Influencing factors and mechanism of high-temperature oxidation of high-entropy alloys: A review

1.   Introduction

High-temperature oxidation refers to metal corrosion in which metal materials react with oxygen to form oxides at high temperatures. In the process of the high-temperature oxidation of metal, a complete and dense oxide film is formed that can have a protective effect on the metal. Superalloys have a wide range of applications, and the more common are Ni-based [1–3], which are mainly used in aerospace propulsion and aircraft engines. Cr₂O₃ and Al₂O₃ are common protective films for high-temperature oxidation processes. Studies have shown that Ni-based superalloys with Cr content greater than 30wt% have a protective effect [4]. Although nickel-based superalloys meet the requirements for use under high-temperature conditions [5–7], they are expensive and their use temperature limit is typically 1000°C. Price should not be ignored while pursuing the material performance. For Ni-based superalloys, the content of nickel is usually greater than 50wt%. Some high-entropy alloys, such as FeCoNiCrAl, perform well under high-temperature oxidation but are less expensive. Among pure metals with the same particle size and purity, the unit price of Fe, Co, Cr, and Al are all less expensive than Ni; therefore, high-entropy alloys have certain cost advantages and have emerged as alternatives.

High-entropy alloys have evolved from multi-principal alloy elements, which first appeared in Cantor’s undergraduate thesis [8], and high-entropy alloys were first introduced by Yeh et al. in 2004 [9]. High-entropy alloys generally comprise five or more alloy elements, and the content of each element is between 5at% and 35at%. The ideal mixed entropy of a high-entropy alloy is ${\Delta S}_{\mathrm{m}\mathrm{i}\mathrm{x}}\ge 1.5R$ (R = 8.314 $\mathrm{J} \cdot {\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$) [10]. A high-entropy alloy has four characteristics [11]: high-entropy effect in thermodynamics [10,12], the lattice distortion effect in the structure [13], the hysteresis diffusion effect in dynamics [14], and the cocktail effect in the properties [15–17]. The high mixing entropy of high-entropy alloys tends to form a solid solution; however, it is not easy to form an intermediate phase, which is important for the excellent properties of high-entropy alloys. High-entropy alloys have excellent thermal stability [18–22], radiation resistance [23–26], corrosion resistance [27–35], and mechanical properties [36–42]. High-entropy alloys are expected to be used in turbine blades and high-speed cutting tools, and lightweight high-entropy alloys [43] may be used in energy and transportation fields.

Current research on the oxidation of high-entropy alloys is primarily divided into two categories, FeCoNiCr-based and refractory. FeCoNiCr-based studies mainly evaluate the effects of alloy elements, such as Al and Cr, on oxidation. Most of the early studies on high-entropy alloys are based on equal molar ratio; however, most researchers now use different element ratios to study the oxidation properties of high-entropy alloys, which can fully explore the specific role of alloy elements in the oxidation process and provide guidance for optimizing their composition. For refractory high-entropy alloys, the role of Mo, Ta, and other elements in the oxidation process is typically studied. The oxidation resistance of refractory metals is poor; however, with the deepening understanding of the high-temperature oxidation process, some high-entropy alloys with good oxidation properties have been identified.

This paper provides a specific discussion on recent research on the influence of high-entropy alloy high-temperature oxidation from the aspects of the service environment preparation process, phase structure, and alloy elements. By analyzing the influencing factors of high-entropy alloys, several mechanisms of oxidation of high-entropy alloys were obtained, and the future development of high-entropy alloys was proposed.

2.   Main factors of high-temperature oxidation

2.1   Influence of the service environment on the high-temperature oxidation of a high-entropy alloy

The FeCoNiCrMn high-entropy alloy was oxidized in different gas mixtures at 700 and 950°C. The higher the partial pressure of the CO₂/CO gas mixture corresponds to the faster the oxidation rate at the same temperature. When the partial pressure of the CO₂/CO gas mixture was the same, the higher the temperature was, the faster the oxidation rate was [44]. The oxidation kinetics curve of the Ni₂FeCoCrAl_0.5 high-entropy alloy is shown in Fig. 1 [45]. When the oxygen partial pressure was small, the oxidation rate increased with an increase in the oxygen partial pressure, and the oxidation weight kept increasing. A single α-Al₂O₃ layer was formed on the surface of the alloy, and its thickness increased as the oxygen partial pressure increased. When the oxygen partial pressure was 1.0 × 10⁵ Pa, the oxidation weight gain increased at first and then decreased, which may be caused by the exfoliation of the oxide layer. At the oxygen partial pressure ${P}_{{\mathrm{O}}_{2}}$ = 1.0 × 10⁵ Pa, the heterogeneous ratio of α-Al₂O₃, Cr₂O₃, and M–Cr spinel (M = Fe, Co, or Ni) was observed.

Fig. 1.  Oxidation kinetic curves of Ni₂FeCoCrAl_0.5 high-entropy alloys at 900°C with different oxygen pressures. Reprinted from Corros. Sci., 158, W. Kai, F.P. Cheng, F.C. Chien, Y.R. Lin, D. Chen, J.J. Kai, C.T. Liu, and C.J. Wang, The oxidation behavior of a Ni₂FeCoCrAl_0.5 high-entropy superalloy in O₂-containing environments, 108093, Copyright 2019, with permission from Elsevier.

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The oxidation of HEA-1 (Al_7.9Cr_23.2Fe_34.1Ni_34.8), HEA-2 (Al_8.9Cr_23.1Fe_33.7Ni_34.3), HEA-3 (Al_8.2Cr_21.4Fe_30.3Ni₃₅Nb_5.1), and HEA-4 (Al_7.9Cr₂₂Fe_31.9Ni_33.2Ti₅) in a steam environment at 1200°C mainly came from the decomposition of water vapor [46]. The mass increases after oxidation are shown in Table 1, and the oxidation weight gain of HEA-4 was the greatest, indicating that the antioxidant performance of HEA-4 was the worst. Although HEA-4 contained Al, there was no Al₂O₃ protective film formed during the oxidation process, so the oxidation performance of HEA-1 to HEA-3 was better than that of HEA-4. After oxidation, HEA-4 formed a multi-layer oxide film, and internal oxidation occurred, resulting in a decrease in antioxidant performance [46].

Table  1.  Mass gains of high-entropy alloys at 1200°C. Adapted from Corros. Sci., 170, H. Shi, C. Tang, A. Jianu, R. Fetzer, A. Weisenburger, M. Steinbrueck, M. Grosse, R. Stieglitz, and G. Müller, Oxidation behavior and microstructure evolution of alumina-forming austenitic & high-entropy alloys in steam environment at 1200°C, 108654, Copyright 2020, with permission from Elsevier mg/cm²

HEA-1	HEA-2	HEA-3	HEA-4
0.39	0.24	0.72	2.66

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Generally, the higher the oxidation temperature, the faster the oxidation rate of the alloy. A higher temperature corresponds to a greater slope of the oxidation parabola, indicating that the oxidation rate is faster [47]. The kinetic curve of the oxidation behavior of the Ni₂FeCoCrAl_x high-entropy alloys in dry air at 600–900°C is similar to a previous report [48]. However, there are also special cases, such as CoCrFeMnNb_xNi (x = 0 and 0.25, recorded as Nb₀ and Nb_0.25, respectively) [49]. At low temperatures, the antioxidant capacity of Nb_0.25 is better than that of Nb₀; however, its antioxidant capacity is worse than that of Nb₀ at 900°C. The reason for this phenomenon illuminates the effect of the Nb element on high-temperature oxidation.

2.2   Effect of the phase structure on high-temperature oxidation

The high mixing entropy of high-entropy alloys contributes to the formation of solid solutions and inhibits the formation of intermetallic compounds. Yang and Zhang [50] summarized the law of phase formation by proposing a new parameter $\varOmega$ and the mean square deviation of atomic radius difference $\delta$: $\varOmega ={T}_{\mathrm{m}}{\Delta S}_{\mathrm{m}\mathrm{i}\mathrm{x}}/\left|{\Delta H}_{\mathrm{m}\mathrm{i}\mathrm{x}}\right|$, where ${T}_{\mathrm{m}}$ is the melting temperature of an alloy element, ${\Delta S}_{\mathrm{m}\mathrm{i}\mathrm{x}}$ is the mixed entropy, and ${\Delta H}_{\mathrm{m}\mathrm{i}\mathrm{x}}$ is the mixing enthalpy; $\delta =\sqrt{\sum\nolimits_{i=1}^{n}{c}_{i}{\Bigg(1-\dfrac{{r}_{i}}{\bar{r}}\Bigg)}^{2}}$, where ${c}_{i}$ is the atomic percentage of the ith element, ${r}_{i}$ is the atomic radius of the ith element, and $\bar{r}$ is the average radius of an element atom. When $\varOmega \ge 1.1\;\mathrm{a}\mathrm{n}\mathrm{d}\;\delta \le 6.6$, the stable solid solution phase of the high-entropy alloy is easily formed, and the formation of a single solid solution plays an important role in the oxidation of a high-entropy alloy.

Through the calculation of 20Mo–20W–20Al–20Cr–20Ti in the range of 200–2000°C, only the body-centered cubic (bcc) phase can exist stably at high temperature [51], as shown in Fig. 2(a). Pacheco et al. [52] found a similar rule in the study of HfNbTiVZr, as shown in Fig. 2(b), which provides guidance for the selection of materials for the study of oxidation of high-entropy alloys. A stable phase of bcc or materials converted to bcc at high temperatures should be chosen for oxidation experiments. With the addition of Al, the microstructure of the alloy changes from a face-centered cubic (fcc) phase to a mix of fcc phase and bcc phase. When the Al content continues to increase, the bcc structure of high-entropy alloy will continue to increase [53–57]. Uporov et al. [58] also determined that the bcc phase increases with the anneal time of the AlCoCrFeNiMn high-entropy alloy.

Fig. 2.  (a) Equilibrium phase distribution of 20Mo–20W–20Al–20Cr–20Ti at different temperatures; (b) equilibrium phase distribution of HfNbTiVZr at different temperatures (hcp—Hexagonal closed packed; HT—High temperature; LT—Low temperature). (a) Reprinted from J. Alloys Compd., 624, B. Gorr, M. Azim, H.J. Christ, T. Mueller, D. Schliephake, and M. Heilmaier, Phase equilibria, microstructure, and high-temperature oxidation resistance of novel refractory high-entropy alloys, 270-278, Copyright 2015, with permission from Elsevier; (b) reprinted with permission from V. Pacheco, G. Lindwall, D. Karlsson, J. Cedervall, S. Fritze, G. Ek, P. Berastegui, M. Sahlberg, and U. Jansson, Inorg Chem., 58, 811-820, Copyright 2019 American Chemical Society.

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Al_0.7CoCrFeNi alloy with Y/Hf has excellent high-temperature oxidation resistance. The addition of elements can inhibit defects and reduce segregation. Fewer interface defects can reduce the oxidation rate during oxidation, thereby improving oxidation resistance [59]. An extremely low oxidation rate and strong interfacial bonding make the Y/Hf-doped high-entropy alloy a promising high-temperature oxidation resistance material [60].

2.3   Effect of alloy elements on high-temperature oxidation

2.3.1   Effect of FeCoNiCr-based alloy elements on high-temperature oxidation

(1) Effect of the Al element on high-temperature oxidation.

The isothermal oxidation curve of the Al_xCr_0.4CuFe_0.4MnNi high-entropy alloy at 800°C consists of two stages [61]. The first part follows the oxidation parabola law: the oxidation is mainly controlled by solid state diffusion, and the second part is the interface control process controlled by linear oxidation.

Experiments were conducted on the oxidation behavior of the Al_xCr_0.4CuFe_0.4MnNi high-entropy alloy, and scanning electronic microscopy (SEM) images of the cross section of Cr_0.4CuFe_0.4MnNi alloy without Al were obtained, as shown in Fig. 3(a). The components of the corresponding points are shown in Table 2. The thickness of the oxide film is approximately 20 μm. Mn is oxidized preferentially because of less Cr, and the oxide film formed is not protective. The content of Cr in the alloy is limited (9.3at%). Hence, the protective layer of the alloy system is thin and the oxidation rate is high. The outermost oxide film is composed of a large number of Cu, Ni, and Mn, and some of the oxides are easy to peel off. Fig. 3(b) shows an SEM cross-sectional image of an Al_0.3Cr_0.4CuFe_0.4MnNi alloy containing Al for comparison, and the components of the corresponding points are shown in Table 2 [61]. The oxide scale is only approximately 7 μm, indicating that the antioxidant capacity has been greatly improved with the addition of Al. Energy dispersive spectrometer (EDS) analysis shows that the innermost layer of the oxide film is a thin and dense protective layer of Cr and Al oxides; the rest is mainly rich manganese oxides. The contents of Ni and Cu in the outermost oxide film are very low, and the spalling phenomenon is still observed. The typical microstructure shows that manganese oxide has a negative effect on oxidation resistance because manganese oxide is unprotected and easy to peel off. The oxidation resistance of the high-entropy alloy is improved with the addition of Al because of the formation of Al₂O₃ [62–63].

Fig. 3.  SEM micrographs of (a) Cr_0.4CuFe_0.4MnNi and (b) Al_0.3Cr_0.4CuFe_0.4MnNi alloys. Z.Y. Rao, X. Wang, Q.J. Wang, T. Liu, X.H. Chen, L. Wang, and X.D. Hui, Adv. Eng. Mater., 19, 1600726 (2017) [61]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

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Table  2.  Components for the corresponding points in Fig. 3. Z.Y. Rao, X. Wang, Q.J. Wang, T. Liu, X.H. Chen, L. Wang, and X.D. Hui, Adv. Eng. Mater., 19, 1600726 (2017) [61]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission at%

Point	Fe	Cr	Ni	Mn	Cu	Al	O
A (Cr + Mn OL)	0.3 ± 0.1	3.4 ± 0.2	0.2 ± 0.1	42.8 ± 0.3	0.2 ± 0.1	0	53.4 ± 0.3
B (Mn OL)	0.2 ± 0.1	0.2 ± 0.1	0.2 ± 0.1	51.1 ± 0.2	0.2 ± 0.1	0	48.1 ± 0.3
C (Mn + Cu + Ni OL)	0.2 ± 0.1	0.2 ± 0.1	1.9 ± 0.2	45.8 ± 0.4	15.3 ± 0.2	0	37.0 ± 0.4
D (Cr + Al + Mn OL)	0.2 ± 0.1	1.2 ± 0.1	0.2 ± 0.1	47.1 ± 0.1	0.2 ± 0.1	6.1 ± 0.1	44.3 ± 0.3
E (Mn OL)	0.1 ± 0.1	0.3 ± 0.1	0.2 ± 0.1	45.3 ± 0.3	0.2 ± 0.1	0.3 ± 0.1	53.7 ± 0.4
F (Mn + Cu + Ni OL)	1.6 ± 0.1	0.1 ± 0.1	0.8 ± 0.1	54.8 ± 0.3	1.4 ± 0.1	0.3 ± 0.1	41.7 ± 0.3
Note: OL—Oxide layer.

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The valence electron concentration (VEC) has a great influence on the crystal structure:

where ${c}_{i}$ and ${\left(\mathrm{V}\mathrm{E}\mathrm{C}\right)}_{i}$ are the atomic percentage and VEC of a single element, respectively. When VEC $\ge$ 8, the phase tends to be a fcc structure, and when VEC $<$ 6.87, it is bcc. When VEC is 6.87–8, a mixed phase of fcc and bcc exists. The value of ${\left(\mathrm{V}\mathrm{E}\mathrm{C}\right)}_{\mathrm{A}\mathrm{l}}$ is 3, such that the crystal structure can be transformed into a bcc structure [64]. The study of Al_x(NiCoCrFe)_100−x using scanning electron microscopy shows that when x > 15, the fcc structure disappears [65].

The oxidation behavior of Al_xCoCrCuFeNi at high temperatures was studied [66]. The interface between matrix and oxide film is no longer smooth after oxidation of Al₀CoCrCuFeNi at 1000°C for 100 h because Al₀CoCrCuFeNi has Cr without Al, so it is difficult to form a dense oxide film. According to our previous discussion, the outer layer is composed of hollow, loose Cr₂O₃ with poor adhesion. Unlike the Al₀CoCrCuFeNi high-entropy alloy, the oxide layer and element distribution of the Al_0.5, Al_1.0, Al_1.5, and Al_2.0 high-entropy alloy vary with the increase in Al composition. The distribution of alloy elements becomes more uniform with increasing Al content, and the formed Al₂O₃ film becomes denser. Therefore, the oxidation resistance becomes better.

The continuity of Al₂O₃ becomes better with an increase in Al content; however, when the x of Al_x(NiCoCrFe)_100−x is less than 15, the oxidation rate increases significantly at the initial stage of oxidation. The Al₂O₃ formed with low Al content is discontinuous, thus reducing the antioxidant activity [67–68].

Ye et al. [69] found that the oxide film is mainly divided into three layers through the study of Al_xCoCrFeMnNi. When Al is not added, the oxidation is very serious. When the Al element is added, a dense aluminum oxide protective film is formed, and the total oxide film thickness is reduced, indicating that the Al element improves the oxidation resistance. The EDS scan of the oxide film in Fig. 4 shows that the Mn content in the oxide film is gradually reduced, and the Al content is gradually increased. As shown in Fig. 4(e), at point 13, the Al content is 29.49at% and the Mn content is 27.08at%. At point 15, the Al content is 33.42at% and the Mn content is 4.76at%. The dense alumina formed during the oxidation prevents the diffusion of Mn elements, which reduces the Mn content of the inner layer and improves the oxidation resistance.

Fig. 4.  Microstructure of the oxide film after 100 h of oxidation at 900°C: (a) Al_0.0; (b) Al_0.5; (c) Al_1.0; (d) Al_1.5; (e) Al_2.0. Reprinted from Vacuum, 174, F.X. Ye, Z.P. Jiao, S. Yan, L. Guo, L.Z. Feng, and J.X. Yu, Microbeam plasma arc remanufacturing: Effects of Al on microstructure, wear resistance, corrosion resistance and high-temperature oxidation resistance of Al_xCoCrFeMnNi high-entropy alloy cladding layer, 109178, Copyright 2020, with permission from Elsevier.

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Recent studies have also shown that high-entropy alloys without Al have good high-temperature oxidation resistance. Through the comparative study of MoTaTiCrAl and MoTaTiCr, Li et al. [70] found that MoTaTiCr without Al forms a continuous CrTaO₄ layer in the process of high-temperature oxidation, which plays a protective role in the process of high-temperature oxidation and makes the high-temperature oxidation resistance of MoTaTiCr better than that of MoTaTiCrAl. A continuous CrTaO₄ layer can also effectively inhibit the volatilization of Mo, which is also the reason for the improvement of the antioxidant capacity of MoTaTiCr. This discovery also contradicts the traditional belief that adding Al can improve the antioxidant capacity at high temperatures, which is worthy of attention in future research.

(2) Effect of Ti element on high-temperature oxidation.

Ti can affect the oxidation properties of high-entropy alloys containing Al. Among the constituent elements of the CoCrFeNiAl_xTi_y [71] alloy system, Al has the highest affinity for oxygen. In the oxidation process, Al forms Al₂O₃, the most stable oxide in the CoCrFeNiAl_xTi alloy. The weight gain of a high-entropy alloy is greater with the increase in Ti content. Although CoCrFeNiAlTi_0.5 has a high content of Al, it does not show good antioxidant activity. The oxidation behavior of the alloy does not conform to the parabolic oxidation law. The oxidation process changes nearly linearly, indicating that the oxidation rate is very large, which shows that the oxide layer may peel off and the alloy may be oxidized again. Compared with other high-entropy alloys, the content of Ti in this alloy is the highest. As shown by the phase distribution below the oxide layer, Al is oxidized at the very beginning. A large amount of TiO₂ is formed above this region and a Cr₂O₃ phase is formed on the TiO₂ oxide film. In addition, a thin Al₂O₃ oxide film is formed on top of the internal alumina oxide. X-ray diffraction (XRD) analysis shows that there is a Fe₂TiO₅ phase in the initial stage after oxidation for 5 h, which may be formed by the reaction of Fe₂O₃ and TiO₂. The phase is not clearly observed in the elemental atlas and scanning electron microscopy, which may be because the phase is unstable and difficult to capture. In the following stages, a large weight change at the end of the oxidation test also reveals the existence of this phase, which is unstable and grows rapidly. The increase in oxidation rate is because of the presence of Al₂O₃ oxide film. Ti ions will diffuse through Al₂O₃, which promotes the formation of oxides, resulting in the decrease in oxidation resistance of high-entropy alloys containing Ti.

Ham et al.’s research on AlCoCrFeNiTi_x [72] shows that Ti can improve the adhesion of Al₂O₃. As shown in Fig. 5, through the observation of the oxidation surface, both materials have Al₂O₃ oxides. The components of the corresponding points in Fig. 5 are listed in Table 3. The Ti_0.0 material shows exfoliation of Al₂O₃ oxide (yellow area) in most areas, and the adhesion between Al₂O₃ and the high-entropy alloy is very low. The Ti_1.0 alloy only exfoliates Al₂O₃ oxide in some areas and has better adhesion to Al₂O₃ than the Ti_0.0 alloy. According to these results, adding a small amount of Ti to an AlCoCrFeNi high-entropy alloy can improve the adhesion of the oxide. Therefore, when we design high-entropy alloy materials containing Ti, we should consider the dual factors that Ti improves the adhesion ability of oxides and leads to a decline in oxidation resistance of high-entropy alloys to produce better oxidation resistance materials.

Fig. 5.  SEM micrographs of (a) AlCoCrFeNiTi_0.0 oxidation and (b) AlCoCrFeNiTi_1.0 oxidation. Reprinted by permission from Springer Nature: Met. Mater. Int., Effect of Ti addition on the microstructure and high-temperature oxidation property of AlCoCrFeNi high-entropy alloy, G.S. Ham, Y.K. Kim, Y.S. Na, and K.A. Lee, Copyright 2021.

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Table  3.  Corresponding point components in Fig. 5. Reprinted by permission from Springer Nature: Met. Mater. Int., Effect of Ti addition on the microstructure and high-temperature oxidation property of AlCoCrFeNi high-entropy alloy, G.S. Ham, Y.K. Kim, Y.S. Na, and K.A. Lee, Copyright 2021 wt%

Point	O	Al	Ti	Cr	Fe	Co	Ni
Point in Fig. 5(a)	50.48	26.62	0	7.91	6.52	5.36	3.12
Point in Fig. 5(b)	63.75	35.29	0.08	0.45	0.24	0.18	0

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CoCrCuFeNi is a fcc solid solution and CoCrCuFeNi–TiO is composed of a fcc solid solution and TiO crystals. When a specific amount of TiO is added, TiO₂ is formed during the oxidation process, which improves the oxidation resistance. However, with the increase in TiO, the precipitation of TiO₂ leads to a decrease in oxidation resistance [73].

(3) Effect of Cu element on high-temperature oxidation.

The AlCoCrCu_xFeNi [74] alloy was oxidized for 100 and 500 h, and the morphology of the obtained oxide film was evaluated by backscattered electron (BSE), as shown in Fig. 6. As shown in the oxidation kinetics curve, the spallation of two high-entropy alloys AlCoCrCu_0.5FeNi and AlCoCrCuFeNi containing Cu is more serious, and the spallation is more obvious with the increase in Cu content. However, the ${K}_{\mathrm{p}}$ (growth rate constant) values of AlCoCrFeNi, AlCoCrCu_0.5FeNi, and AlCoCrCuFeNi alloys are $9.39\times {10}^{-13}$, $3.40\times {10}^{-13}$, and $8.00\times {10}^{-14}$, respectively. The oxidation rate decreases with an increase in Cu content. For general high-entropy alloys, the lower oxidation rate generally corresponds to better oxidation resistance; however, the opposite result appears after the addition of Cu, which may be because of the improvement in the overall thermal expansion coefficient of Cu. Because of the large volume change in the oxidation process, the Al₂O₃ film in the oxidation process is discontinuous, which does not serve as a sufficient protective, so the oxidation resistance becomes worse with the increase of Cu content.

Fig. 6.  BSE micrographs oxidized at 1273 K for (a–c) 100 and (e–f) 500 h. Reprinted from Intermetallics, 84, J. Dąbrowa, G. Cieślak, M. Stygar, K. Mroczka, and M. Danielewski, Influence of Cu content on the high-temperature oxidation behavior of AlCoCrCu_xFeNi high-entropy alloys (x=0; 0.5; 1), 52-61, Copyright 2017, with permission from Elsevier.

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(4) Effect of Cr element on high-temperature oxidation.

The role of Cr in the oxidation process of high-entropy alloy FeCr_xCoNiB was recently studied [75]. With the increase in Cr content, the grain size of the high-entropy alloy decreases, indicating that Cr can inhibit coarsening, which is also an important reason for the improvement in oxidation resistance of the high-entropy alloy. Boride is also easily oxidized at high temperatures to increase the oxidation trend. The increase in Cr will reduce coarsening and form dense Cr₂O₃, reducing the contact between oxygen and boron to improve the oxidation resistance in the oxidation process.

Al content can cause the "third element effect", increase the activity of Cr, and promote the formation of protective Cr₂O₃. In high-entropy alloys, once a continuous Cr₂O₃ layer is formed, the oxidation will be hindered. In the future alloy design, the activities of Cr and Al should be considered to form protective oxides more quickly [76].

(5) Effect of Mn element on high-temperature oxidation.

H5M (FeCoNiCrMn) high-temperature oxidation was studied at different temperatures [77]. Although there are some microcracks at the oxide film/substrate interface, especially at 900°C, the oxide film still has a good bond with the substrate. At 700, 800, and 900°C, the thickness of the oxide film of H5M is (10.17 ± 0.53), (13.61 ± 1.44), and (19.24 ± 0.87) μm, respectively. The serious oxidation situation shows that the thickness of the oxide film of H5M increases gradually after oxidation at different temperatures for 48 h. The Mn element forms MnO at the initial stage of oxidation and transforms into Mn₃O₄ in the later stage of oxidation. The outward diffusion rate of Mn ions is much faster than other elements, resulting in the formation of porous Mn films with binary and ternary oxide films on top of the Cr₂O₃ layer. Fig. 7 shows the energy dispersive X-ray spectroscopy (EDX) element diagram of the CrMnFeCoNi high-entropy alloy oxidized at 900°C for 100 h; qualitatively similar element distribution is observed [78]. The enrichment of Mn and Cr in the oxide film is serious. The oxide film is divided into two layers because of the different diffusion rates of the elements. The enrichment of Mn in the outer layer shows that the diffusion rate is greater than that of Cr. As the temperature increases, α-Mn₂O₃ gradually transforms into Mn₃O₄, forming pores near the oxide interface and reducing the oxidation resistance.

Fig. 7.  Backscattered electron micrograph and element distribution map of the oxide film of the CrMnFeCoNi high-entropy alloy oxidized at 900°C for 100 h. Reprinted by permission from Springer Nature: Oxid. Met., Oxidation Behavior of the CrMnFeCoNi High-Entropy Alloy, G. Laplanche, U.F. Volkert, G. Eggeler, and E.P. George, Copyright 2016.

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(6) Effect of Si element on high-temperature oxidation.

The oxidation kinetics of Nb–Mo–Cr–Ti–Al and Nb–Mo–Cr–Ti–Al–1Si shows that the oxidation rate containing Si is significantly less than those without Si at high temperatures greater than 900°C. When the temperature reaches 1000 and 1100°C, the oxide layer rich in Cr and Al is formed in Nb–Mo–Cr–Ti–Al–1Si alloy. Compared with the alloy without Si, the oxide layer rich in Cr and Al in the Nb–Mo–Cr–Ti–Al–1Si alloy is also compact. Therefore, the formation of Al-rich oxide film in the Nb–Mo–Cr–Ti–Al alloy system is affected by the amount of Si added [79].

When the Si content exceeds 0.6at%, the effect of Si on the anti-oxidation weakens [80]. The addition of Si leads to a decrease in the lattice constant, which leads to lattice distortion. Decreasing the lattice parameters will reduce the volume diffusion of oxygen or cations, thus reducing the degree of oxidation. However, with the Si content increasing in the cooling process, a hard-brittle composite phase (σ) appears, resulting in higher porosity. In addition, the lattice parameters of all of the constituent phases decrease with the increase in Si content, which may lead to stomata during the synthesis process. The role of Si slows anion diffusion and contributes to the infiltration of oxygen. This phenomenon leads to a decrease in anti-oxidation when the content of Si is too high, which causes the best antioxidant performance at 0.6at% Si.

The addition of Si will decrease the antioxidant performance. As shown in Fig. 8, in Si-free alloys, relatively small Laves phase particles are preferentially formed at grain boundaries. In the Si alloys, Laves phase particles are obviously larger and uniformly distributed in the matrix. The appearance of internal corrosion after high-temperature oxidation depends largely on the size, shape, and number of Laves phases (Fig. 9), which means that the matrix near the large-size Laves phase particles is more seriously eroded [81]. Wang et al. [82] came to a similar conclusion.

Fig. 8.  BSE micrographs of the substrate oxidized after 20 h at 1400°C: (a) Ta–Mo–Cr–Ti–Al; (b) Ta–Mo–Cr–Ti–Al–1Si. Copyright 2018 from Effect of microalloying with silicon on the high-temperature oxidation resistance of novel refractory high-entropy alloy Ta–Mo–Cr–Ti–Al by F. Müller, B. Gorr, H.J. Christ, H. Chen, A. Kauffmann, and M. Heilmaier. Reproduced by permission of Taylor and Francis Group, LLC, a division of Informapic.

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Fig. 9.  BSE cross-section images of (a) Ta–Mo–Cr–Ti–Al and (b) Ta–Mo–Cr–Ti–Al–1Si oxidized at 1000°C for 48 h. Copyright 2018 from Effect of microalloying with silicon on the high-temperature oxidation resistance of novel refractory high-entropy alloy Ta–Mo–Cr–Ti–Al by F. Müller, B. Gorr, H.J. Christ, H. Chen, A. Kauffmann, and M. Heilmaier. Reproduced by permission of Taylor and Francis Group, LLC, a division of Informapic.

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(7) Effect of Nb element on high-temperature oxidation.

In the high-temperature oxidation process, Nb plays a dual role in the Nb element in CoCrFeMnNb_xNi: the new Laves phase hinders the outward diffusion of metal ions and also becomes a channel for the inward diffusion of oxygen ions. The combined effect of these two factors will lead to resistance oxidation fluctuations with changes in temperature, which supports why the antioxidant capacity of Nb_0.25 is better than that of Nb₀ at low temperature and worse at high temperature [49]. The high-temperature oxidation resistance of the Al_0.2Co_1.5CrFeNi_1.5Ti_0.3 high-entropy alloy is significantly improved after adding a trace amount of Nb [83]. During the oxidation process, a 4-μm thick Nb-rich layer is formed between Cr₂O₃ and Al₂O₃, which acts as a diffusion barrier, avoids internal diffusion of oxygen, and greatly improves high-temperature oxidation resistance.

2.3.2   Effect of refractory high-entropy alloy elements on high-temperature oxidation

(1) Effect of Zr and Hf elements on high-temperature oxidation.

Compared with Fig. 10(a), Fig. 10(b) shows that there is obvious internal oxidation [84]. The enrichment and dissolution of oxygen on the surface lead to an increase in the lattice constant, resulting in volume expansion and internal stress. As the oxidation process progresses, stress begins to accumulate; after reaching a certain level, the stress releases and causes fracture. Hf and Zr play an inseparable role in accelerating oxidation because Zr and Hf have high oxygen affinity and a large atomic radius, which leads to the weakening of interatomic bonds and an increase in intermetallic diffusion coefficient at high temperatures. Therefore, it is easier for oxides to form and cause the material to peel off.

(2) Effect of Mo element on high-temperature oxidation.

The oxide film of high-entropy alloy TiNbTa_0.5ZrAlMo_0.5 containing Mo becomes weaker and separates from the matrix during oxidation at 1000°C [85]. As shown in Fig. 11(a), vertical cracks are observed through the oxide film, which leads to the diffusion of oxygen into the substrate. A large number of nanoscale pores are observed around the cracks. The dispersed phase with the same pore size is molybdenum-rich oxide. According to the XRD results, the molybdenum-rich oxide is MoO₃. Fig. 11(e) shows the subsurface of the oxide scale, with a total of three layers: the first layer contains many pores and defects, the second layer shows a transition oxide, as well as dispersed MoO₃ and many nano-sized pores, and the third layer is a Mo-rich defect layer. The oxide scale of the TiNbTa_0.5ZrAlMo_0.5 alloy is closely related to MoO₃. The melting point of MoO₃ is the lowest (1000°C), which leads to the liquefaction of oxides at high temperatures. MoO₃ has a high saturated vapor pressure and fast evaporation rate. Therefore, the large number of nanopores are considered evidence of MoO₃ evaporation, which leads to the decrease in oxidation resistance. Waseem and Ryu [86] also confirmed that the oxidation resistance of high-entropy alloys containing Mo decreases because of evaporation.

Fig. 10.  Microstructure of a Hf_0.5Nb_0.5Ta_0.5Ti_1.5Zr alloy when oxidized at 800°C for 5 h: (a) base metal; (b) oxide layer cracked after oxidation. Reprinted from Intermetallics, 97, S. Sheikh, M.K. Bijaksana, A. Motallebzadeh, S. Shafeie, A. Lozinko, L. Gan, T.K. Tsao, U. Klement, D. Canadinc, H. Murakami, and S. Guo, Accelerated oxidation in ductile refractory high-entropy alloys, 58-66, Copyright 2018, with permission from Elsevier.

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Fig. 11.  Oxide film diagrams of TiNbTa_0.5ZrAlMo_0.5 high-entropy alloy oxidized at 1000°C for 10 h: (a) macro morphology; (b) partially enlarged view; (c) oxide distribution and morphology; (d) surface oxidation; (e) oxides in the subsurface. Reprinted from Trans. Nonferrous Met. Soc. China, 29, Y.K. Cao, Y. Liu, B. Liu, W.D. Zhang, J.W. Wang, and M. Du, Effects of Al and Mo on the high-temperature oxidation behavior of refractory high-entropy alloys, 1476-1483, Copyright 2019, with permission from Elsevier.

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(3) Effect of Ta element on high-temperature oxidation.

The microstructure of Ta–Mo–Cr–Ti–Al is analyzed in Fig. 12 [87]. After oxidation of the Ta–Mo–Cr–Ti–Al high-entropy alloy, the outer layer forms a continuous TiO₂ layer and a continuous CrTaO₄ layer at the bottom; however, the Cr₂O₃ and Al₂O₃ particles are discontinuous. TiO₂ has limited protective ability, so CrTaO₄ is the main reason for its high oxidation resistance. During the growth of CrTaO₄-based oxides, a large number of elements are dissolved, which potentially inhibits these elements from reacting with oxygen and improves the oxidation resistance [88]. Lo et al. [89] also confirmed the antioxidant capacity of CrTaO₄.

Fig. 12.  Microscopic images of a Ta–Mo–Cr–Ti–Al high-entropy alloy after oxidation for 12 h: (a) 1300°C (b) 1500°C. Reprinted from Corros. Sci., 166, B. Gorr, F. Müller, S. Schellert, H. J. Christ, H. Chen, A. Kauffmann, and M. Heilmaier, A new strategy to intrinsically protect refractory metal based alloys at ultra high temperatures, 108475, Copyright 2020, with permission from Elsevier.

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2.4   Effect of the preparation process on high-temperature oxidation of high-entropy alloys

The high-temperature oxidation process of high-entropy alloys is most commonly performed in a vacuum arc furnace [90–91]. High-entropy alloys are also prepared by mechanical alloying [92–93], laser cladding [68,94], magnetron sputtering [95–99], and 3D printing [100]. High-entropy alloys for high-temperature oxidation research are generally vacuum arc furnace smelting or laser cladding. The oxidation of high-entropy alloys in vacuum arc smelting is outside the scope of this review.

The pre-oxidation conditions of the traditional thermal spraying of a MCrAlY (M = Ni, Co, Ni, and Co) bonding layer has high-speed laser cladding [22]. Compared with traditional thermal spraying, the high-entropy alloy bonding layer of high-speed laser cladding forms a denser aluminum oxide protective film during the oxidation process, which improves the high-temperature oxidation resistance. The non-bonding interface from thermal spraying causes the Al element to diffuse slowly, which does not form a continuous aluminum oxide protective film, reducing the oxidation resistance.

3.   Mechanism of oxidation of high-entropy alloys

3.1   Influence of thermodynamic factors on high-temperature oxidation

The oxide binding energy and electronegativity of elements are very important for the high-temperature oxidation of high-entropy alloys because it is related to the order of oxide formation. If the oxidation resistance of the oxides formed at first is good, it is possible to prevent oxygen from continuing to react with other alloy elements, thus improving the oxidation resistance of high-entropy alloys. If the oxidation resistance of the oxides formed before is relatively poor, it will accelerate the reaction of oxygen with other alloy elements, thus worsening the oxidation resistance of high-entropy alloys. For example, Fig. 13 shows the relationship between the oxide binding energy of the Al_0.3CoCrFeNiCu high-entropy alloy and the electronegativity of elements [101]. For metallic materials, the fewer valence electrons, the greater the tendency of oxidation because it is easier to break away from the parent atom. Therefore, for the Al_0.3CoCrFeNiCu high-entropy alloy, Al and Cr have fewer valence electrons and lower binding energy. Al₂O₃ and Cr₂O₃ are first formed to cover the surface of the matrix metal during high-temperature oxidation, which improves the oxidation resistance.

Fig. 13.  (a) Normalized cohesive energies (E_c) of different oxides in the Al_0.3CoCrFeNiCu high-entropy alloy; (b) number of valence electrons and electronegativity values of the elements in Al_0.3CoCrFeNiCu. Reprinted from Scripta Mater., 168, Y. Hong, M.B. Kivy, and M. A. Zaeem, Competition between formation of Al₂O₃ and Cr₂O₃ in oxidation of Al_0.3CoCrCuFeNi high-entropy alloy: A first-principles study, 139-143, Copyright 2019, with permission from Elsevier.

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The formation of oxide film during high-temperature oxidation is also closely related to Gibbs free energy. Generally, oxides with lower Gibbs free energy are preferentially formed during oxidation. The Gibbs free energy during the oxidation process of common elements is shown in Table 4.

Table  4.  Standard Gibbs free energies ($\Delta \mathit{G}$) of formation at different temperatures [77, 102] kJ/mol

Oxide	${\Delta G}_{800^ \circ \mathrm{C}}$	${\Delta G}_{1000^ \circ \mathrm{C}}$	${\Delta G}_{1200^ \circ \mathrm{C}}$
Al2O3	−891.4	−847.4	−803.4
MnO	−612.8	−583.2	−553.6
Cr2O3	−571.9	−538.3	−504.7
FeO	−404.8	−379.8	−354.8
NiO	−286.0	−251.8	−217.6
CoO	−316.9	−289.1	−261.3
SiO2	−717.5	−682.5	−647.5

 | Show Table

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The Gibbs free energy shows that the order of oxide formation during oxidation is Al₂O₃ > SiO₂ > MnO > Cr₂O₃ > FeO > CoO > NiO; Al is considered to have excellent high-temperature oxidation resistance during high-temperature oxidation. The Gibbs free energy of Mn is also relatively low and easy to form during the oxidation process. However, the oxide of Mn is usually relatively loose, so high-temperature oxidation resistance is not very good.

3.2   Effect of ion diffusion on high-temperature oxidation

The diffusion sequence of ions is also an important factor in the study of the oxidation process of high-entropy alloys. If the diffusion rate of ions is large, the reaction rate of the ions with oxygen in the process of high-temperature oxidation will also be accelerated. If the ions with high diffusion speed react with oxygen to form a dense oxide film, the oxidation resistance of high-entropy alloys will be improved. Alternatively, if the oxide film is discontinuous after the reaction of the ions with high diffusion velocity with oxygen, or even forms relatively large voids, the oxidation resistance of the high-entropy alloy will be greatly reduced. The diffusion order of ions in FeCoNiCrMn high-entropy alloy is Mn⁺² > Fe⁺² > Ni⁺² > Cr⁺³, so the diffusion rate of Mn⁺² is the fastest, while the Mn₃O₄ formed by Mn element in the outer layer is discontinuous, leading to oxidation voids and accelerated oxidation in Mn-containing alloys [103–104]. The schematic diagram of FeCoNiCrMn oxidation is shown in Fig. 14 [45]. MnO is formed in the early stage of oxidation, and (Mn,Cr)₃O₄ is formed in the middle stage of oxidation. In the later stage of oxidation, (Mn,Fe)₃O₄ and MnO grow mixed in the outer layer.

3.3   Effect of lattice distortion and slow diffusion on high-temperature oxidation of high-entropy alloys

Severe lattice distortion will introduce slow diffusion of high-entropy alloys, which will also have a certain effect on the high-temperature oxidation. Currently, serious local lattice distortion only appears in high-entropy alloys containing Zr or Hf [105–106]. According to the dislocation tube theory, when heated at 1400°C, the defects in the dislocation can promote the atom to move. The dislocation can act as a pipe and the atom can move rapidly along the pipe. During the oxidation process, there are two obvious zones in the TiZrNbTa high-entropy alloy, the TiZr-rich region and the TaNb-rich region [106]. The region containing Zr will have serious lattice distortion, resulting in obvious lattice distortion and the dislocation tube effect. The more serious the lattice distortion in the TiZr enrichment region and more dislocations, the higher the diffusion rate and oxidation rate of oxygen atoms. The uneven diffusion of oxygen atoms at 800°C creates stress concentration and cracks along the TiZr zone. When the temperature is greater than 1000°C, the oxygen atom diffuses uniformly, and the degree of lattice distortion is less than the threshold of the dislocation tube effect, which slows the diffusion rate of oxygen atoms and leads to uniform oxidation.

Fig. 14.  Schematic diagram of the transformation of a FeCoNiCrMn high-entropy alloy during oxidation: (a) initial oxidation; (b) following oxidation; (c) later oxidation. Reprinted from Corros. Sci., 153, W. Kai, F.C. Chien, F.P. Cheng, R.T. Huang, J.J. Kai, and C.T. Liu, The corrosion of an equimolar FeCoNiCrMn high-entropy alloy in various CO₂/CO mixed gases at 700 and 950°C, 150-161, Copyright 2019, with permission from Elsevier.

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There is no systematic research on the mechanism of high-temperature oxidation. The research has been mainly experiments to evaluate the anti-oxidation performance. The essence of the high-temperature oxidation resistance of high-entropy alloys and nickel-based superalloys is the formation of oxide films. One of the potential advantages of alloys is the slow diffusion effect. The slow diffusion effect can reduce the combination speed between oxygen and element, increasing the activation energy of diffusion and reducing the diffusion rate, thereby improving the oxidation resistance.

4.   Future directions and perspectives

The main challenges presenting high-temperature oxidation are that the upper limit temperature of high-temperature service is not high enough and there are few studies on high-temperature oxidation in complex gas environments. It is possible to explore the high-temperature oxidation behavior under ultra-high temperature (greater than 1500°C) conditions. The emergence of high-entropy alloys allows us to make new attempts in this field. Research on the complex gas environment is also of great practical significance. Our country’s aerospace industry is developing rapidly; as a result, we will need to study the oxidation behavior in the air at room temperature, as well as explore the changes in superalloys under conditions of the low oxygen content of space. Regarding the theoretical research and actual development needs of high-entropy alloys, theoretical research should start with alloying elements, lattice distortion, and slow diffusion. For the actual development needs of high-entropy alloys, the oxidation performance of high-entropy alloys can be optimized in consideration of coatings, heat treatment processes, and preparation methods.

(1) On the basis of the above analysis, alloy elements play a decisive role in the oxidation properties of high-entropy alloys. Therefore, we should start to look for alloy elements with better oxidation resistance. For example, there are a few studies on the Au element, which can enhance the properties in combination with Co, Cr, and other elements. We can use high-throughput technology to prepare different kinds of high-entropy alloys containing Au element, as well as study the effect of the proportion of Au element on the oxidation properties.

(2) Lattice distortion can affect the thermodynamics and kinetics of high-entropy alloys, which will be reflected in the high-temperature oxidation process of high-entropy alloys. The change of structure will affect the diffusion of oxygen, and even transform the diffusion path of oxygen in high-entropy alloys. When the atomic radius of metal element is large, it will lead to lattice distortion in high-entropy alloys such as Ti, Zr, and Mo. We can seek an alloying element to produce obvious lattice distortion, leading oxygen to the vicinity of Al, Cr, and other alloy elements with good oxidation resistance similar to drainage to form Al₂O₃ and Cr₂O₃, improving the oxidation resistance.

(3) The slow diffusion effect is a typical characteristic of high-entropy alloys. The diffusion rate of ions in FeCoNiCrMnis Mn⁺² > Fe⁺² > Ni⁺² > Cr⁺³, which leads to Mn₃O₄ discontinuity formed by Mn elements in the outer layer, forming pores, and accelerating oxidation. Combining theory with experiment can be used in future research, and a potential high-entropy alloy model can be constructed through the first principle. The diffusion rates of various elements in high-entropy alloy at a high temperature can be roughly predicted, and the sequence of oxidation can be determined. Finally, experiments can be designed to identify high-entropy alloys with high-temperature oxidation resistance.

(4) Although high-entropy alloys have some price advantages, high-entropy alloy coatings on the surface of a substrate can be considered to reduce costs.

(5) Different heat treatment processes also have a great influence on the oxidation resistance of high-entropy alloys. The composition of high-entropy alloys can be more uniform through solid solution and other methods. In addition, the dense oxide film formed on the surface of the high-entropy alloy through heat treatment at a certain temperature may improve the high-temperature oxidation resistance of high-entropy alloys.

(6) Different preparation processes have different effects on high-temperature oxidation, and the preparation process has made great progress in recent years. Different preparation processes, such as casting, 3D printing, magnetron sputtering, and other processes, can be used to identify the high-entropy alloy with the best performance.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 52071014), the Fundamental Research Funds for the Central Universities (No. FRF-GF-19-033BZ), and the National Key Research and Development Program of China (No. 2020YFB0704501).