce lattice defects and dislocations, which may serve as nucleation sites for corrosion initiation. These defects can accelerate corrosion by compromising the integrity of the passive layer.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n\u003ch2\u003e3.5 Near-surface oxidation states after polarization\u003c/h2\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e XANES spectra of the Al₀.₃CoCrFeNi alloy after PDP testing in 3.5 wt% NaCl solution, comparing the effects of different heat treatments on corrosion behavior. Since the phonon energy of the main peak is similar across all three heat-treated samples before corrosion, only the data after corrosion is shown. XANES results include total electron yield (TEY) and total fluorescence yield (TFY) data, with TEY (probe depth: 5\u0026ndash;10 nm) being surface-sensitive and TFY (probe depth: ~150 nm) being bulk-sensitive. As the TFY data shows minimal differences between corroded and uncorroded regions, only the TEY data is presented in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eThe TEY spectrum in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ea for Cr shows a sharp peak in the base condition (0 h), indicating a significant surface oxide layer (e.g., Cr₂O₃) formed during corrosion. [\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e] This high intensity reflects Cr\u0026rsquo;s high reactivity, establishing a protective oxide within the 5\u0026ndash;10 nm surface region. After 2 h annealing at 620\u0026deg;C, the peak intensity decreases slightly, suggesting an initial thinning or restructuring of the surface oxide, possibly due to microstructural changes or partial oxide dissolution. At 50 h, the intensity drops more noticeably, indicating a significant reduction in surface oxide concentration. This decreasing trend suggests that prolonged annealing weakens Cr\u0026rsquo;s surface oxide stability, potentially due to diffusion of corrosive species into deeper layers or oxide breakdown, while the +\u0026thinsp;3 oxidation state remains stable as no peak shifts are observed.\u003c/p\u003e\n\u003cp\u003eThe TEY spectrum in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eb for Fe exhibits a prominent peak in the base condition (0 h), indicating surface oxidation (e.g., Fe₂O₃ or Fe₃O\u003csub\u003e4\u003c/sub\u003e). [\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e] After 2 h annealing at 620\u0026deg;C, the peak intensity increases slightly, suggesting an enhancement in surface oxide formation or thickening, possibly due to improved Fe diffusion or oxide stabilization with initial heat treatment. At 50 h, the intensity rises further, reflecting a significant increase in surface oxide concentration. This increasing trend indicates that prolonged annealing strengthens Fe\u0026rsquo;s surface oxide layer, potentially due to sustained reaction with the corrosive NaCl environment, with the +\u0026thinsp;2/+3 oxidation state remaining stable as no edge shifts occur.\u003c/p\u003e\n\u003cp\u003eBased on the L-XAS TEY data in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ec, with peaks at 853.5 eV (L₃-edge) and 873 eV (L₂-edge) and a slight intensity increase at 50 h, the Ni is likely predominantly in an oxide state (e.g., Ni\u0026sup2;⁺ as NiO) at the surface (5\u0026ndash;10 nm), rather than metallic Ni⁰. The stable intensity from 0 h to 2 h suggests an initial oxide layer formed during corrosion, and the slight increase at 50 h indicates minor oxide growth or thickening due to annealing. The edge positions (853.5 eV and 873 eV) support Ni\u0026sup2;⁺, and the intensity trend aligns with surface oxidation. The lack of a significant shift reflects a consistent oxidation state, with the 50 h increase driving limited oxide enhancement.\u003c/p\u003e\n\u003cp\u003eBased on the L-XAS TEY data in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ed, with peaks at 779 eV (L₃-edge) and 795 eV (L₂-edge) and an intensity increase at 50 h, the Co is likely predominantly in an oxide state (e.g., Co\u0026sup2;⁺ as CoO, possibly Co\u0026sup3;⁺ as Co₂O₃) at the surface (5\u0026ndash;10 nm), rather than metallic Co⁰. The stable intensity from 0 h to 2 h suggests an initial oxide layer formed during corrosion, and the increase at 50 h indicates enhanced oxide formation or thickening due to annealing. The L₃-edge at 779 eV aligns with Co\u0026sup2;⁺, and the intensity rise supports oxide growth, though a minor metallic contribution is possible if the oxide is thin. The lack of a significant shift may reflect a consistent oxidation state, with the 50 h increase driving further oxidation.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAlthough XANES is effective for detecting oxidation states and shifts in Fe, Ni, and Co, it is less sensitive to Al signals due to the lower X-ray absorption cross-section of Al in the relevant energy ranges, particularly in HEAs. XPS and TEM, however, are more effective at detecting Al, especially when Al oxides like Al₂O₃ form during corrosion, as they show clear changes in peak positions corresponding to different oxidation states. While XANES reveals oxidation and reduction processes through peak shifts, XPS mainly provides chemical state information and may not detect such changes unless large oxidation state variations occur. Therefore, combining multiple techniques offers a more comprehensive understanding of the corrosion behavior in HEAs.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n\u003ch2\u003e3.6 Corrosion product morphology through microstructure characterization\u003c/h2\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e shows the TEM images of the different heat-treated Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi HEA alloys after corrosion. For short annealing, two distinct layers are observed on the surface of the CR90-620C/2h sample as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e(a). Note that the specimens were subjected to anodic polarization up to the pitting potential, resulting in localized breakdown of the passive film and initiation of stable pitting corrosion. Consequently, the protective oxide layers originally present on the surface have been extensively damaged or dissolved in the pitted regions. The observed surface features (e.g., corrosion product aggregates) are characteristic of post-pitting degradation and do not represent the intact passive film formed under open-circuit or early-stage polarization conditions. These images serve to illustrate the morphology and subsurface damage induced by pitting attack rather than the structure of the protective film.\u003c/p\u003e\n\u003cp\u003eThe outermost layer is rich in Cr (60 at%), while the second layer consists of Al₂O₃, as confirmed by characteristic Al 2p and O 1s binding energies in XPS spectra and composition from EDS analysis in Table.2.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn contrast, the CR90-620\u0026deg;C/16h sample does not exhibit the presence of Al₂O₃ as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e (b). Instead, a Cr-rich oxide layer forms on the surface. Furthermore, the matrix of the 16-hour annealed sample contains B2 phases, with B2 phases forming along specific grain boundaries\u0026mdash;phases that are not observed in the 2-hour annealed sample. For the 2-hour annealed sample, the rapid formation of Al₂O₃ suggests a kinetic pathway where aluminum preferentially reacts with oxygen rather than diffusing to form the B2 phase. This preference indicates that the alloy may not reach the equilibrium state required to stabilize the B2 phase during the shorter annealing time.\u003c/p\u003e\n\u003cp\u003eLi et al. [\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e] proposed that in Al\u003csub\u003e1.0\u003c/sub\u003eCoCrFeNi and Al\u003csub\u003e1.3\u003c/sub\u003eCoCrFeNi alloys, Al reacts with NaCl to form AlCl₃, which subsequently leads to the formation of a protective Al₂O₃ layer on the surface. This mechanism effectively protects the alloy from corrosion. In the current study, the presence of Al₂O₃ in the CR90-620C/2h sample suggests that it also plays a protective role, contributing to better corrosion resistance in shorter annealing times (1\u0026ndash;2 hours) compared to longer times (16 hours or more). The presence of a Cr-rich layer above the Al₂O₃ has also been reported in previous studies. Gwalani et al. [\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e] used a combination of in situ atom probe tomography and thermo-kinetic treatment applying preferential interactivity parameter approach to investigate the oxidation mechanism/sequence. They reported that the early stages of oxidation can be interpreted through a multi-pronged coupling of thermodynamic parameters such as atomic size, cohesive energy, electronegative and so on.\u003c/p\u003e\n\u003cp\u003eAn aluminum oxide (Al₂O₃) layer tends to form first on HEAs due to the higher affinity of aluminum for oxygen, leading to its preferential oxidation over elements like Cr, Fe, Co, and Ni. According to the Ellingham diagram, aluminum has a significantly lower Gibbs free energy of oxidation than these elements, making the formation of Al₂O₃ thermodynamically more favorable. This behavior is particularly advantageous in HEAs, as the rapid formation of a protective Al₂O₃ layer can enhance the oxidation resistance of the material.\u003c/p\u003e\n\u003cp\u003eTable.2 Atomic ratio of metals in Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi HEA alloy (a) annealing at 620℃ for 2h and (b) 16 h after corroding in 3.5wt% NaCl solution\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cdiv class=\"colspec\" align=\"char\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tabb\" border=\"1\"\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eatomic ratio for metal\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eAl\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eCr\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eFe\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eCo\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eNi\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd rowspan=\"2\" align=\"left\"\u003e\n\u003cp\u003e(a)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e620C-2h (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e9.7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.0\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.0\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e620C-2h Cr-rich oxide\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3.4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.0\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e(b)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e620C-16h Cr-rich oxide\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.0\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.0\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.2\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eassuming iron as 1, and other components are the ratio to iron.\u003c/p\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e shows the Cr and Al signals in the differently heat-treated HEAs after corrosion. The data indicate that protective oxide layers containing Cr₂O₃ and Al₂O₃ formed after corrosion. All raw XPS spectra are provided in the supplementary information (Fig.sS5 and S6) for comprehensive detail. It appears that Al₂O₃ reaches its maximum intensity in the HEA after cold rolling and annealing at 620\u0026deg;C for2 hours. However, the intensity of Cr₂O₃ decreases after corrosion in the HEA subjected to longer annealing.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eChange in Corrosion Mechanisms as a Function of Microstructural Change\u003c/h3\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e shows the schematic illustration of the effect of annealing time on corrosion behaviors of Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi HEA alloy. Before corrosion, annealing at 620\u0026deg;C for 2 hours results in partial recrystallization, especially in areas that experienced significant strain during cold rolling. These recrystallized regions exhibit equiaxed grains with low dislocation density. However, most regions remain un-recrystallized due to the high degree of plastic deformation caused by cold rolling, retaining elongated grains and a higher dislocation density. Moreover, Ni/Al-rich domains act as nuclei for B2 phase formation.\u003c/p\u003e\n\u003cp\u003eAfter corrosion, NaCl solution dissolves the metal matrix, leading to the formation of a protective Al₂O₃ oxide layer due to its thermodynamic stability. The Cr-rich phase forms on top of Al₂O₃ because chromium also has lower redox potential compared to other metal elements (Fe, Co and Ni) [\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e] and higher diffusion coefficients in aluminum oxide [\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eRoy et al. [\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e] studied cationic and anionic diffusion through stable aluminum oxides in a model HEA using atomistic simulations.. They found that metal ions such as Cr, Fe, Co, and Ni have higher diffusion coefficients than Al or O ions, indicating that these metal ions move more easily or rapidly than Al or O ions. This finding aligns with our observations, where we identified a Cr-rich phase on top of the Al₂O₃ layer.\u003c/p\u003e\n\u003cp\u003eFor longer annealing time at 16 h, more regions were recrystallized compared to the 2-hour annealing condition, primarily exhibiting FCC phases. In addition, B2 and sigma phases formed in some regions due to their thermodynamic stability. After corrosion, due to the formation of the B2 phase, aluminum preferentially formed B2 in the matrix rather than Al₂O₃ on the surface, compared to the 2-hour condition. Cr₂O₃ and FeCr₂O₄ spinel oxides were formed on the surface due to their thermodynamic stability [\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e]. The previous work on stainless steels and nickel-based alloys in simulated pressurized water reactor conditions has consistently shown the presence of these stable oxide phases contributing to corrosion performance. [\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study investigates the impact of various annealing times on the corrosion resistance and mechanical properties of the Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi alloy. Annealing at 620\u0026deg;C transformed the cold-rolled Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi alloy from a deformed structure into a heterogeneous microstructure featuring both recrystallized and un-recrystallized regions.\u003c/p\u003e\u003cp\u003eInitially (1\u0026ndash;4h), the recrystallized areas contained a mix of FCC and B2 phases, while un-recrystallized regions retained high dislocation density and nano-sized Ni-Al-rich domains. This annealing-induced microstructure optimizes the balance between mechanical strength and corrosion resistance. By 50h, B2 and sigma phases dominated, with Al favoring B2 formation over Al₂O₃, which significantly impacted both mechanical and corrosion properties.\u003c/p\u003e\u003cp\u003eShort-term annealing (1\u0026ndash;2h) notably improved corrosion resistance due to the formation of Al-rich oxide films. However, longer annealing times increased the fractions of B2 and sigma phases, consequently reducing the Al available for protective oxide formation. While this led to lower corrosion resistance compared to short-term annealing, the HEA still outperformed stainless steel.\u003c/p\u003e\u003cp\u003eUltimately, this study highlights that optimizing microstructure through thermal treatments is crucial for enhancing the Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi HEA's performance in corrosive environments and improving its mechanical strength.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eCompeting interest\u003c/h2\u003e\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eThe research was supported (in part) by ONR under Grant N00014-23-1-2758.\u003c/p\u003e\u003cp\u003eInstrumentation at the AIF was acquired with support from the National Science Foundation (DMR-1726294).\u003c/p\u003e\u003cp\u003eThe AIF is supported by the National Science Foundation (award number ECCS-1542015).\u003c/p\u003e\u003cp\u003eAlice Pandaleon was supported by the NSF-funded MAT-DAT REU Site (DMR-2150360).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eF.Y.T., A.P., S.O., A.M., A.Pa., F.I., M.L., Z.Z., and M.J.U. performed experiments, data collection, and preliminary analysis.J.G., M.M.T., R.G., R.B., and E.K. contributed to the interpretation of results and methodology development.F.Y.T., A.P., S.O., A.M., A.Pa., F.I., and M.L. assisted with data curation, visualization, and figure preparation.B.G. led project conceptualization, experimental design, and overall supervision.F.Y.T. and B.G. prepared the initial manuscript draft.All authors (F.Y.T., A.P., S.O., A.M., A.Pa., F.I., M.L., M.J.U., Z.Z., J.G., M.M.T., R.G., R.B., E.K., B.G.) reviewed and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe TEM characterization work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina. The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI). The authors thank the AIF staff for their assistance.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eAll data were available in the main text or the supplementary materials.\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMiracle, D.B., \u003cem\u003eCritical Assessment 14: High entropy alloys and their development as structural materials\u003c/em\u003e. Materials Science and Technology, 2024. 31(10): p. 1142\u0026ndash;1147.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eStepanov, N.D., et al., \u003cem\u003eEffect of Al on structure and mechanical properties of AlxNbTiVZr (x\u0026thinsp;=\u0026thinsp;0, 0.5, 1, 1.5) high entropy alloys\u003c/em\u003e. Materials Science and Technology, 2015. 31(10): p. 1184\u0026ndash;1193.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKumar, A. and M. Gupta, \u003cem\u003eAn Insight into Evolution of Light Weight High Entropy Alloys: A Review\u003c/em\u003e. Metals, 2016. 6(9): p. 199.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSonal, S. and J. Lee, \u003cem\u003eRecent Advances in Additive Manufacturing of High Entropy Alloys and Their Nuclear and Wear-Resistant Applications.\u003c/em\u003e Metals, 2021. 11(12): p. 1980.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYeh, J.W., et al., \u003cem\u003eNanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes\u003c/em\u003e. Advanced Engineering Materials, 2004. 6(5): p. 299\u0026ndash;303.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eCantor, B., et al., \u003cem\u003eMicrostructural development in equiatomic multicomponent alloys\u003c/em\u003e. Materials Science and Engineering: A, 2004. 375\u0026ndash;377: p. 213\u0026ndash;218.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGwalani, B., et al., \u003cem\u003eTensile yield strength of a single bulk Al0.3CoCrFeNi high entropy alloy can be tuned from 160 MPa to 1800 MPa\u003c/em\u003e. Scripta Materialia, 2019. 162: p. 18\u0026ndash;23.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMiracle, D.B. and O.N. Senkov, \u003cem\u003eA critical review of high entropy alloys and related concepts\u003c/em\u003e. Acta Materialia, 2017. 122: p. 448\u0026ndash;511.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eShi, Z.J., et al., \u003cem\u003eEffect of thermally induced B2 phase on the corrosion behavior of an Al0.3CoCrFeNi high entropy alloy\u003c/em\u003e. Journal of Alloys and Compounds, 2022. 903: p. 163886.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eQiu, Y., et al., \u003cem\u003eCorrosion of high entropy alloys\u003c/em\u003e. npj Materials Degradation, 2017. 1(1).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eShi, Y., et al., \u003cem\u003eCorrosion of Al CoCrFeNi high-entropy alloys: Al-content and potential scan-rate dependent pitting behavior\u003c/em\u003e. Corrosion Science, 2017. 119: p. 33\u0026ndash;45.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGwalani, B., et al., \u003cem\u003eCompositionally graded high entropy alloy with a strong front and ductile back\u003c/em\u003e. Materials Today Communications, 2019. 20: p. 100602.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLi, L., et al., \u003cem\u003eAl CoCrFeNi high entropy alloys with superior hot corrosion resistance to Na2SO4\u0026thinsp;+\u0026thinsp;25% NaCl at 900\u0026deg;C\u003c/em\u003e. Corrosion Science, 2021. 187: p. 109479.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eFirouzdor, V., et al., \u003cem\u003eCorrosion of a stainless steel and nickel-based alloys in high temperature supercritical carbon dioxide environment\u003c/em\u003e. Corrosion Science, 2013. 69: p. 281\u0026ndash;291.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang, X., et al., \u003cem\u003eInfluence of remelting and annealing treatment on corrosion resistance of AlFeNiCoCuCr high entropy alloy in 3.5% NaCl solution\u003c/em\u003e. Journal of Alloys and Compounds, 2019. 775: p. 565\u0026ndash;570.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLu, P., et al., \u003cem\u003eComputational materials design of a corrosion resistant high entropy alloy for harsh environments\u003c/em\u003e. Scripta Materialia, 2018. 153: p. 19\u0026ndash;22.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLiang, J., et al., \u003cem\u003eCorrosion resistance and mechanism of high-entropy alloys: A review\u003c/em\u003e. Materials and Corrosion, 2024. 75(4): p. 424\u0026ndash;432.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eChen, M., et al., \u003cem\u003ePhase, Microstructure and Corrosion Behaviour of Al(0.3)FeCoNiCr(x) High-Entropy Alloys via Cr Addition\u003c/em\u003e. Materials (Basel), 2024. 17(21).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSahu, S., et al., \u003cem\u003eLocalized Corrosion Behavior of Non-Equiatomic NiFeCrMnCo Multi-Principal Element Alloys\u003c/em\u003e. Electrochimica Acta, 2020. 354: p. 136749.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLiu, Z., et al., \u003cem\u003eMicrostructure, mechanical properties and corrosion resistance of direct energy deposited AlCu0.25CoCrFeNi2.1 high entropy alloy: Tailored by annealing heat treatment\u003c/em\u003e. Materials Characterization, 2024. 213: p. 114035.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMartin, A. and M. Thuo, \u003cem\u003ePredicting Emergence of Nanoscale Order in Surface Oxides through Preferential Interactivity Parameter\u003c/em\u003e. ACS Nano, 2024. 18(9): p. 6740\u0026ndash;6747.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGwalani, B., et al., \u003cem\u003eMechanistic understanding of speciated oxide growth in high entropy alloys\u003c/em\u003e. Nat Commun, 2024. 15(1): p. 5026.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLee, C.P., et al., \u003cem\u003eEnhancing pitting corrosion resistance of AlxCrFe1.5MnNi0.5 high-entropy alloys by anodic treatment in sulfuric acid\u003c/em\u003e. Thin Solid Films, 2008. 517(3): p. 1301\u0026ndash;1305.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGwalani, B., et al., \u003cem\u003eModifying transformation pathways in high entropy alloys or complex concentrated alloys via thermo-mechanical processing\u003c/em\u003e. Acta Materialia, 2018. 153: p. 169\u0026ndash;185.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSeong, S., et al., \u003cem\u003eExperimental evidence for mixed-valent Cr ions in half-metallic CrO2: Temperature-dependent XMCD study\u003c/em\u003e. Journal of Magnetism and Magnetic Materials, 2018. 452: p. 447\u0026ndash;450.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eFonin, M., et al., \u003cem\u003eSurface electronic structure of theFe3O4(100): Evidence of a half-metal to metal transition\u003c/em\u003e. Physical Review B, 2005. 72(10).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eRoy, I., P.K. Ray, and G. Balasubramanian, \u003cem\u003eDiffusion of multi-principal elements through stable Cr2O3 and Al2O3 scales\u003c/em\u003e. Materialia, 2022. 24: p. 101497.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePuigdomenech, B.B.a.I., \u003cem\u003ePourbaix diagrams for the ternary system of Fe-Cr-Ni\u003c/em\u003e. Corrosion. 55(11): p. 1077\u0026ndash;1087.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTsai, F.Y., \u003cem\u003eCorrosion Sensitivity of Stainless Steels in Pressurized Water Reactor Water Chemistry: Can KOH replace LiOH in PWRs.\u003c/em\u003e Doctoral dissertation, North Carolina State University https://\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003erepository.lib.ncsu.edu/items/937cd4c\u003c/span\u003e\u003cspan address=\"http://repository.lib.ncsu.edu/items/937cd4c\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e1-72a3-4d73-b5d8-8def6b7670ac, 2022.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTsai, F.-Y., et al., \u003cem\u003eCorrosion sensitivity of nickel-based Alloy Inconel 600 in pressurized water reactor water chemistry: Can KOH replace LiOH?\u003c/em\u003e Corrosion Science, 2025. 255: p. 113052.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTsai, F.-Y., et al., \u003cem\u003eEffect of irradiation on the corrosion of 304 stainless steel in pressurized water reactor (PWR) simulated water chemistry\u003c/em\u003e. Corrosion Science, 2024. 240: p. 112454.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"npj-materials-degradation","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjmatdeg","sideBox":"Learn more about [npj Materials Degradation](http://www.nature.com/npjmatdeg/)","snPcode":"41529","submissionUrl":"https://submission.springernature.com/new-submission/41529/3","title":"npj Materials Degradation","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8223266/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8223266/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe effects of transitional phase states on corrosion behavior in the equiatomic-derived high-entropy alloy Al₀.₃CoCrFeNi were investigated following 90% cold-rolling and subsequent isothermal annealing at 620\u0026deg;C for durations ranging from 1 to 50 hours. Electron and X-ray diffraction analysis reveal a progressive transformation from single-phase FCC to an FCC\u0026thinsp;+\u0026thinsp;B2\u0026thinsp;+\u0026thinsp;σ trinity. Nano-scale B2 and σ nuclei emerge within partially recrystallized grains after 1\u0026ndash;2 h, lowering the cold rolling micro-strain from 0.26 to 0.13. This heterogeneous state has the maximum hardness (552 VHN) while enlarging the chloride-passive window by \u0026gt;\u0026thinsp;200 mV relative to the solutionized material. Prolonged annealing (\u0026gt;\u0026thinsp;10 h) coarsens the intermetallics and exhausts Al from the matrix, thereby reducing pitting resistance due to the loss of protective Al₂O₃ film. Characterization by electron microscopy and atom probe tomography reveals that Al-rich B2 nuclei are the primary Al sink that compromises Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e formation at long times. The results demonstrate that a narrowly defined heat-treatment window balances strengthening with passivation in Al\u003csub\u003e0.3\u003c/sub\u003eCoCrFeNi, providing a mechanistic blueprint for designing corrosion-tolerant high-entropy alloys (HEAs).\u003c/p\u003e","manuscriptTitle":"Effects of Annealing on Phase Transformation and Corrosion Mechanism of Severely Deformed Al 0.3 CoCrFeNi High-Entropy Alloy","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-04 13:12:05","doi":"10.21203/rs.3.rs-8223266/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-30T23:02:00+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-30T03:27:58+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"30034702017483874592342458647687168579","date":"2026-03-17T09:41:12+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"85741332216498781264878764793160416566","date":"2026-01-12T00:56:06+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-08T18:45:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"60574895970034740815759239335458535654","date":"2025-12-17T14:47:43+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-02T10:35:11+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-02T07:37:37+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-12-02T06:10:44+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj Materials Degradation","date":"2025-11-27T14:44:12+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-materials-degradation","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjmatdeg","sideBox":"Learn more about [npj Materials Degradation](http://www.nature.com/npjmatdeg/)","snPcode":"41529","submissionUrl":"https://submission.springernature.com/new-submission/41529/3","title":"npj Materials Degradation","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"da7b5dd5-3f8d-4688-8c41-459b025f0e01","owner":[],"postedDate":"December 4th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":59042498,"name":"Physical sciences/Chemistry"},{"id":59042499,"name":"Physical sciences/Engineering"},{"id":59042500,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2026-05-18T23:53:32+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-04 13:12:05","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8223266","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8223266","identity":"rs-8223266","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}