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Jhuo-Lun Lee, An-Chou Yeh, [Hideyuki Murakami](https://orcid.org/0000-0001-8220-5816)

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This version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1007/s11085-024-10313-3.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Oxidation Properties of Additively Manufactured High Entropy Alloys: A Short Review](https://mdr.nims.go.jp/datasets/f684720b-069d-4547-89b9-50aa6222ee71)

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Title Goes HereOxidation Properties of Additively Manufactured High Entropy Alloys -A Short Review Jhuo-Lun Lee a, b, c, d, An-Chou Yeh a, b, c*, Hideyuki Murakami d*a High Entropy Materials Center, National Tsing Hua University (NTHU), Hsinchu 30013, Taiwan ROCb Ph.D. Program in Prospective Functional Materials Industry, NTHU, Hsinchu 30013, Taiwan ROCc Department of Materials Science and Engineering, NTHU, Hsinchu, 30013, Taiwan ROC d Research Center for Structural Materials, National Institute for Materials Science, Tsukuba, 305-0047, Japan*Corresponding authors Email address: a0920989910@gmail.com; yehac@mx.nthu.edu.tw; murakami.hideyuki@nims.go.jpAbstract: High entropy alloys (HEAs) challenge conventional alloy design by incorporating five or more principal elements in near-equal atomic proportions, forming random solid solutions with simple phases. HEAs exhibit exceptional properties such as high phase stability, mechanical strength, corrosion, oxidation, wear, fatigue resistance, and notable thermal stability. While traditional methods like arc melting and casting are often used for HEA preparation, they pose limitations due to cost and processing challenges. Additive Manufacturing(AM) has emerged as a transformative technique, enabling the cost-effective fabrication of complex structures with customized properties. Here, we summarized the following “state-of-the-art” additively manufactured alloy systems: AlCrCoNiX (X=Fe, Si, Ti, etc.) HEAs, CoCrFeMnNi HEAs, and refractory HEAs. This review focused on elucidating their oxidation properties, emphasizing key findings, challenges, and opportunities. It also discussed the potential strategies for enhancing oxidation resistance. Additionally, it highlighted research gaps and underscored the urgent need for further exploration to meet the demands for high-temperature applications.Keywords: Oxidation properties, Additive manufacturing, High entropy alloys1. IntroductionHigh entropy alloys (HEAs) redefine traditional alloy design by incorporating five or more principal elements in near-equal atomic proportions[1,2]. HEAs, originating from pioneering research at the University of Taiwan, form random solid solutions during solidification and exhibit simple solid solution phases such as BCC, FCC, or HCP structures[3]. HEAs demonstrate exceptional properties, including high phase stability, remarkable mechanical strength, excellent resistance to corrosion, oxidation, wear, and fatigue, and notable thermal stability[4-12]. Most studies prepare HEAs using traditional processing routes like arc melting and casting. However, these methods are often expensive and time-consuming, hindering their practical applications due to the challenges in achieving desired shapes and chemical homogeneity.In recent years, additive manufacturing (AM) has revolutionized material fabrication by enabling the cost-effective creation of complex structures with excellent forming quality and customized properties[13,14]. Typically, AM can be seen as a layer-by-layer deposition process using a high-energy source with computer-aided design (CAD). Various types of AM methods have been applied to HEAs[15], including selective laser melting (SLM)[16,17], electron-beam-melting (EBM)[18,19], directed energy deposition (DED)[20,21], and other methods. Recently, AM-HEAs have gained increasing interest in the research community due to their exceptional high-temperature properties, making them promising structural materials for high-temperature environments [12,22-24].In high-temperature applications, superior oxidation resistance is crucial to prevent surface degradation that could compromise mechanical performance. AM-HEAs possess distinctive features, such as refined dendritic structures and metallurgical imperfections, which may induce different oxidation kinetics than traditional processes[25]. Also, prior studies have indicated that HEAs exhibit a favorable combination of specific strength and oxidation resistance compared to conventional alloys[26]. Furthermore, it has been reported that AM-HEAs exhibit slower oxidation kinetics than those produced using traditional methods. The parabolic rate constant of AlCrCoNiFe series alloys in AM specimens (ground) exposed to 1100°C for 200 hours was lower than that of arc-melted specimens (ground) exposed to 1050°C for 100 hours[27,28]. However, previous research has primarily focused on the microstructures and high-temperature mechanical properties of AM-HEAs. Therefore, it is crucial to comprehend the oxidation resistance of AM-HEAs fully. Hence, there is a compelling need for a comprehensive review explicitly focusing on the oxidation properties of AM-HEAs. This review critically surveys the existing literature and highlights key findings, challenges, and opportunities in the oxidation properties of AM-HEAs. We will discuss crucial aspects such as types of thermally grown oxide, phase evolution, oxidation kinetics, and mechanisms under oxidative conditions. Additionally, we will explore potential strategies for enhancing the oxidation resistance of AM-HEAs, encompassing alloy design, process optimization, and surface modification. By consolidating the current state-of-the-art knowledge, this review seeks to provide valuable insights for materials science and engineering researchers, fostering the development of robust, oxidation-resistant HEA-based materials for advanced engineering applications and contributing to innovation and sustainability across various industrial sectors. In the following text, alloys composed of element A in x at% and element B in y at% will be denoted as AxBy. ​If no numerical values are specified, the alloys are assumed to be in an equimolar ratio.2. AlCrCoNiX (X=Fe, Si, Ti, etc.) HEAsAccording to traditional oxidation theory, the ability of an alloy to resist oxidation largely relies on the ability of protective oxides like Al2O3 and Cr2O3 to form[29]. As a result, many studies have focused on incorporating Al and Cr into the AM-HEAs systems to investigate whether this enhances their resistance to oxidation at high temperatures.The first attempts were made by Mohanty et al.[27]. Two AlCoCrFeNi HEAs with varying Al content, which are Al8Co23Cr23Fe23Ni23 and Al14Co21.5Cr21.5Fe21.5Ni21.5, prepared by direct laser deposition(DLD), were investigated. Specimens were ground till 2000 grade emery paper. Cyclic oxidation tests were conducted at 1100 °C for 200 hours. After an early transition stage, quasi-parabolic oxide growth was observed in Al8Co23Cr23Fe23Ni23 and Al14Co21.5Cr21.5Fe21.5Ni21.5 HEAs. A more significant mass gain was observed for Al8Co23Cr23Fe23Ni23 compared to Al14Co21.5Cr21.5Fe21.5Ni21.5  throughout the testing duration. The oxide scale comprises an external Cr2O3 scale and an internal  Al2O3 subscale for all oxidized samples. Al content significantly influenced the thickness and uniformity of the oxide scale(Fig. 1[27]). The higher aluminum content in Al14Co21.5Cr21.5Fe21.5Ni21.5 promoted a continuous Al2O3 oxide layer, offering better protection against oxidation than the Al8Co23Cr23Fe23Ni23 alloy. Fig. 1. Cross-sectional SEM micrographs of the oxide scale morphology developed on (a-c) Al8Co23Cr23Fe23Ni23 and (d-e) Al14Co21.5Cr21.5Fe21.5Ni21.5 at 1100 ºC after (a, d) 20 h, (b, c) 100 h, and (c, f) 200 h. Modified after ref.[27].Kuzminova et al.[30] studied the effect of Al content in the (CrFeCoNi)100-xAlx system on the oxidation resistance. Three alloys, namely CoCrFeNi, Al2.4Co24.4Cr24.4Fe24.4Ni24.4, and Al10Co22.5Cr22.5Fe22.5Ni22.5, were prepared by powder bed fusion(PBF) from Al and CrFeCoNi powder blends. Isothermal oxidation tests were conducted at 800 and 1000 ℃ for up to 500 hours. The resulting oxide scale mainly comprises MnO, Cr2O3, and Al2O3. In addition, AlN was found beneath the Al2O3 layer in Al10Co22.5Cr22.5Fe22.5Ni22.5 at 1000 ℃. It was found that without Al addition, the oxide scale delamination is severe. The addition of Al in the CrFeCoNi system could promote the formation of the Al2O3 layer predominantly along the columnar grain boundaries and improve the oxidation properties. The issue of powder impurity is also reported herein. On the one hand, the original CrFeCoNi powder contains ~1 wt.% of Mn, leading to unprotective Mn-rich oxides. On the other hand, the formation of nitrides was attributed to the retained nitrogen in the as-built sample conveyed from the powder.Lee et al.[31] investigated the oxidation kinetics of novel Al2O3-forming L12-strengthened high entropy alloys, comparing them with the traditional 3D-printable Ni-based superalloy IN718. Pre-alloyed gas-atomized powders with nominal compositions of Al8Co35Cr18Ni34Ti3Nb2 (HEA4) and Al9Co25Cr15Ni43.7Ti3Nb2Mo1.5W0.8 (HEAX) are prepared. Alloys were fabricated by SLM from pre-alloyed powders. Specimens were ground till #1200 SiC abrasive paper. Isothermal oxidation tests were conducted at 900 °C to 1100 °C for 24 hours, and a cyclic test at 1100 °C. The resulting isothermal weight gain behavior and parabolic constant highlighted the superior oxidation resistance of HEAs compared to IN718, attributed to the formation of a protective α-Al2O3 layer in HEAs (as shown in Fig. 2[31]). Also, it was suggested that the superior isothermal weight gain behavior of HEAX could be related to higher Al content. Higher Al content increases Al activity, facilitating alumina formation to inhibit oxygen ingress. Regarding the cyclic oxidation test, the stability of protective oxide is critical.  IN718 shows significant weight loss, suggesting oxide spallation. By contrast, HEAX shows limited weight change. Previous studies have found that AM IN718 demonstrated severe spallation attributed to the segregation of impurity elements, such as sulfur[32,33]. Despite the detailed quantification of the powder composition not being reported in this research, future works are encouraged to investigate the effects of impurity elements on the cyclic oxidation behavior of AM-HEAs. Fig. 2. Isothermal oxidation kinetics of HEAs and IN718 at 1100 ℃ and corresponding cross-sectional SEM micrographs of the oxide scale morphology after testing[31].The above studies show that increasing Al content can improve oxidation resistance. However, Yan et al.[34] investigated two types of AlCrCoNiSi-based HEAs, namely Al10Cr17Co34Ni34Si5 (Al10) and Al15Cr16Co32Ni32Si5 (Al15), prepared by laser powder-based directed energy deposition. The deposited microstructures of both alloys consist of Al-depleted FCC and Al-rich BCC phases. The surface oxidation behaviors of these two alloys were studied at 1100 ℃ for 100h. Specimens were ground and polished. The results show that the oxidation kinetics of Al10 and Al15 alloys follow linear rate law at early-stage oxidation and parabolic rate law at prolonged oxidation times. The linear and parabolic oxidation rate constants of Al10 alloy are lower than that of Al15, which indicates a better oxidation resistance in alloy with lower Al content. The oxide scales for Al10 and Al15 alloys comprise mainly α-Al2O3, with a small fraction of (Ni, Co)Al2O4 spinel and θ-Al2O3. Moreover, a newly formed FCC layer was found beneath the oxide scale in the Al-depletion zone. The author attributed the formation of this FCC layer to the phase transformation of BCC, which occurs due to the preferential oxidation of Al, leading to a reduction in Al content in the BCC phases. The newly formed FCC layer is almost twice as thick in Al10 as in Al15 after 100h of exposure. It is concluded that the thicker newly-formed FCC layer in the Al10 alloy effectively hinders the outward diffusion of Al, resulting in lower oxidation kinetics, as shown in Fig. 3[34].Fig. 3. Schematic diagram showing the formation of oxide scales on Al10 and Al15 alloys[34].Surface modification by surface plastic deformation treatments, such as laser shock peening (LSP), can significantly improve the oxidation resistance of conventional alloys prepared by AM (for example,  Ti6Al4V alloy[35] and IN625 composites[36]). Considering this, Tong et al.[37] studied the high-temperature oxidation behaviors of SLM-fabricated AlCoCrCuFeNi HEA with and without LSP. Isothermal oxidation tests were conducted at 700, 900, and 1100 ℃ for 100 hours. Specimens were ground and polished to a 3 μm diamond suspension. The weight gain curves indicate that all samples initially exhibited linear growth, transitioning to parabolic growth as the testing time increased. This linear phase corresponds to the transient oxidation stage of the reaction control process. Additionally, samples treated with LSP consistently showed lower weight gain and a constant reduced oxidation rate across all tests and temperatures. In contrast, untreated specimens displayed rough spinel oxide scales, which detached from the matrix and experienced significant spallation after prolonged exposure. It was concluded that the combined effects of grain refinement and compressive residual stress contributed to the enhanced oxidation resistance observed in the LSP-treated samples. The detailed oxidation mechanism is illustrated in Fig. 4[37].Fig. 4. Schematic diagrams of the high-temperature oxidation process: (a) without LSP and (b) with LSP. Modified after ref.[37]. 3. CoCrFeMnNi HEAsAnother alloy system, CoCrFeMnNi HEAs, has garnered significant attention since its discovery in 2004[38]. Various AM methods have been employed for processing CoCrFeMnNi HEAs[18,39,40], although the emphasis has not been on oxidation properties. Previous studies have been published investigating the oxidation mechanism of CoCrFeMnNi HEAs through traditional processes [41,42]. However, variations in oxidation mechanisms have been observed across different fabrication processes[25].Tong et al.[25] studied CoCrFeMnNi HEAs utilizing laser additive manufacturing (LAM) and conducted high-temperature isothermal oxidation tests at 800 to 1000 °C for 100 hours. Pre-alloyed powders with chemical compositions such as Co20.5Cr19.8Fe19.4Mn20.1Ni20.1O0.1 are used to fabricate the bulk material. Specimens were ground and polished to a 0.25 μm diamond suspension. The oxidation kinetics of the LAM-fabricated specimens adhered to the parabolic rate law. Different testing temperatures induce notable changes in oxide scales. At 800 °C, the oxide scale comprises Mn2O3 and Cr2O3, whereas Mn3O4 and (Mn, Cr)3O4 at 900 and 1000 °C. It was found that the Mn-rich and Cr-rich oxides formed due to the rapid outward diffusion of Mn and Cr along the dendrite grain boundaries reacting with inward diffusing oxygen. With the experiment results, the author further elaborates on the oxidation kinetics of LAM-fabricated CoCrFeMnNi HEAs. Also, the metallurgical defects and thermal stress within LAM-fabricated specimens were found to be detrimental factors affecting high-temperature oxidation properties. The defects could act as rapid diffusion channels and stress concentration points to promote the spallation of oxides. These observations demonstrate the influence of sample specificity of the AM route could indeed affect the oxidation properties of materials.Similarly, Jia et al.[43] studied the oxidation behavior of the CoCrFeMnNi HEAs prepared by SLM with and without annealing. Annealing was set as 900 °C heat treatment for 4 hours. Isothermal oxidation tests were conducted for 8 hours at 800, 900, and 1000 °C. Specimens were ground and polished to a 0.5 μm diamond suspension. The parabolic law could describe the oxidation kinetics for samples with and without annealing throughout the testing temperature. The oxide products were in accordance with previous studies, i.e., Cr-oxides and Mn-oxides. A notable finding was that the annealing process significantly enhanced oxidation resistance by impeding preferential oxidation and reducing oxide spallation risk. This was attributed to removing melt pool boundaries and residual stress caused by the AM process after annealing.4. Refractory HEAsRefractory HEAs(RHEAs), introduced by Senkov in 2010, are metallic materials crafted for extreme heat environments.[44] Composed of Cr, Mo, Ta, Hf, Nb, V, W, and additional elements in equimolar or near-equimolar ratios, RHEAs exhibit exceptional strength at high temperatures[45-48]. However, the high melting points of different elements also pose challenges to preparing RHEAs using traditional methods, especially complex shaping. Therefore, many efforts have been made to fabricate RHEAs by AM[49-53]. On the other hand, despite their promise for advanced structural applications in gas turbines, RHEAs suffer from poor oxidation resistance, mainly due to their inability to form a protective oxide layer at high temperatures[54,55].With a particular focus on the formability of a protective oxide layer at high temperatures, Zhou et al.[56] investigated the oxidation resistance of a series of AlCrMoTaTi RHEAs manufactured by laser melting deposition(LMD). Three alloys, namely Al5.6Cr30.4Mo18.2Ta19.7Ti26.1 (Al5), Al10Cr31.3Mo22.5Ta19.5Ti16.7 (Al10), and Al15.4Cr27.5Mo19.2Ta20.7Ti17.2 (Al15) were studied. The as-prepared RHEA primarily comprises two BCC phases, namely BCC1  and BCC2. The highest (Cr+Al) concentrations were found in the phase boundary, followed by the BCC2 phase and BCC1 phase. The BCC2 phase volume fraction and the phase boundary area are increased with increasing Al content. Oxidation kinetics were evaluated by isothermal oxidation at 1300 °C for 20 hours. Al15 shows superior oxidation performance with the lowest weight gain after testing. Further investigation reveals that the oxide scale of Al5 and Al10 comprises a mixture of TiO2, Ta2O5, Cr2O3, and α-Al2O3. Interestingly, Al15 can form a pure external α-Al2O3 scale. The above results show that Al content plays a significant role in the oxidation process of the AlCrMoTaTi RHEA. This work also highlights that AM-RHEAs can be a potential candidate for achieving excellent high-temperature oxidation resistance.Table 1. High-temperature oxidation properties of additively manufactured HEAs Alloy Processing Temperature (ºC) Weight gain(mg/cm2) Kp(g2 /cm4s) Ref.    8h 20h 100h   Al8Co23Cr23Fe23Ni23  DLD 1100 - 0.17 0.82 1.7×10-12 [27] Al14Co21.5Cr21.5Fe21.5Ni21.5  1100 - 0.03 0.44 3.8×10-13  CoCrFeNi PBF 800 - 0.09 0.13 - [30]   1000 - 0.38 -0.17 -  Al2.4Co24.4Cr24.4Fe24.4Ni24.4  800 - 0.15 0.18 1.5×10-13    1000 - 1.00 1.76 6.8×10-12  Al10Co22.5Cr22.5Fe22.5Ni22.5  800 - 0.04 0.15 1.4×10-13    1000 - 1.46 2.77 2.6×10-11  Al8Co35Cr18Ni34Ti3Nb2  SLM 900 0.11 0.19 - 5.710-13 [31]   1000 0.42 0.60 - 4.410-12    1100 1.25 1.98 - 5.310-11  Al9Co25Cr15Ni43.7Ti3Nb2Mo1.5W0.8  900 0.09 0.13 - 2.710-13    1000 0.35 0.50 - 2.910-12    1100 0.78 1.19 - 1.810-11  AlCoCrCuFeNi SLM 700 - 0.13 0.29 2.5×10-13 [37]   900 - 0.21 0.38 3.8×10-13    1100 - 0.27 0.50 6.1×10-13   SLM+LSP 700 - 0.08 0.22 1.3×10-13    900 - 0.11 0.25 1.5×10-13    1100 - 0.14 0.31 2.7×10-13  CoCrFeMnNi LAM 800 - 1.2 3.2 2.9×10-11 [25]   900 - 1.9 4.9 7.0×10-11    1000 - 4.0 8.7 2.1×10-10  CoCrFeMnNi SLM 800 0.67 - - 1.6×10-11 [43]   900 1.02 - - 3.7×10-11    1000 4.01 - - 5.7×10-10   SLM+annealing 800 0.62 - - 1.3×10-11    900 0.91 - - 2.9×10-11    1000 3.02 - - 3.2×10-10  Al5.6Cr30.4Mo18.2Ta19.7Ti26.1    3.72 9.22 - 3.0×10-9 [56] Al10Cr31.3Mo22.5Ta19.5Ti16.7  LMD 1300 3.04 5.12 - 4.1×10-10  Al15.4Cr27.5Mo19.2Ta20.7Ti17.2    0.91 1.12 - 1.3×10-11 5. Conclusions and future worksTable 1 presents a comprehensive overview of the high-temperature oxidation characteristics of AM-HEAs, detailing weight gain and parabolic constants sourced from various authors. Analysis of Table 1 reveals several key insights. Firstly, AlCrCoNiX (X=Fe, Si, Ti, etc.) HEAs exhibit superior oxidation resistance compared to CoCrFeMnNi HEAs, with the most notable weight gain performance observed in AlCoCrCuFeNi HEAs post-treated with the LSP process. This underscores the beneficial impact of aluminum additions on oxidation resistance, mirroring trends seen in traditional alloys. Furthermore, the potential efficacy of stress alleviation on sample surfaces in further enhancing AM-HEAs oxidation resistance is expected, as emphasized by multiple studies[25,37,43]. Secondly, an evident research gap exists regarding the oxidation properties of AM-RHEAs, with only a single publication addressing this aspect thus far. Numerous possible future works are discussed as follows. Firstly, it is pertinent to note that current oxidation investigations of AM-HEAs primarily rely on traditional theoretical frameworks applied to empirical data. Given the multi-element composition of HEAs and the complex microstructures induced by AM, an urgent need arises for a comprehensive model capable of accurately predicting oxide formation and oxidation kinetics. Secondly, the effect of impurity elements in the alloy powders should be carefully investigated since they could be detrimental to oxidation performance[30,32]. Thirdly, all studies reviewed herein used ground or polished specimens for oxidation tests (not specified in ref. [30,56]). However, previous studies suggest that the surface conditions of AM specimens also affect oxidation properties[32,57,58]. Therefore, it is crucial to understand how the unique microstructural characteristics, such as surface roughness, grain texture, and residual stress induced by AM, impact the oxidation properties of HEAs. The above-mentioned issues are worth concerted efforts from researchers to delve deeper into this area in forthcoming studies. The oxidation behavior of AM-HEAs stands as a crucial research domain poised to address application requirements. A pressing demand exists for materials exhibiting high-temperature stability, particularly in terms of oxidation resistance. Consequently, further research and development efforts are warranted to comprehensively explore the oxidation behavior of AM-HEAs, with the ultimate aim of meeting these critical application demands.AcknowledgmentsThis work was financially supported by the “High Entropy Materials Center” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. This work was also supported by the National Science and Technology Council (NSTC) in Taiwan under Grant NSTC 112-2927-I-007-504, NSTC 110-2221-E-007-020-MY3, NSTC 112-2224-E-007-003. The authors would like to thank Drs. Takanobu Hiroto and Akira Ishida, both NIMS, for their academic support, and Jhuo-Lun Lee would like to thank NIMS for the provision of the international collaborative graduate program (ICGP) scholarship.Declaration of Conflict of InterestThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.References1. Yen H, Yeh A-C, Yeh J-W. High-entropy alloys: An overview on the fundamentals, development, and future perspective. 2023.2. Yeh A-C, Gorsse S, Keppens V, et al. Design and development of high entropy materials. APL Materials. 2023;11(3).3. Yeh JW, Chen SK, Lin SJ, et al. Nanostructured high‐entropy alloys with multiple principal elements: novel alloy design concepts and outcomes. Advanced engineering materials. 2004;6(5):299-303.4. Gorsse S, Chen Y-T, Hsu W-C, et al. 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