Comparison of the Oxidation Behaviour of Wrought and Additively Manufactured Alloy 625 in a High Temperature CO2 Environment

Comparison of the Oxidation Behaviour of Wrought and Additively Manufactured Alloy 625 in a High Temperature CO2 Environment | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Comparison of the Oxidation Behaviour of Wrought and Additively Manufactured Alloy 625 in a High Temperature CO 2 Environment Boma Phoebe Norman, Joy Sumner, Stefano Mori, Nigel Simms, Emily Rose Lewis, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8175421/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 9 You are reading this latest preprint version Abstract In pursuit of cleaner and more sustainable power generation, gas turbine power cycles using supercritical carbon dioxide (sCO 2 ) as a working fluid have emerged as an option to meet emissions targets. The Allam cycle achieves emission reduction through high-pressure sCO 2 in an oxy-combusted, highly recuperated cycle, operating at 300 bar and up to 800°C. Alloy 625, known for its high corrosion resistance, is used for high-temperature components, including heat exchangers, which can be manufactured traditionally or via additive manufacturing (AM) techniques, such as laser powder bed fusion (LPBF). This study investigates Alloy 625’s microstructural influence on resistance to simulated Allam cycle conditions; comparing as-built and solution heat-treated LPBF AM samples with conventionally manufactured wrought. Isothermal oxidation tests were conducted at 800 ° C for 1000h in a CO 2 rich environment (CO 2 + 2.7mol% H 2 O + 1.43mol% N 2 + 0.17mol% O 2 + 300ppm SO 2 ). Wrought Alloy 625 demonstrated the lowest oxidation rate, attributed to its homogenized microstructure and fine grain size. Solution heat-treated AM samples exhibited a continuous oxide layer due to grain boundary changes, enhancing oxide scale formation. The K p values of as-built, solution heat-treated and wrought Alloy 625 were 4.8 x 10 − 5 , 5.3×10 − 5 , and 4.0×10 − 5 mg 2 /cm 4 respectively. Grain morphology and heat treatment influenced oxidation. As-built LPBF samples formed ridges on the oxide scale and with subsurface voids, while wrought samples displayed uniform oxide layers without subsurface voids. These findings highlight how manufacturing techniques and post-processing affect Alloy 625’s high-temperature performance, crucial for turbine casing and heat exchangers in a baseline Allam cycle environment. supercritical carbon dioxide oxidation kinetics additive manufacturing wrought manufacturing Alloy 625 Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction The global push towards cleaner and more sustainable power generation aims to reduce greenhouse gas emissions by at least 55% by 2030 [ 1 ]. Utilizing supercritical carbon dioxide (sCO 2 ) as a working fluid in power plant cycles is a potential strategy for the energy industry to enhance turbine efficiency and overall plant performance [ 2 ]. The Allam cycle, employs high-pressure sCO 2 in an oxy-combustion, highly recuperative cycle with a turbine inlet temperature of up to 800°C and pressures of up to 300 bar [ 3 ]. As well as being highly efficient and having a small turbomachinery footprint, this cycle addresses emission reduction objectives by integrating CO 2 capture [ 3 ]. However, the unique characteristics of sCO 2 introduce new challenges for turbomachinery components such as heat exchanges, requiring materials that balance cost, mechanical performance, and manufacturability[ 4 ]. In high temperature and pressure working conditions, nickel-based superalloys such as Alloy 625 have been used for manufacturing turbomachinery such as turbine casing, and heat exchangers etc [ 5 ]. Alloy 625 is typically utilized in a gas turbine for components such as swirlers in combustion systems [ 6 ], heat exchangers and for turbine casing. Alloy 625 (see Table 2 for composition) is a Ni-based superalloy known for its excellent corrosion resistance at temperatures up to 980ºC. Alloy 625 is a chromia-former, which is strengthened by both precipitation hardening [ 7 ], and also by solid-solution hardening from the Nb and Mo in the Ni-Cr matrix [ 8 ]. Precipitation hardening of Alloy 625 is typically dependent on to the formation of a number of phases to help minimise creep at high temperatures. These include a δ-Ni 3 Nb strengthening phase, gamma double prime (- Ni 3 Nb) and various carbides (M 23 C 6 , M 6 C, MC) [ 9 , 10 ]. The dominant form of MC in Alloy 625 is niobium carbide (NbC). However, with exposure to high temperatures, NbC may decompose, leading to the formation of secondary carbides such as M 6 C (Mo, W and Nb rich) and M 23 C 6 (Cr-rich). This formation of secondary carbides can be beneficial or detrimental, depending on their distribution and morphology. Finely dispersed and located along grain boundaries can improve creep strength through prevention of grain boundary sliding, however, the formation of continuous films at the grain boundaries can be detrimental for ductility [ 11 , 12 ]. The heat required for such phase transformation may be introduced during standard processing or post-treatment, however, this is important to consider when exploring newer manufacturing routes such as additive manufacturing (AM). In addition to phases that benefit performance, Ni-based superalloys can form undesirable phases such as laves, µ, and δ phases; after prolonged exposures to high temperatures [ 13 ]. As such, Alloy 625’s manufacturing methods are important when considering high temperature applications. Conventional processing techniques include casting, forging and machining, but additive manufacturing (AM) enables the production of complex geometries, faster production time, autonomous production capabilities, the creation of lightweight structures, and topology optimization [ 14 , 15 ]. AM is the process of building up materials to create objects from 3D model data, typically layer by layer, as opposed to conventional subtractive manufacturing methodologies [ 16 ]. Despite challenges such as cost of 3D machines and materials, and extensive post-processing to meet industrial standards increases the cost and manufacturing time [ 17 ]. Among AM techniques, Laser Powder Bed Fusion (LPBF) offers fine microstructural control, high density, and near-net-shape capability, making it promising for components like heat exchangers [ 18 ]. However, due to the energy used, these techniques result in components with complex thermal histories, which contribute to producing components with unique microstructure & performance characteristics [ 19 – 22 ], including for Alloy 625. Indeed, research into Alloy 625 indicates that the following microstructural variations may exist as a result of AM: The segregation of Nb, Mo and other alloying elements [ 23 ], which accumulate in interdendritic regions forms laves phases (Nb-rich) and δ phases (Ni 3 Nb) [ 24 , 25 ]. The as-built microstructure in AM has been found to be more prone to forming laves (Nb-rich) and δ (Ni 3 Nb) phases due to the rapid cooling and repeated remelting associated with powder processing. The residual stresses and microstructural exposure to high temperature contribute to laves and δ phases formation. Post-build heat treatments help reduce these internal stresses, as well as homogenize the microstructure. However, Alloy 625 is less susceptible than many other alloys to the formation of ‘hot cracks’ when produced by AM. As an indication, Fig. 1 shows a weldability diagram (with weldability being a good indicator of suitability for AM production) of various Ni-based superalloys, indicating the ease of fabrication of nickel-based superalloys as a function of their Ti and Al contents. Alloys above the red line have high Ti and Al contents, and present manufacturing challenges for AM fabrication. Alloy 625, with its low levels of 0.10% Ti and 0.11% Al and higher Nb, 3.65% content compared to other Inconel alloys, has been noted to impede rapid age hardening post-welding. Additionally, weld cracking is prevented by inducing chromium carbide precipitation at grain boundaries [ 26 ]. LPBF is widely utilised across industries including automotive, energy, and medical sectors [ 28 – 30 ]. Ni-based superalloys produced by LPBF such as Haynes 230 [ 31 ], Alloy 718[ 32 ] and Alloy 625[ 33 – 35 ] have been studied for their mechanical strength and oxidation behaviour at elevated temperatures. However, LPBF components can exhibit defects influenced by process parameters and the material’s thermophysical properties such as balling [ 19 ], denudation [ 36 ], surface roughness [ 37 ], porosities (lack-of-fusion, keyhole, and gas porosity) [ 19 , 38 , 39 ], cracking (solidification[ 40 ] and liquation) and delamination [ 41 ]. Minimisation of defects through adjusting process parameters has been attempted, for example Chowdury et al.[ 43 ], found that surface roughness can be reduced by increasing energy density, using higher laser power at lower scan speeds, and optimizing hatch spacing. Despite potential defects, LPBF offers advantages over other AM technologies due to its high precision ( \(\:\pm\:\) 0.1–0.3%) [ 44 ], ability to fabricate complex geometries, and full melting of metal powders resulting into 99.9% density and maximizing tensile strength [ 17 ]. Hager et al.[ 45 ] were able to print LPBF AF9628 steel with density > 99.5%. Studies have been conducted on LPBF Alloy 625, including microstructural changes [ 46 ], manufacturing techniques [ 47 ], mechanical properties and surface morphology on oxidation behaviour. One area which still needs study is the influence of the Allam cycle’s working fluid (sCO 2 ) on the AM Alloy 625. In particular, as the cycle is a direct-fired system, the working fluid may contain contaminants such as nitrogen and sulphur. For high-temperature oxidation, sulphur can influence the formation of alumina on Ni-based alloys and consequently increases oxidation kinetics as the sulphur content increases [ 49 ]. As such, the impact of these in trace levels on the long-term performance of any AM components must be understood While the oxidation behaviour of traditional wrought Alloy 625 in environments such as air and CO 2 is well documented, the microstructural characteristics of AM material, such as dendritic structure, columnar grains, and fine grain sizes, can influence oxidation resistance. Ramenatte et al.[ 13 ] highlights that laser beam manufacturing (LBM) Alloy 625 exhibits lower oxidation resistance than its wrought counterpart, despite identical compositions, primarily due to the difference in microstructure and surface quality. Edwards & Ramulu [ 50 ], identifies residual stress microstructure, porosity, and surface finish as key factors influencing fatigue performance and subsequently oxidation behaviour. This study investigates the role of grain size, phase distribution, and precipitate formation in influencing the oxidation resistance of AM Alloy 625. To address current knowledge gaps, this work examines the oxidation and carburization behaviour of LPBF Alloy 625 in a simulated baseline Allam cycle environment (800°C) in comparison to the wrought samples. Additionally, it provides an in-depth analysis of the oxidation resistance of both as-built and solution heat-treated (1177°C, 1 hr, water-quenched) AM microstructures after 1000h exposure. Metallic AM components may exhibit microstructure and properties distinct conventionally manufactured counterparts, which must be fully understood to build industrial confidence. As the Allam cycle’s sCO 2 working fluid typically contains impurities such as H 2 O, COS, NH 3 , and HCN, which when combusted, are converted to SO 2 , NO, H 2 O, and N 2 , AM Alloy 625’s performance in this test atmosphere is also studied and compared to conventional manufacturing routes. 2. Materials and Exposure Procedure 2.1. Materials As-received wrought Alloy 625 supplied by Alleima, and LPBF AM manufactured material supplied by the University of Birmingham [ 46 ] manufactured by the Manufacturing Technology Centre, have been compared (Table 1 ). Wrought manufactured (WM) Alloy 625 was machined into cylindrical rods. The dimensions of the cylindrical samples were ∅ 14mm x 10-11mm. The chemical composition of the wrought material is presented in Table 2 . WM samples underwent no heat treatment. The chemical composition of the powder used for AM, given in Table 2 , was determined using a combination of LECO ICP-MS and XRF. The powder has a size range of 15–45 µm and was manufactured into cylindrical samples using the laser-powder bed fusion process in an EOS M280 machine with proprietary parameters. The cylindrical rods produced were 14 mm in diameter x 125 mm in length. To adequately analyse the AM samples’ final microstructures, two post-printing procedures were followed. The AM samples were either exposed in an as-built format or underwent solution heat treatment as shown in Table 1 . Table 1 Sample codes Code Manufacturing Process Heat Treatment As-built (AB) LPBF AM None Solution heat treated (SH) 1177°C, 1hr, water quenched Wrought manufactured (WM) Wrought None Table 2 Chemical composition of alloys, AM powder composition measured using LECO-ICM XP and WM composition obtained from SEM for Alloy 625 samples Alloy 625 Elemental composition (% wt .) C Mn Si P S Cr Co Mo Nb Ti Al Fe Ni AM - < 0.01 0.07 < 0.01 0 21.27 0.04 9.02 3.65 0.10 0.11 3.66 B WM 0.025 0.15 0.2 \(\:\le\:\) 0.015 \(\:\le\:\) 0.015 21.5 - 8.7 3.5 - - 4 61 2.2. Pre-exposure Treatment of Samples The samples were cleaned with isopropanol and acetone in an ultrasonic bath for 15 mins each to remove any surface contamination. Figure 2 shows the samples before exposure. Dimensions of the cylindrical samples were measured using a digital micrometre with a 0.001mm resolution, and mass was recorded for comparison after the oxidation test (with a 0.01 mg resolution). After each exposure cycle’s completion, sample masses were taken to create a mass change plot. 2.3. High Temperature Exposure of Samples Isothermal oxidation tests were performed in a vertical controlled atmosphere furnace at 800°C, for up to 1000h in CO 2 + 2.7mol% H 2 O + 1.43mol% N 2 + 0.17mol% O 2 + 300ppm SO 2 . The gas flow rate was set at 88.78cm/min using a mass flow controller, and automated injection was used to inject deionised water at a rate of 0.003ml/min into the furnace. The test was conducted in 200 hour cycles. 2.4. Post-exposure Treatment of Samples Samples were mounted in a low-shrinkage epoxy resin with ballotini glass spheres in a 60:40 and cross-sectioned in the XY plane perpendicular to the Z-axis of LPBF build or the cylindrical axis for WM. Samples were ground to 1200 grit size using SiC paper and then polished to 1µm using oil-based diamond polishing medium. 2.5. Microstructural Analysis A VEGA L scanning electron microscope (SEM) was used to capture backscattered electron (BSE) images of cross-sectioned samples and the chemical composition of the oxide scale on alloy was ascertained using elemental dispersive X-ray spectroscopy (EDX). The combination of electron microscopy and EDX techniques was used to provide a comprehensive analysis of the sample’s microstructure, and oxide scale composition. Samples were electrolytically etched in 10% oxalic acid at 4V for 4 seconds to reveal grain boundary carbides. 3. Effect of Manufacturing Technique on the Oxidation Behaviour Parameters including heat treatment and microstructural characteristics, influence the oxidation behaviour of LPBF Alloy 625. To assess the impact of manufacturing solution-treated and as-built LPBF samples were compared with wrought Alloy 625 to evaluate how factors like solution treatment influence phase formation, grain structure, and oxide formation. Figure 3 shows the mass change data for Alloy 625. All samples demonstrated less than < 1.3g/cm 2 mass gain following 1000h exposure time. The low mass gain indicates Alloy 625 exhibits low oxidation rates in the tested exposure condition. At 1000h, the wrought (WM) solution heat-treated (SH), and as-built (AB) samples show no significant difference in change in mass. Given the overlapping error bars, no significant differences in oxidation behaviour can be concluded between the manufacturing conditions. Noticeably, the mass fluctuations ie. the decreases at 800h points indicate spallation, which was noticeable in the crucible, which should be considered in interpreting the plot [ 49 ]. The three production variants showed little difference in mass change, notably, solution heat-treated LPBF has shown the highest mass gain, 0.26 mg.cm 2 /hr while the wrought has shown the least mass gain, 0.18 mg.cm 2 /hr. Table 3 K p value of as-built, solution heat-treated and wrought manufactured Alloy 625 over 1000h exposure Time (h) K p (mg 2 /cm 4 hr) Error (mg 2 /cm 4 hr) AB 48.4 x 10 − 6 8.8 x 10 − 6 SH 52.9 x 10 − 6 9.2 x 10 − 6 WM 40 x 10 − 6 8 x 10 − 6 The parabolic oxidation behaviour of as-built LPBF, solution heat-treated LPBF and wrought manufactured Alloy 625 was quantified based on mass change per unit area \(\:\varDelta\:{m}^{2}={K}_{p}t\) . Where K p is the parabolic rate constant, \(\:\varDelta\:m\) is mass change and t is the exposure time. The parabolic rate constants (K p ) exhibited similar oxidation resistance across the variations in manufacturing techniques as seen in Table 3 . The as-built microstructure, Fig. 4 a shows fine grain structure which Emily et al.[ 46 ] EBSD analysis shows this as-built structure was a fine columnar dendrites which extended parallel to the build direction, typically due to rapid solidification. The EBSD also showed very few twin grain boundaries (TGBs). LPBF typically produces components with preferential crystallographic orientation, resulting in columnar grains aligned with the building direction [ 56 ]. The solution heat-treated sample shown in Fig. 4 b, shows a more homogenized grain structure and EBSD in Emily et al.[ 46 ], shows 30% more TGBs. The wrought sample shows microstructure with fully equiaxed grains, EBSD maps of alloy 625 seen in Chyrkin et al.[ 51 ]. As-built AM samples typically demonstrate more pronounced crystallographic texture and grain distribution [ 52 , 53 ]. However, studies like Qi et al.[ 54 ] postulate that the grain orientation and texture are controlled by the direction of heat flow and temperature gradient, which depend on process parameters. Grain size data from the BSE images in Fig. 4 were analyzed using the ASTM E112 intercept method in Image J, and the results summarized in Table 5 . This intercept line was taken in the horizontal direction, and this reveals a significant difference in grain size as a result of processing. Table 4 ASTM E112 intercept method grain size analysis Sample Name Average Grain Size (µm) Standard deviation Error (µm) AB 33.5 9.3 SH 26.1 4.9 WM 13.7 1.9 The relationship between grain characteristics and oxidation behaviour was evident in this study. Microstructural analysis of the as-built (AB), solution heat-treated (SH) LPBF and wrought manufactured (WM) Alloy 625 samples revealed notable differences in grain morphology and size, which correlated with oxidation performance. The WM samples exhibit the finest average size of 13.7 µm and thus promoting accelerated oxidation as the higher grain boundary density acts as an enhanced diffusion path, which is evident in the BSE images in Fig. 5 . While the AB samples show considerably coarser grains averaging 33.5 µm. The SH samples display an intermediate grain of 26.1 µm due to partial recrystallization. Previous research indicates grain sizes could be important to oxidation behaviour, higher mass gain exhibited in LPBF samples in comparison to wrought samples has been associated with the grain size [ 55 ]. The finer-grained WM samples in Fig. 5 developed a thicker continuous oxide layer, indicative of good oxidation resistance. The dense grain boundary network enhanced Cr diffusion, enabling the rapid formation of a continuous and adherent Chromia scale. This promotes efficient selective oxidation of chromium while minimising internal oxidation and maintaining stable oxide/metal interfaces. Such behaviour is consistent with previous studies on chromia-forming alloys manufactured via laser beam processes, including Alloy 718 [ 57 ], Alloy 625[ 58 ] and Hastelloy X[ 59 ], which also all exhibit parabolic oxidation kinetics. These experimental observations are consistent with previous findings linking grain boundary characteristics to oxidation kinetics in chromia-forming alloys in Sanviemvongsak et al.[ 60 ], and Ramenatte et al.[ 61 ] works. These results confirm that the refined microstructure of WM Alloy 625 facilitates a more effective selective oxidation of Cr. The improved oxidation resistance observed in wrought specimens can be attributed to their refined grain structure, which shows through multiple protective mechanisms. The finer grain size of WM (13.7 µm) promotes more efficient chromium diffusion to the surface through shortened diffusion pathways and an enhanced grain boundary network, facilitating the rapid formation of a protective chromia layer. The presence of voids and surface ridges in the oxidized as-built LPBF samples shown in Fig. 6 indicates the development of stress and non-uniform scale growth. In contrast, the absence of such features in the WM and solution heat-treated LPBF samples supports the more stable oxide/metal interface and improved scale adherence. The EDX maps as-built sample shows the formation of an almost continuous chromia rich layer, the Al signals have a very low intensity and do not show any significant alumina looking at the O maps as well in Fig. 6 . Mo and Nb show enrichment near the oxide/substrate interface which indicates the formation of Nb and Mo rich phase. The oxide layer in the as-built appears less uniform than in the wrought counterpart, displaying localised thickening and areas of spallation. This suggests less efficient chromium transportation and delayed chromia formation. The coarser and columnar grains provide few grain boundary diffusion paths compared with the finer grain wrought microstructure, leading to less efficient chromia diffusion towards the surface. Sanviemvongsak et al.[ 60 ] identified a microstructural feature in as-built samples: the absence of special grain boundaries with specific crystallographic aligned features that typically enhance oxidation resistance. The absence of such boundaries also combined with higher residual stresses and elementary segregation, leads to discontinuous chromia formation and reduced oxide adhesion. These microstructural features explain the less protective and more irregular oxide scale observed in the as-built Alloy 625 samples compared to the uniform and adherent chromia layer of the wrought. The solution heat treated sample in Fig. 7 shows a more continuous chromia layer. Ni-Cr alloys like Alloy 625 form duplex oxide structure characterized by a Ni rich δ phase near the metal-oxide and Cr 2 O 3 layer near the oxide-alloy interface. These findings align with Parozia et al.[ 62 ], who investigated LPBF Alloy 625, comparing solution annealed (2h at 1150°C) and as-built at 900°C up to 100h. Their investigation revealed higher mass gain in heat-treated samples attributed to the formation of NiCr 2 O 4 and Nb 2 O 5 clusters inside the oxide scale. In the as-built AM sample (Fig. 6 ), a duplex oxide structure comprising Ni and Cr oxides is observed at the oxide-alloy interface. In contrast, the solution heat-treated sample (Fig. 7 ), subjected to oxidation at 800°C for 1000 hours after being heat-treated at 1100°C for 1 hour and water-quenched, exhibits a well-developed and continuous chromia layer. Beneath the chromia layer, an intermetallic Ni and Nb rich phase, expected to be δ-Ni₃Nb phase, was detected in both the as-built and solution heat-treated samples. 3.1. Effect of Heat Treatment on the Oxidation Behaviour. Figure 8 illustrates cross-sectional SEM images of LPBF as-built, solution heat-treated and wrought manufactured Alloy 625 samples after exposure to CO 2 + 2.7mol% H 2 O + 1.43mol% N 2 + 0.17mol% O 2 + 300ppm SO 2 for 200, 600, and 1000h. Cross-sectional SEM analysis after 200h of exposure demonstrates the initial formation of a chromia, likely Cr 2 O 3, protective layer in all conditions, accompanied by Nb-rich δ-phase and Nb carbides, though the protective oxide layer is thin, suggesting the initial stage of oxide formation. The solution heat-treated samples exhibited enhanced precipitate structure visibility even at this early stage. Compared to both LPBF samples, the wrought Alloy 625 has a denser, more continuous oxide layer and a thinner Nb-rich δ-phase below the oxide layer. As exposure time increased to 600 hours, both microstructures developed thicker oxide scales while maintaining the presence of possible Nb-rich phase and carbides. The as-built sample uniquely developed characteristic ridge formations, marking a divergence in oxidation behaviour between the as-built and solution heat-treated samples. Furthermore, void formation in the subsurface region is noticed in the as-built microstructure after 600h and 1000h. The void formation can be attributed to vacancy injection or internal oxidation of carbon, a phenomenon consistent with the Kirkendall effect. As proposed by Bricknell et al.[ 63 ], the outward diffusion of metal cations during oxidation must be balanced by an inward flux of metal vacancies. These vacancies migrate to grain boundaries, where they agglomerate, resulting in the formation of voids and grain boundary deterioration. The as-built also formed ridges may be linked to the stress concentration in the sample, which could lead to acceleration of spallation [ 64 ]. The most substantial differences emerged after 1000 hours of exposure, where solution heat-treated samples demonstrated superior oxidation resistance through the formation of a thick, continuous oxide scale and the presence of internal Al. These samples also developed needle-like δ-phase precipitates throughout the bulk material, while both conditions showed Mo- and Nb-rich precipitate formation along grain boundaries, confirmed through EDX mapping, possibly a Ni 3 Nb 2 δ-phase. These findings have direct industrial applications in gas turbine manufacturing, chemical processing equipment, and advanced manufacturing protocols. The better resistance in terms of formation of ridge void, lesser spallation, and denser oxide layer of solution heat-treated samples (as seen in Fig. 7 ) suggests that post-processing heat treatments for AM samples could eliminate the formation of ridges and reduction in spallation, which could improve component durability in high-temperature CO 2 -rich environments. Spallation was observed in LPBF materials, with the as-built exhibiting more pronounced spallation after exposure. The absence of Ridge-Void (RV) formations in wrought samples, as noted by Chyrkin et al.[ 64 ] indicates that RV morphology is not inherently linked to AM processing and appears independent of reaction conditions. This independence from processing method suggests that RV formation is a fundamental material response to environmental conditions rather than a manufacturing artifact when the as-built and solution heat-treated micrographs in Fig. 7 are compared. The formation of RV on the as-built can act as stress concentration sites, which can influence surface degradation. The increased spallation in as-built samples may be linked to higher residual stresses, or microstructural heterogeneities specific to the LPBF process. The absence of RV morphology could benefit component performance through improved mechanical integrity, enhanced oxidation resistance due to more uniform oxide layer formation, and potentially extended component lifetime through reduced degradation mechanisms. Further investigation of the relationship between precipitation kinetics and oxidation behaviour could yield valuable insights for optimizing industrial applications of AM-manufactured Alloy 625 components. Figure 9 shows the locations corresponding to the points analysed in Table 5 for As-built LPBF Alloy 625, where precipitates were observed along grain boundaries to identify the precipitates that formed. Table 5 Point analysis of as-built LPBF alloy 625 after 1000h exposure in CO 2 + 2.7mol% H 2 O + 1.43mol% N 2 + 0.17mol% O 2 + 300ppm SO 2 at 800°C Point(wt%) Ni Cr Mo Nb Fe Si S 1 53.6 21.1 15.9 4.6 3.6 0.4 0.3 2 45.7 25.1 19.8 4.7 3.2 0.6 0.6 3 58.5 21.3 7.9 5.4 3.8 4 58 20 9.5 7.8 3.4 0.2 The point analysis of precipitates after 1000h oxidation reveals the formation of Nb- rich and Cr- rich carbides. The elevated Nb concentrations at Points 3 (5.4wt%) and 4 (7.8wt%), suggest the formation of niobium carbides (NbC), consistent with the known carbide-forming tendency of Nb in Ni-based superalloys under thermal exposure [65]. The Cr enrichment at point 2 (25.1wt%) suggests the presence of Cr 23 C 6 carbides, which are thermodynamically favoured in high-Cr alloys under oxidizing conditions [66]. The significant Mo levels (up to 19.8% at Point 2) suggest the potential for complex mixed-metal carbides (e.g., M 6 C, where M = Ni, Mo, Cr). Variations in elemental distribution reflect localized phase formation and diffusion induced segregation during oxidation. Additionally, Xuebing et al.[67] observed that M 23 C 6 was not found in the as-cast specimen, suggesting that M 23 C 6 was exclusively formed during the solution heat treatment process. The presence of carbon in the environment during high-temperature oxidation potentially accelerates carbide formation through carbon diffusion into the alloy surface, leading to enhanced precipitation. Notably, despite the presence of sulphur and nitrogen in the testing environment, these elements were not detected during point analysis, suggesting potential limited interaction with the alloy surface or delocalised distribution. These findings have significant implications for industrial applications, particularly in component lifetime prediction in high-temperature environments and the optimization of heat treatment protocols for desired carbide distributions, while also enhancing our understanding of degradation mechanisms in service conditions. The comprehensive understanding of microstructural evolution in additively manufactured superalloys under oxidizing conditions provided by this analysis suggests that further investigation of the carbon environment's role in precipitation behaviour would be valuable for optimizing material performance in industrial applications, particularly in high-temperature and oxidizing environments where carbide stability plays a crucial role in component longevity. The absence of detectable sulphur and nitrogen indicates favourable environmental conditions for oxide scale formation and stability, contributing to the overall oxidation resistance of the alloy. 4. Conclusions This study investigates the oxidation behaviour of LPBF in comparison with wrought Alloy 625, highlighting the influence of manufacturing parameters such as heat treatment, and microstructural characteristics on its high-temperature oxidation resistance. Heat treatment of LPBF, particularly solution heat treatment, influenced oxidation resistance by promoting a microstructure which was demonstrated to support the development of a protective chromium oxide layer. The study demonstrates that solution heat treatment forms adherent chromia layer which is important for high temperature exposures. Careful consideration must be given to managing microstructural defects such as voids and ridges that can compromise long-term stability. These defects highlight spallation noticeable during cyclic oxidation and the need for post-processing optimization to mitigate the impact of these microstructural flaws. The study examines the role of microstructure, and grain boundary in the formation of a protective oxide layer. Wrought Alloy 625, with its fine and uniform grain structure, demonstrated a thicker protective oxide layer, indicating better oxidation resistance compared to the LPBF variants, both as-built and solution heat-treated. Despite this, both LPBF variants also developed Cr oxide layers, indicating baseline suitability for high temperature applications. Notably, the solution heat treated LPBF sample displayed a thicker continuous oxide layer with the absence of ridge like features when compared to as-built. By 1000 h, the wrought alloy had already formed a much thicker oxide layer, suggesting that prolonged exposure may lead to the depletion of protective elements such as Al, Cr. In contrast, the solution-treated LPBF sample is expected to continue developing its oxide layer at a more gradual rate, reducing the risk of significant alloy degradation or alloy strength during extended service. The findings hold particular relevance for Allam cycle power plants operating in supercritical CO 2 environments, where component durability under extreme conditions directly impacts system reliability and efficiency. Future work should focus on extended cyclic oxidation studies beyond 1000 hours, and investigation of combined high-pressure and temperature effects on oxidation behaviour. Additional research directions include exploring modified alloy compositions and understanding the impact of thermal and pressure cycling on oxide scale adherence. These investigations would further enhance our understanding of material behaviour in supercritical CO 2 environments and contribute to the advancement of Allam cycle technology. Declarations Author Contribution B.P.N wrote the main manuscriptA.P. assisted in experimental workE.R.L, N.C and S.C supplied materials for experimental workAll authors excluding B.P.N. reviewed and provided feedback on the manuscript Funding Acknowledgement The Authors would like to thank EPSRC for funding, also thank Alleima in Germany for the provision of the Alloy 625 wrought samples and the University of Birmingham, United Kingdom, for providing the LPBF samples made by the Manufacturing Technology Centre. Data Availability Data supporting this study are openly available from the CORD data repository at [https://dspace.lib.cranfield.ac.uk/handle/1826/20712](https:/dspace.lib.cranfield.ac.uk/handle/1826/20712) References United Nations. Net Zero Coalition | United Nations. Available at: https://www.un.org/en/climatechange/net-zero-coalition (Accessed: 12 October 2023) Weiland NT., Dennis RA., Ames R., Lawson S., Strakey P. Fossil energy. 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16:53:49","extension":"png","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":53105,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/a0ed424c6482c433d53add1f.png"},{"id":97280777,"identity":"ec8667b2-234f-43cb-8148-549a1a999c91","added_by":"auto","created_at":"2025-12-02 16:53:47","extension":"png","order_by":20,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":224325,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/0ba601160d839762abd605e0.png"},{"id":97280788,"identity":"c4e462b9-d189-4c2d-9b15-b36a11c1d47e","added_by":"auto","created_at":"2025-12-02 16:53:48","extension":"xml","order_by":21,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":147229,"visible":true,"origin":"","legend":"","description":"","filename":"0eea540d44484f319a27040993908eb71structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/d99f9bea315c59a12d5e0a81.xml"},{"id":97280803,"identity":"fb9dcacf-ee3e-4275-afd4-ed1366a95d47","added_by":"auto","created_at":"2025-12-02 16:53:49","extension":"html","order_by":22,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":159618,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/0fb44d509e2d368453ac54a0.html"},{"id":97280793,"identity":"189061d7-e9f3-414a-8004-e4fac347aff8","added_by":"auto","created_at":"2025-12-02 16:53:48","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":33015,"visible":true,"origin":"","legend":"\u003cp\u003eWeldability diagram of nickel-based superalloys with IN625, an Alloy 625 variant, highlighted in \u0026nbsp;[27].\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/8bc630015953d4d69e02a021.jpg"},{"id":97280766,"identity":"de5f9615-9782-4f3e-b6cb-1e975cf3cc14","added_by":"auto","created_at":"2025-12-02 16:53:46","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":19179,"visible":true,"origin":"","legend":"\u003cp\u003eImages of as-built (AB), solution heat-treated (SH) and wrought manufactured (WM) samples before exposure\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/0926194535a71bdb6f148e02.jpg"},{"id":97280794,"identity":"642c5a6b-c887-4614-bd8d-bd721d69eb36","added_by":"auto","created_at":"2025-12-02 16:53:48","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":42005,"visible":true,"origin":"","legend":"\u003cp\u003eMass change plot of as-built (AB), solution heat treated (SH) AM and wrought manufactured (WM) Alloy 625 exposed at 800ºC for up to 1000h. Error bars indicate the maximum and minimum values from up to 5 samples.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/b8d76ec6fb81926b0ca7f7fd.jpg"},{"id":97280771,"identity":"e6b964ce-f02e-4c7c-97a2-42c03f07892a","added_by":"auto","created_at":"2025-12-02 16:53:46","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":90538,"visible":true,"origin":"","legend":"\u003cp\u003eBSE SEM micrographs of polished (a) AM as-built (b) AM solution heat-treated (c) Wrought Alloy 625 after 1000h exposure in CO\u003csub\u003e2\u003c/sub\u003e + 2.7mol% H\u003csub\u003e2\u003c/sub\u003eO + 1.43mol% N\u003csub\u003e2\u003c/sub\u003e + 0.17mol% O\u003csub\u003e2\u003c/sub\u003e + 300ppm SO\u003csub\u003e2\u003c/sub\u003e at 800°C. Samples have been electrolytically etched in 10% oxalic acid at 4V for 4 seconds\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/907ac817415d998149edb005.jpg"},{"id":97280791,"identity":"a4f98b4e-bef3-47ad-8503-28307c9649c3","added_by":"auto","created_at":"2025-12-02 16:53:48","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":117168,"visible":true,"origin":"","legend":"\u003cp\u003eWrought Manufactured Alloy 625 exposed after 1000h oxidation in CO\u003csub\u003e2\u003c/sub\u003e + 2.7mol% H\u003csub\u003e2\u003c/sub\u003eO + 1.43mol% N\u003csub\u003e2\u003c/sub\u003e + 0.17mol% O\u003csub\u003e2\u003c/sub\u003e + 300ppm SO\u003csub\u003e2\u003c/sub\u003e at 800ᵒC\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/e23a33e4d9e982df604e69c1.jpg"},{"id":97280796,"identity":"b88e6291-9609-4e42-afc1-92b4f25f3a89","added_by":"auto","created_at":"2025-12-02 16:53:48","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":94776,"visible":true,"origin":"","legend":"\u003cp\u003eSEM/EDX maps of as-built AM Alloy 625 after 1000h oxidation in CO\u003csub\u003e2\u003c/sub\u003e + 2.7mol% H\u003csub\u003e2\u003c/sub\u003eO + 1.43mol% N\u003csub\u003e2\u003c/sub\u003e + 0.17mol% O\u003csub\u003e2\u003c/sub\u003e + 300ppm SO\u003csub\u003e2\u003c/sub\u003e at 800°C\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/75fd36c0be0394ab693ae2b8.jpg"},{"id":97368661,"identity":"ee48e494-01ef-4a2f-ad82-8b8cf2d33ecd","added_by":"auto","created_at":"2025-12-03 16:22:43","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":113270,"visible":true,"origin":"","legend":"\u003cp\u003eEtched EDX image of solution heat-treated AM Alloy 625 exposed for 1000h in CO\u003csub\u003e2\u003c/sub\u003e + 2.7mol% H\u003csub\u003e2\u003c/sub\u003eO + 1.43mol% N\u003csub\u003e2\u003c/sub\u003e + 0.17mol% O\u003csub\u003e2\u003c/sub\u003e + 300ppm SO\u003csub\u003e2\u003c/sub\u003e at 800ᵒC then electro-etched in 10% oxalic acid.\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/22b310db2a1ddffe4915df16.jpg"},{"id":97368252,"identity":"5fb8742c-6d69-4733-af57-84a389a68a7c","added_by":"auto","created_at":"2025-12-03 16:21:52","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":94415,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of as-built and solution heat-treated LPBF and wrought Alloy 625 after 1000h oxidation in CO\u003csub\u003e2\u003c/sub\u003e + 2.7mol% H\u003csub\u003e2\u003c/sub\u003eO + 1.43mol% N\u003csub\u003e2\u003c/sub\u003e + 0.17mol% O\u003csub\u003e2\u003c/sub\u003e + 300ppm SO\u003csub\u003e2\u003c/sub\u003e at 800°C\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/1590316ff6f5500dd077c797.jpg"},{"id":97280789,"identity":"3ebcc53c-f6b4-415b-b393-541e8e09ac8b","added_by":"auto","created_at":"2025-12-02 16:53:48","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":69569,"visible":true,"origin":"","legend":"\u003cp\u003eAs-built LPBF Alloy 625 after 1000h exposure in CO\u003csub\u003e2\u003c/sub\u003e + 2.7mol% H\u003csub\u003e2\u003c/sub\u003eO + 1.43mol% N\u003csub\u003e2\u003c/sub\u003e + 0.17mol% O\u003csub\u003e2\u003c/sub\u003e + 300ppm SO\u003csub\u003e2\u003c/sub\u003e at 800°C\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/f5c0f1588e4a16bc98c29886.jpg"},{"id":97372934,"identity":"ff62efc9-b934-4cac-946c-b58d4d040171","added_by":"auto","created_at":"2025-12-03 16:33:34","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1526172,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8175421/v1/f8e19fdd-641f-48b1-976f-233d177c02b4.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eComparison of the Oxidation Behaviour of Wrought and Additively Manufactured Alloy 625 in a High Temperature CO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e Environment\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe global push towards cleaner and more sustainable power generation aims to reduce greenhouse gas emissions by at least 55% by 2030 [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Utilizing supercritical carbon dioxide (sCO\u003csub\u003e2\u003c/sub\u003e) as a working fluid in power plant cycles is a potential strategy for the energy industry to enhance turbine efficiency and overall plant performance [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The Allam cycle, employs high-pressure sCO\u003csub\u003e2\u003c/sub\u003e in an oxy-combustion, highly recuperative cycle with a turbine inlet temperature of up to 800\u0026deg;C and pressures of up to 300 bar [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. As well as being highly efficient and having a small turbomachinery footprint, this cycle addresses emission reduction objectives by integrating CO\u003csub\u003e2\u003c/sub\u003e capture [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. However, the unique characteristics of sCO\u003csub\u003e2\u003c/sub\u003e introduce new challenges for turbomachinery components such as heat exchanges, requiring materials that balance cost, mechanical performance, and manufacturability[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn high temperature and pressure working conditions, nickel-based superalloys such as Alloy 625 have been used for manufacturing turbomachinery such as turbine casing, and heat exchangers etc [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eAlloy 625 is typically utilized in a gas turbine for components such as swirlers in combustion systems [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], heat exchangers and for turbine casing. Alloy 625 (see Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e for composition) is a Ni-based superalloy known for its excellent corrosion resistance at temperatures up to 980\u0026ordm;C. Alloy 625 is a chromia-former, which is strengthened by both precipitation hardening [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], and also by solid-solution hardening from the Nb and Mo in the Ni-Cr matrix [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Precipitation hardening of Alloy 625 is typically dependent on to the formation of a number of phases to help minimise creep at high temperatures. These include a δ-Ni\u003csub\u003e3\u003c/sub\u003eNb strengthening phase, gamma double prime (- Ni\u003csub\u003e3\u003c/sub\u003eNb) and various carbides (M\u003csub\u003e23\u003c/sub\u003eC\u003csub\u003e6\u003c/sub\u003e, M\u003csub\u003e6\u003c/sub\u003eC, MC) [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The dominant form of MC in Alloy 625 is niobium carbide (NbC). However, with exposure to high temperatures, NbC may decompose, leading to the formation of secondary carbides such as M\u003csub\u003e6\u003c/sub\u003eC (Mo, W and Nb rich) and M\u003csub\u003e23\u003c/sub\u003eC\u003csub\u003e6\u003c/sub\u003e (Cr-rich). This formation of secondary carbides can be beneficial or detrimental, depending on their distribution and morphology. Finely dispersed and located along grain boundaries can improve creep strength through prevention of grain boundary sliding, however, the formation of continuous films at the grain boundaries can be detrimental for ductility [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. The heat required for such phase transformation may be introduced during standard processing or post-treatment, however, this is important to consider when exploring newer manufacturing routes such as additive manufacturing (AM). In addition to phases that benefit performance, Ni-based superalloys can form undesirable phases such as laves, \u0026micro;, and δ phases; after prolonged exposures to high temperatures [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. As such, Alloy 625\u0026rsquo;s manufacturing methods are important when considering high temperature applications.\u003c/p\u003e\u003cp\u003eConventional processing techniques include casting, forging and machining, but additive manufacturing (AM) enables the production of complex geometries, faster production time, autonomous production capabilities, the creation of lightweight structures, and topology optimization [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. AM is the process of building up materials to create objects from 3D model data, typically layer by layer, as opposed to conventional subtractive manufacturing methodologies [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Despite challenges such as cost of 3D machines and materials, and extensive post-processing to meet industrial standards increases the cost and manufacturing time [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eAmong AM techniques, Laser Powder Bed Fusion (LPBF) offers fine microstructural control, high density, and near-net-shape capability, making it promising for components like heat exchangers [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. However, due to the energy used, these techniques result in components with complex thermal histories, which contribute to producing components with unique microstructure \u0026amp; performance characteristics [\u003cspan additionalcitationids=\"CR20 CR21\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e], including for Alloy 625.\u003c/p\u003e\u003cp\u003eIndeed, research into Alloy 625 indicates that the following microstructural variations may exist as a result of AM:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eThe segregation of Nb, Mo and other alloying elements [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], which accumulate in interdendritic regions forms laves phases (Nb-rich) and δ phases (Ni\u003csub\u003e3\u003c/sub\u003eNb) [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eThe as-built microstructure in AM has been found to be more prone to forming laves (Nb-rich) and δ (Ni\u003csub\u003e3\u003c/sub\u003eNb) phases due to the rapid cooling and repeated remelting associated with powder processing.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eThe residual stresses and microstructural exposure to high temperature contribute to laves and δ phases formation. Post-build heat treatments help reduce these internal stresses, as well as homogenize the microstructure.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eHowever, Alloy 625 is less susceptible than many other alloys to the formation of \u0026lsquo;hot cracks\u0026rsquo; when produced by AM. As an indication, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows a weldability diagram (with weldability being a good indicator of suitability for AM production) of various Ni-based superalloys, indicating the ease of fabrication of nickel-based superalloys as a function of their Ti and Al contents. Alloys above the red line have high Ti and Al contents, and present manufacturing challenges for AM fabrication. Alloy 625, with its low levels of 0.10% Ti and 0.11% Al and higher Nb, 3.65% content compared to other Inconel alloys, has been noted to impede rapid age hardening post-welding. Additionally, weld cracking is prevented by inducing chromium carbide precipitation at grain boundaries [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eLPBF is widely utilised across industries including automotive, energy, and medical sectors [\u003cspan additionalcitationids=\"CR29\" citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Ni-based superalloys produced by LPBF such as Haynes 230 [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], Alloy 718[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e] and Alloy 625[\u003cspan additionalcitationids=\"CR34\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] have been studied for their mechanical strength and oxidation behaviour at elevated temperatures. However, LPBF components can exhibit defects influenced by process parameters and the material\u0026rsquo;s thermophysical properties such as balling [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], denudation [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e], surface roughness [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e], porosities (lack-of-fusion, keyhole, and gas porosity) [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e], cracking (solidification[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e] and liquation) and delamination [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Minimisation of defects through adjusting process parameters has been attempted, for example Chowdury et al.[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e], found that surface roughness can be reduced by increasing energy density, using higher laser power at lower scan speeds, and optimizing hatch spacing. Despite potential defects, LPBF offers advantages over other AM technologies due to its high precision (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\pm\\:\\)\u003c/span\u003e\u003c/span\u003e0.1\u0026ndash;0.3%) [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e], ability to fabricate complex geometries, and full melting of metal powders resulting into 99.9% density and maximizing tensile strength [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Hager et al.[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e] were able to print LPBF AF9628 steel with density \u0026gt;\u0026thinsp;99.5%.\u003c/p\u003e\u003cp\u003eStudies have been conducted on LPBF Alloy 625, including microstructural changes [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e], manufacturing techniques [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e], mechanical properties and surface morphology on oxidation behaviour. One area which still needs study is the influence of the Allam cycle\u0026rsquo;s working fluid (sCO\u003csub\u003e2\u003c/sub\u003e) on the AM Alloy 625. In particular, as the cycle is a direct-fired system, the working fluid may contain contaminants such as nitrogen and sulphur. For high-temperature oxidation, sulphur can influence the formation of alumina on Ni-based alloys and consequently increases oxidation kinetics as the sulphur content increases [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. As such, the impact of these in trace levels on the long-term performance of any AM components must be understood\u003c/p\u003e\u003cp\u003eWhile the oxidation behaviour of traditional wrought Alloy 625 in environments such as air and CO\u003csub\u003e2\u003c/sub\u003e is well documented, the microstructural characteristics of AM material, such as dendritic structure, columnar grains, and fine grain sizes, can influence oxidation resistance. Ramenatte et al.[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] highlights that laser beam manufacturing (LBM) Alloy 625 exhibits lower oxidation resistance than its wrought counterpart, despite identical compositions, primarily due to the difference in microstructure and surface quality. Edwards \u0026amp; Ramulu [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e], identifies residual stress microstructure, porosity, and surface finish as key factors influencing fatigue performance and subsequently oxidation behaviour.\u003c/p\u003e\u003cp\u003eThis study investigates the role of grain size, phase distribution, and precipitate formation in influencing the oxidation resistance of AM Alloy 625. To address current knowledge gaps, this work examines the oxidation and carburization behaviour of LPBF Alloy 625 in a simulated baseline Allam cycle environment (800\u0026deg;C) in comparison to the wrought samples. Additionally, it provides an in-depth analysis of the oxidation resistance of both as-built and solution heat-treated (1177\u0026deg;C, 1 hr, water-quenched) AM microstructures after 1000h exposure. Metallic AM components may exhibit microstructure and properties distinct conventionally manufactured counterparts, which must be fully understood to build industrial confidence.\u003c/p\u003e\u003cp\u003eAs the Allam cycle\u0026rsquo;s sCO\u003csub\u003e2\u003c/sub\u003e working fluid typically contains impurities such as H\u003csub\u003e2\u003c/sub\u003eO, COS, NH\u003csub\u003e3\u003c/sub\u003e, and HCN, which when combusted, are converted to SO\u003csub\u003e2\u003c/sub\u003e, NO, H\u003csub\u003e2\u003c/sub\u003eO, and N\u003csub\u003e2\u003c/sub\u003e, AM Alloy 625\u0026rsquo;s performance in this test atmosphere is also studied and compared to conventional manufacturing routes.\u003c/p\u003e"},{"header":"2. Materials and Exposure Procedure","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Materials\u003c/h2\u003e\u003cp\u003eAs-received wrought Alloy 625 supplied by Alleima, and LPBF AM manufactured material supplied by the University of Birmingham [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e] manufactured by the Manufacturing Technology Centre, have been compared (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Wrought manufactured (WM) Alloy 625 was machined into cylindrical rods. The dimensions of the cylindrical samples were \u0026empty; 14mm x 10-11mm. The chemical composition of the wrought material is presented in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. WM samples underwent no heat treatment.\u003c/p\u003e\u003cp\u003eThe chemical composition of the powder used for AM, given in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, was determined using a combination of LECO ICP-MS and XRF. The powder has a size range of 15\u0026ndash;45 \u0026micro;m and was manufactured into cylindrical samples using the laser-powder bed fusion process in an EOS M280 machine with proprietary parameters. The cylindrical rods produced were 14 mm in diameter x 125 mm in length. To adequately analyse the AM samples\u0026rsquo; final microstructures, two post-printing procedures were followed. The AM samples were either exposed in an as-built format or underwent solution heat treatment as shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eSample codes\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCode\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eManufacturing Process\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eHeat Treatment\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAs-built (AB)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eLPBF AM\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eNone\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSolution heat treated (SH)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1177\u0026deg;C, 1hr, water quenched\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eWrought manufactured (WM)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eWrought\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eNone\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eChemical composition of alloys, AM powder composition measured using LECO-ICM XP and WM composition obtained from SEM for Alloy 625 samples\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"14\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c12\" colnum=\"12\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c13\" colnum=\"13\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c14\" colnum=\"14\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colspan=\"14\" nameend=\"c14\" namest=\"c1\"\u003e\u003cp\u003eAlloy 625 Elemental composition (%\u003csub\u003ewt\u003c/sub\u003e.)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMn\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSi\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eS\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003eCr\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003eCo\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003eMo\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003eNb\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003eTi\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003eAl\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003eFe\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c14\"\u003e\u003cp\u003eNi\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eAM\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.07\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e21.27\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e9.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e3.65\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003e0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003e0.11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003e3.66\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c14\"\u003e\u003cp\u003eB\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eWM\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.025\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\le\\:\\)\u003c/span\u003e\u003c/span\u003e0.015\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\le\\:\\)\u003c/span\u003e\u003c/span\u003e0.015\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e21.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e8.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c10\"\u003e\u003cp\u003e3.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c11\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c12\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c13\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c14\"\u003e\u003cp\u003e61\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Pre-exposure Treatment of Samples\u003c/h2\u003e\u003cp\u003eThe samples were cleaned with isopropanol and acetone in an ultrasonic bath for 15 mins each to remove any surface contamination. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the samples before exposure. Dimensions of the cylindrical samples were measured using a digital micrometre with a 0.001mm resolution, and mass was recorded for comparison after the oxidation test (with a 0.01 mg resolution). After each exposure cycle\u0026rsquo;s completion, sample masses were taken to create a mass change plot.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3. High Temperature Exposure of Samples\u003c/h2\u003e\u003cp\u003eIsothermal oxidation tests were performed in a vertical controlled atmosphere furnace at 800\u0026deg;C, for up to 1000h in CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2.7mol% H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;1.43mol% N\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;0.17mol% O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;300ppm SO\u003csub\u003e2\u003c/sub\u003e. The gas flow rate was set at 88.78cm/min using a mass flow controller, and automated injection was used to inject deionised water at a rate of 0.003ml/min into the furnace. The test was conducted in 200 hour cycles.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4. Post-exposure Treatment of Samples\u003c/h2\u003e\u003cp\u003eSamples were mounted in a low-shrinkage epoxy resin with ballotini glass spheres in a 60:40 and cross-sectioned in the XY plane perpendicular to the Z-axis of LPBF build or the cylindrical axis for WM. Samples were ground to 1200 grit size using SiC paper and then polished to 1\u0026micro;m using oil-based diamond polishing medium.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.5. Microstructural Analysis\u003c/h2\u003e\u003cp\u003eA VEGA L scanning electron microscope (SEM) was used to capture backscattered electron (BSE) images of cross-sectioned samples and the chemical composition of the oxide scale on alloy was ascertained using elemental dispersive X-ray spectroscopy (EDX). The combination of electron microscopy and EDX techniques was used to provide a comprehensive analysis of the sample\u0026rsquo;s microstructure, and oxide scale composition. Samples were electrolytically etched in 10% oxalic acid at 4V for 4 seconds to reveal grain boundary carbides.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Effect of Manufacturing Technique on the Oxidation Behaviour","content":"\u003cp\u003eParameters including heat treatment and microstructural characteristics, influence the oxidation behaviour of LPBF Alloy 625. To assess the impact of manufacturing solution-treated and as-built LPBF samples were compared with wrought Alloy 625 to evaluate how factors like solution treatment influence phase formation, grain structure, and oxide formation.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the mass change data for Alloy 625. All samples demonstrated less than \u0026lt;\u0026thinsp;1.3g/cm\u003csup\u003e2\u003c/sup\u003e mass gain following 1000h exposure time. The low mass gain indicates Alloy 625 exhibits low oxidation rates in the tested exposure condition. At 1000h, the wrought (WM) solution heat-treated (SH), and as-built (AB) samples show no significant difference in change in mass. Given the overlapping error bars, no significant differences in oxidation behaviour can be concluded between the manufacturing conditions. Noticeably, the mass fluctuations ie. the decreases at 800h points indicate spallation, which was noticeable in the crucible, which should be considered in interpreting the plot [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. The three production variants showed little difference in mass change, notably, solution heat-treated LPBF has shown the highest mass gain, 0.26 mg.cm\u003csup\u003e2\u003c/sup\u003e/hr while the wrought has shown the least mass gain, 0.18 mg.cm\u003csup\u003e2\u003c/sup\u003e/hr.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eK\u003csub\u003ep\u003c/sub\u003e value of as-built, solution heat-treated and wrought manufactured Alloy 625 over 1000h exposure\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTime (h)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eK\u003csub\u003ep\u003c/sub\u003e(mg\u003csup\u003e2\u003c/sup\u003e/cm\u003csup\u003e4\u003c/sup\u003ehr)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eError (mg\u003csup\u003e2\u003c/sup\u003e/cm\u003csup\u003e4\u003c/sup\u003ehr)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e48.4 x 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e8.8 x 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e52.9 x 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e9.2 x 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eWM\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e40 x 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e8 x 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe parabolic oxidation behaviour of as-built LPBF, solution heat-treated LPBF and wrought manufactured Alloy 625 was quantified based on mass change per unit area \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:{m}^{2}={K}_{p}t\\)\u003c/span\u003e\u003c/span\u003e. Where K\u003csub\u003ep\u003c/sub\u003e is the parabolic rate constant, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:m\\)\u003c/span\u003e\u003c/span\u003e is mass change and t is the exposure time.\u003c/p\u003e\u003cp\u003eThe parabolic rate constants (K\u003csub\u003ep\u003c/sub\u003e) exhibited similar oxidation resistance across the variations in manufacturing techniques as seen in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eThe as-built microstructure, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea shows fine grain structure which Emily et al.[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e] EBSD analysis shows this as-built structure was a fine columnar dendrites which extended parallel to the build direction, typically due to rapid solidification. The EBSD also showed very few twin grain boundaries (TGBs). LPBF typically produces components with preferential\u0026thinsp;\u0026lt;\u0026thinsp;001\u0026thinsp;\u0026gt;\u0026thinsp;crystallographic orientation, resulting in columnar grains aligned with the building direction [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. The solution heat-treated sample shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, shows a more homogenized grain structure and EBSD in Emily et al.[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e], shows 30% more TGBs. The wrought sample shows microstructure with fully equiaxed grains, EBSD maps of alloy 625 seen in Chyrkin et al.[\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. As-built AM samples typically demonstrate more pronounced crystallographic texture and grain distribution [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. However, studies like Qi et al.[\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e] postulate that the grain orientation and texture are controlled by the direction of heat flow and temperature gradient, which depend on process parameters.\u003c/p\u003e\u003cp\u003eGrain size data from the BSE images in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e were analyzed using the ASTM E112 intercept method in Image J, and the results summarized in Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. This intercept line was taken in the horizontal direction, and this reveals a significant difference in grain size as a result of processing.\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eASTM E112 intercept method grain size analysis\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSample Name\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAverage Grain Size (\u0026micro;m)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eStandard deviation Error (\u0026micro;m)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAB\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e33.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e9.3\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e26.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e4.9\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eWM\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e13.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.9\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe relationship between grain characteristics and oxidation behaviour was evident in this study. Microstructural analysis of the as-built (AB), solution heat-treated (SH) LPBF and wrought manufactured (WM) Alloy 625 samples revealed notable differences in grain morphology and size, which correlated with oxidation performance.\u003c/p\u003e\u003cp\u003eThe WM samples exhibit the finest average size of 13.7 \u0026micro;m and thus promoting accelerated oxidation as the higher grain boundary density acts as an enhanced diffusion path, which is evident in the BSE images in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. While the AB samples show considerably coarser grains averaging 33.5 \u0026micro;m. The SH samples display an intermediate grain of 26.1 \u0026micro;m due to partial recrystallization. Previous research indicates grain sizes could be important to oxidation behaviour, higher mass gain exhibited in LPBF samples in comparison to wrought samples has been associated with the grain size [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe finer-grained WM samples in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e developed a thicker continuous oxide layer, indicative of good oxidation resistance. The dense grain boundary network enhanced Cr diffusion, enabling the rapid formation of a continuous and adherent Chromia scale. This promotes efficient selective oxidation of chromium while minimising internal oxidation and maintaining stable oxide/metal interfaces. Such behaviour is consistent with previous studies on chromia-forming alloys manufactured via laser beam processes, including Alloy 718 [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e], Alloy 625[\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e] and Hastelloy X[\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e], which also all exhibit parabolic oxidation kinetics. These experimental observations are consistent with previous findings linking grain boundary characteristics to oxidation kinetics in chromia-forming alloys in Sanviemvongsak et al.[\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e], and Ramenatte et al.[\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e] works. These results confirm that the refined microstructure of WM Alloy 625 facilitates a more effective selective oxidation of Cr. The improved oxidation resistance observed in wrought specimens can be attributed to their refined grain structure, which shows through multiple protective mechanisms. The finer grain size of WM (13.7 \u0026micro;m) promotes more efficient chromium diffusion to the surface through shortened diffusion pathways and an enhanced grain boundary network, facilitating the rapid formation of a protective chromia layer.\u003c/p\u003e\u003cp\u003eThe presence of voids and surface ridges in the oxidized as-built LPBF samples shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e indicates the development of stress and non-uniform scale growth. In contrast, the absence of such features in the WM and solution heat-treated LPBF samples supports the more stable oxide/metal interface and improved scale adherence.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe EDX maps as-built sample shows the formation of an almost continuous chromia rich layer, the Al signals have a very low intensity and do not show any significant alumina looking at the O maps as well in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Mo and Nb show enrichment near the oxide/substrate interface which indicates the formation of Nb and Mo rich phase. The oxide layer in the as-built appears less uniform than in the wrought counterpart, displaying localised thickening and areas of spallation. This suggests less efficient chromium transportation and delayed chromia formation. The coarser and columnar grains provide few grain boundary diffusion paths compared with the finer grain wrought microstructure, leading to less efficient chromia diffusion towards the surface. Sanviemvongsak et al.[\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e] identified a microstructural feature in as-built samples: the absence of special grain boundaries with specific crystallographic aligned features that typically enhance oxidation resistance. The absence of such boundaries also combined with higher residual stresses and elementary segregation, leads to discontinuous chromia formation and reduced oxide adhesion. These microstructural features explain the less protective and more irregular oxide scale observed in the as-built Alloy 625 samples compared to the uniform and adherent chromia layer of the wrought.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe solution heat treated sample in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows a more continuous chromia layer. Ni-Cr alloys like Alloy 625 form duplex oxide structure characterized by a Ni rich δ phase near the metal-oxide and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e layer near the oxide-alloy interface. These findings align with Parozia et al.[\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e], who investigated LPBF Alloy 625, comparing solution annealed (2h at 1150\u0026deg;C) and as-built at 900\u0026deg;C up to 100h. Their investigation revealed higher mass gain in heat-treated samples attributed to the formation of NiCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and Nb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e clusters inside the oxide scale.\u003c/p\u003e\u003cp\u003eIn the as-built AM sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e), a duplex oxide structure comprising Ni and Cr oxides is observed at the oxide-alloy interface. In contrast, the solution heat-treated sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e), subjected to oxidation at 800\u0026deg;C for 1000 hours after being heat-treated at 1100\u0026deg;C for 1 hour and water-quenched, exhibits a well-developed and continuous chromia layer. Beneath the chromia layer, an intermetallic Ni and Nb rich phase, expected to be δ-Ni₃Nb phase, was detected in both the as-built and solution heat-treated samples.\u003c/p\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.1. Effect of Heat Treatment on the Oxidation Behaviour.\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e illustrates cross-sectional SEM images of LPBF as-built, solution heat-treated and wrought manufactured Alloy 625 samples after exposure to CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2.7mol% H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;1.43mol% N\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;0.17mol% O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;300ppm SO\u003csub\u003e2\u003c/sub\u003e for 200, 600, and 1000h. Cross-sectional SEM analysis after 200h of exposure demonstrates the initial formation of a chromia, likely Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3,\u003c/sub\u003e protective layer in all conditions, accompanied by Nb-rich δ-phase and Nb carbides, though the protective oxide layer is thin, suggesting the initial stage of oxide formation. The solution heat-treated samples exhibited enhanced precipitate structure visibility even at this early stage. Compared to both LPBF samples, the wrought Alloy 625 has a denser, more continuous oxide layer and a thinner Nb-rich δ-phase below the oxide layer.\u003c/p\u003e\u003cp\u003eAs exposure time increased to 600 hours, both microstructures developed thicker oxide scales while maintaining the presence of possible Nb-rich phase and carbides. The as-built sample uniquely developed characteristic ridge formations, marking a divergence in oxidation behaviour between the as-built and solution heat-treated samples. Furthermore, void formation in the subsurface region is noticed in the as-built microstructure after 600h and 1000h. The void formation can be attributed to vacancy injection or internal oxidation of carbon, a phenomenon consistent with the Kirkendall effect. As proposed by Bricknell et al.[\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e], the outward diffusion of metal cations during oxidation must be balanced by an inward flux of metal vacancies. These vacancies migrate to grain boundaries, where they agglomerate, resulting in the formation of voids and grain boundary deterioration.\u003c/p\u003e\u003cp\u003eThe as-built also formed ridges may be linked to the stress concentration in the sample, which could lead to acceleration of spallation [\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e]. The most substantial differences emerged after 1000 hours of exposure, where solution heat-treated samples demonstrated superior oxidation resistance through the formation of a thick, continuous oxide scale and the presence of internal Al. These samples also developed needle-like δ-phase precipitates throughout the bulk material, while both conditions showed Mo- and Nb-rich precipitate formation along grain boundaries, confirmed through EDX mapping, possibly a Ni\u003csub\u003e3\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003e δ-phase.\u003c/p\u003e\u003cp\u003eThese findings have direct industrial applications in gas turbine manufacturing, chemical processing equipment, and advanced manufacturing protocols. The better resistance in terms of formation of ridge void, lesser spallation, and denser oxide layer of solution heat-treated samples (as seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) suggests that post-processing heat treatments for AM samples could eliminate the formation of ridges and reduction in spallation, which could improve component durability in high-temperature CO\u003csub\u003e2\u003c/sub\u003e-rich environments. Spallation was observed in LPBF materials, with the as-built exhibiting more pronounced spallation after exposure. The absence of Ridge-Void (RV) formations in wrought samples, as noted by Chyrkin et al.[\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e] indicates that RV morphology is not inherently linked to AM processing and appears independent of reaction conditions. This independence from processing method suggests that RV formation is a fundamental material response to environmental conditions rather than a manufacturing artifact when the as-built and solution heat-treated micrographs in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e are compared. The formation of RV on the as-built can act as stress concentration sites, which can influence surface degradation. The increased spallation in as-built samples may be linked to higher residual stresses, or microstructural heterogeneities specific to the LPBF process.\u003c/p\u003e\u003cp\u003eThe absence of RV morphology could benefit component performance through improved mechanical integrity, enhanced oxidation resistance due to more uniform oxide layer formation, and potentially extended component lifetime through reduced degradation mechanisms. Further investigation of the relationship between precipitation kinetics and oxidation behaviour could yield valuable insights for optimizing industrial applications of AM-manufactured Alloy 625 components.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e shows the locations corresponding to the points analysed in Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e for As-built LPBF Alloy 625, where precipitates were observed along grain boundaries to identify the precipitates that formed.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003ePoint analysis of as-built LPBF alloy 625 after 1000h exposure in CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2.7mol% H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;1.43mol% N\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;0.17mol% O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;300ppm SO\u003csub\u003e2\u003c/sub\u003e at 800\u0026deg;C\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePoint(wt%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNi\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCr\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eMo\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eNb\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eFe\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eSi\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003eS\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e53.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e21.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e15.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e4.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e3.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0.3\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e45.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e25.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e19.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e4.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e3.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0.6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e58.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e21.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e7.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e5.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e3.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e58\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e9.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e7.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e3.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe point analysis of precipitates after 1000h oxidation reveals the formation of Nb- rich and Cr- rich carbides. The elevated Nb concentrations at Points 3 (5.4wt%) and 4 (7.8wt%), suggest the formation of niobium carbides (NbC), consistent with the known carbide-forming tendency of Nb in Ni-based superalloys under thermal exposure [65]. The Cr enrichment at point 2 (25.1wt%) suggests the presence of Cr\u003csub\u003e23\u003c/sub\u003eC\u003csub\u003e6\u003c/sub\u003e carbides, which are thermodynamically favoured in high-Cr alloys under oxidizing conditions [66]. The significant Mo levels (up to 19.8% at Point 2) suggest the potential for complex mixed-metal carbides (e.g., M\u003csub\u003e6\u003c/sub\u003eC, where M\u0026thinsp;=\u0026thinsp;Ni, Mo, Cr). Variations in elemental distribution reflect localized phase formation and diffusion induced segregation during oxidation. Additionally, Xuebing et al.[67] observed that M\u003csub\u003e23\u003c/sub\u003eC\u003csub\u003e6\u003c/sub\u003e was not found in the as-cast specimen, suggesting that M\u003csub\u003e23\u003c/sub\u003eC\u003csub\u003e6\u003c/sub\u003e was exclusively formed during the solution heat treatment process. The presence of carbon in the environment during high-temperature oxidation potentially accelerates carbide formation through carbon diffusion into the alloy surface, leading to enhanced precipitation. Notably, despite the presence of sulphur and nitrogen in the testing environment, these elements were not detected during point analysis, suggesting potential limited interaction with the alloy surface or delocalised distribution. These findings have significant implications for industrial applications, particularly in component lifetime prediction in high-temperature environments and the optimization of heat treatment protocols for desired carbide distributions, while also enhancing our understanding of degradation mechanisms in service conditions. The comprehensive understanding of microstructural evolution in additively manufactured superalloys under oxidizing conditions provided by this analysis suggests that further investigation of the carbon environment's role in precipitation behaviour would be valuable for optimizing material performance in industrial applications, particularly in high-temperature and oxidizing environments where carbide stability plays a crucial role in component longevity. The absence of detectable sulphur and nitrogen indicates favourable environmental conditions for oxide scale formation and stability, contributing to the overall oxidation resistance of the alloy.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThis study investigates the oxidation behaviour of LPBF in comparison with wrought Alloy 625, highlighting the influence of manufacturing parameters such as heat treatment, and microstructural characteristics on its high-temperature oxidation resistance. Heat treatment of LPBF, particularly solution heat treatment, influenced oxidation resistance by promoting a microstructure which was demonstrated to support the development of a protective chromium oxide layer. The study demonstrates that solution heat treatment forms adherent chromia layer which is important for high temperature exposures. Careful consideration must be given to managing microstructural defects such as voids and ridges that can compromise long-term stability. These defects highlight spallation noticeable during cyclic oxidation and the need for post-processing optimization to mitigate the impact of these microstructural flaws.\u0026nbsp;The study examines the role of microstructure, and grain boundary in the formation of a protective oxide layer. Wrought Alloy 625, with its fine and uniform grain structure, demonstrated a thicker protective oxide layer, indicating better oxidation resistance compared to the LPBF variants, both as-built and solution heat-treated. Despite this, both LPBF variants also developed Cr oxide layers, indicating baseline suitability for high temperature applications. Notably, the solution heat treated LPBF sample displayed a thicker continuous oxide layer with the absence of ridge like features when compared to as-built. By 1000 h, the wrought alloy had already formed a much thicker oxide layer, suggesting that prolonged exposure may lead to the depletion of protective elements such as Al, Cr. In contrast, the solution-treated LPBF sample is expected to continue developing its oxide layer at a more gradual rate, reducing the risk of significant alloy degradation or alloy strength during extended service. The findings hold particular relevance for Allam cycle power plants operating in supercritical CO\u003csub\u003e2\u003c/sub\u003e environments, where component durability under extreme conditions directly impacts system reliability and efficiency.\u003c/p\u003e\n\u003cp\u003eFuture work should focus on extended cyclic oxidation studies beyond 1000 hours, and investigation of combined high-pressure and temperature effects on oxidation behaviour. Additional research directions include exploring modified alloy compositions and understanding the impact of thermal and pressure cycling on oxide scale adherence. These investigations would further enhance our understanding of material behaviour in supercritical CO\u003csub\u003e2\u003c/sub\u003e environments and contribute to the advancement of Allam cycle technology.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eB.P.N wrote the main manuscriptA.P. assisted in experimental workE.R.L, N.C and S.C supplied materials for experimental workAll authors excluding B.P.N. reviewed and provided feedback on the manuscript\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eFunding Acknowledgement\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe Authors would like to thank EPSRC for funding, also thank Alleima in Germany for the provision of the Alloy 625 wrought samples and the University of Birmingham, United Kingdom, for providing the LPBF samples made by the Manufacturing Technology Centre.\u0026nbsp;\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eData supporting this study are openly available from the CORD data repository at [https://dspace.lib.cranfield.ac.uk/handle/1826/20712](https:/dspace.lib.cranfield.ac.uk/handle/1826/20712)\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eUnited Nations. 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Available at: DOI:\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/S0167-577X(98)00028-7\u003c/span\u003e\u003cspan address=\"10.1016/S0167-577X(98)00028-7\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (Accessed: 16 October 2023)\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":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"high-temperature-corrosion-of-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [High Temperature Corrosion of Materials](https://www.springer.com/journal/11085)","snPcode":"11085","submissionUrl":"https://submission.nature.com/new-submission/11085/3","title":"High Temperature Corrosion of Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"supercritical carbon dioxide, oxidation kinetics, additive manufacturing, wrought manufacturing, Alloy 625","lastPublishedDoi":"10.21203/rs.3.rs-8175421/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8175421/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn pursuit of cleaner and more sustainable power generation, gas turbine power cycles using supercritical carbon dioxide (sCO\u003csub\u003e2\u003c/sub\u003e) as a working fluid have emerged as an option to meet emissions targets. The Allam cycle achieves emission reduction through high-pressure sCO\u003csub\u003e2\u003c/sub\u003e in an oxy-combusted, highly recuperated cycle, operating at 300 bar and up to 800\u0026deg;C. Alloy 625, known for its high corrosion resistance, is used for high-temperature components, including heat exchangers, which can be manufactured traditionally or via additive manufacturing (AM) techniques, such as laser powder bed fusion (LPBF).\u003c/p\u003e\u003cp\u003eThis study investigates Alloy 625\u0026rsquo;s microstructural influence on resistance to simulated Allam cycle conditions; comparing as-built and solution heat-treated LPBF AM samples with conventionally manufactured wrought. Isothermal oxidation tests were conducted at 800\u003csup\u003e\u0026deg;\u003c/sup\u003eC for 1000h in a CO\u003csub\u003e2\u003c/sub\u003e rich environment (CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2.7mol% H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;1.43mol% N\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;0.17mol% O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;300ppm SO\u003csub\u003e2\u003c/sub\u003e). Wrought Alloy 625 demonstrated the lowest oxidation rate, attributed to its homogenized microstructure and fine grain size. Solution heat-treated AM samples exhibited a continuous oxide layer due to grain boundary changes, enhancing oxide scale formation. The K\u003csub\u003ep\u003c/sub\u003e values of as-built, solution heat-treated and wrought Alloy 625 were 4.8 x 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e, 5.3\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e, and 4.0\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e mg\u003csup\u003e2\u003c/sup\u003e/cm\u003csup\u003e4\u003c/sup\u003e respectively.\u003c/p\u003e\u003cp\u003eGrain morphology and heat treatment influenced oxidation. As-built LPBF samples formed ridges on the oxide scale and with subsurface voids, while wrought samples displayed uniform oxide layers without subsurface voids. These findings highlight how manufacturing techniques and post-processing affect Alloy 625\u0026rsquo;s high-temperature performance, crucial for turbine casing and heat exchangers in a baseline Allam cycle environment.\u003c/p\u003e","manuscriptTitle":"Comparison of the Oxidation Behaviour of Wrought and Additively Manufactured Alloy 625 in a High Temperature CO2 Environment","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-02 16:53:39","doi":"10.21203/rs.3.rs-8175421/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-12-17T01:51:13+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-17T01:46:14+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-01T09:13:51+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"55919688396532283027709961536546813905","date":"2025-12-01T08:57:56+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"69003376327156992889214368519990597160","date":"2025-12-01T07:02:09+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-01T06:40:09+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-24T08:24:30+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-24T08:19:09+00:00","index":"","fulltext":""},{"type":"submitted","content":"High Temperature Corrosion of Materials","date":"2025-11-21T16:24:12+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"high-temperature-corrosion-of-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [High Temperature Corrosion of Materials](https://www.springer.com/journal/11085)","snPcode":"11085","submissionUrl":"https://submission.nature.com/new-submission/11085/3","title":"High Temperature Corrosion of Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a7643745-d4c0-4155-88a9-121e6ddcfc88","owner":[],"postedDate":"December 2nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2025-12-17T01:53:40+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-02 16:53:39","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8175421","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8175421","identity":"rs-8175421","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}