High-Temperature Oxidation of High-Entropy FeNiCoCrAl Alloys

. Phase composition and mechanical properties and the formation of oxide layers on Fe 40-x NiCoCrAl x (x = 5 and 10 at.%) alloys in long-term oxidation at 900 and 1000°C were studied. In the initial cast state, depending on the aluminum content and valence electron concentration, the alloys contain only an fcc solid solution (VEC = 8 e/a) or a mixture of fcc and bcc phases (VEC = 7.75 e/a). Thin continuous oxide scales containing Cr 2 O 3 and NiCr 2 O spinel formed on the surface of both alloys oxidized at 900°C for 50 h. A further increase in the annealing time to 100 h leads to the formation of aluminum oxide Al 2 O 3 in the scale on the Fe 30 Ni 25 Co 15 Cr 20 Al 10 alloy, having high protective properties. An increase in the oxidation temperature to 1000°C results in partial failure of the protective layer on the alloy with 10 at.% Al. Long-term holding at 900°C (100 h) + 1000°C (50 h) does not change the phase composition of the Fe 35 Ni 25 Co 15 Cr 20 Al 5 alloy matrix, being indicative of its high thermal stability. In the two-phase Fe 30 Ni 25 Co 15 Cr 20 Al 10 alloy, the quantitative ratio of solid solutions sharply changes: the amount of the bcc phase increases from 4 to 54 wt.% and its B2-type ordering is observed. The mechanical characteristics of the starting alloys and those after long-term high-temperature annealing were determined by automated indentation. The hardness (HIT) and elastic modulus (E) of the cast Fe 35 Ni 25 Co 15 Cr 20 Al 5 alloy are equal to 2 and 147 GPa, respectively, and decrease to 1.8 and 106 GPa after a series of long-term annealing operations. The Fe 30 Ni 25 Co 15 Cr 20 Al 10 alloy shows the opposite dependence: HIT increases from 2.5 in the initial state to 3.1 GPa after annealing and E decreases from 152 to 134 GPa. This indicates that the Fe 30 Ni 25 Co 15 Cr 20 Al 10 alloy is promising as a high-temperature oxidation-resistant and creep-resistant material.


Introduction
A new class of alloys, called high-entropy alloys (HEAs), possessing low free energy and high mixing entropy has been of research focus recently. They contain more than four elements in an equiatomic or close ratio (Yeh et al., 2004). The HEAs are peculiar in that they form simple substitutional solid fcc or bcc solutions or a mixture of bcc + fcc solid solutions (Gao et al., 2013;Sheng et al., 2011). Differently doped alloys in which simple bcc and fcc solid solutions form are among HEAs that have been studied most extensively. Numerous papers focus on the optimization of alloy structures and mechanical properties (Firstov et al., 2013ab;Firstov et al., 2016a;Gumen et al., 2019ab;Wang et al., 2012). There are currently very limited data on the high-temperature oxidation resistance of these materials and the stability of their phase composition and mechanical properties in long-term high-temperature holding (Butler et al., 2016ab;Holcomb et al., 2015;Kim et al., 2018;Wu et al., 2017). Our objective is to examine the effect of aluminum on the stability of phase composition, structure, and mechanical properties and on the formation of oxide layers in long-term temperature oxidation of the Fe40-xNiCoCrAlx alloys (x = 5 and 10 at.% Al) at 900 and 1000°C.

Experimental part
The Fe40-xNiCoCrAlx alloys (x = 5 and 10 at.% Al) were produced by arc melting in a high-purity argon atmosphere. The ingots were melted six to seven times to homogenize their composition. The starting components were high-purity materials (at least 99.98% purity).
The oxidation resistance of the alloys was examined in an electric arc furnace at 900 and 1000°C in air. The samples were periodically weighed in 5, 10, 25, 50, and 100 h. To measure the weight change of the samples, we used a Radwag precision balance (±0.0001 g). The oxidation resistance was assessed from the specific change in weight of the samples q (mg/cm 2 ·h).
The phase composition of the starting alloys and those after high-temperature oxidation was analyzed with an Ultima IV diffractometer in monochromatic Cu-K α radiation. The X-ray diffraction data were processed using the PowderCell 2.4 software for full-profile analysis of Xray spectra for a mixture of polycrystalline phase components. The accuracy of the measured lattice period values is ±0.0001 nm. The morphology and microstructure of scales on the starting and annealing alloys were examined with a Jeol Superprobe 733 scanning electron microscope (SEM) and with a MIM-8 optical microscope. The mechanical properties were determined by automated indentation with a Berkovich pyramid under 3N load employing a Micron Gamma unit.

Results and Discussion
The fcc solid-solution phase is known (Butler et al., 2017;Firstov et al., 2016b) (Fig. 1b) included grains elongated in the crystallization direction and subgrains at grain boundaries and partially inside them. With higher aluminum content of the Fe30Ni25Co15Cr20Al10 alloy, dendritic crystallization leading to light dendrites and a darker phase in the space between the dendrites was observed (Fig. 1d). A greater aluminum content of the alloy reduces VEC, in turn indicating that stability of the fcc solid solution reduces and the bcc phase forms through solid-phase decomposition mechanism.

Fig. 1. X-ray diffraction patterns (a, c) and microstructures (b, d) for the Fe35Ni25Co15Cr20Al5 (a, b) and Fe30Ni25Co15Cr20Al10 (c, d) high-entropy alloys
in starting state (SEM). Figure 2 shows variation in the specific weight of the samples q (mg/cm 2 ) after oxidation of the Fe35Ni25Cr20Co20Al5 and Fe30Ni25Cr20Co15Al10 high-entropy alloys at 900ºC for 100 h and then at 1000°C for 50 h. After short holding for 5 h at 900°C, the specific weight of the alloy samples increased differently. The alloy with 10 at.% Al oxidized much more slowly for the first 5 h than the alloy containing 5 at.% Al.

Fig. 2. Variation in the specific weight of alloy samples after oxidation at 900°C (100 h) and further at 1000°C (50 h).
The specific weight of the Fe35Ni25Cr20Co20Al5 alloy showed a stable gain for 30 h at 900°C and naturally increased further with longer holding as oxide film began to develop and grow. The Fe30Ni25Cr20Co15Al10 alloy behaved differently. The sample significantly gained weight after the first 5 h and then partially lost its weight, which was accompanied by the formation of a thin brittle scale that spalled in some places. A dense scale formed further and oxidation proceeded not only without weight losses but also without weight gain, being indicative of high protective properties of the oxide film. However, a further increase of the oxidation temperature to 1000°C led to a sharp gain in the specific weight of the samples and growth of the scale thickness (Fig. 2). The Fe35Ni25Cr20Co20Al5 alloy showed a continuous increase in the specific weight within entire holding at 1000°C; q was minimum for the first 5 h and the oxidation rate sharply increased in a period between 5 to 10 h. During the next exposure in the interval from 10 to 25 hours, a significant slowdown in the oxidation process is observed, and then there is a significant increase in the increase in the mass of the samples, this may indicate that the scale formed on the surface of the alloy does not have high protective properties at a given temperature.

Fig. 3. Morphology of the scale on the Fe30Ni25Cr20Co15Al10 alloy (a) after oxidation at 900°C (100 h) + 1000°C (50 h) and cross-sectional microstructure (b).
The situation was different for the Fe30Ni25Cr20Co15Al10 alloy that contained more aluminum and less iron. After the first 5 h, the sample weight gain was much lower than that of the Fe35Ni25Cr20Co20Al5 alloy. In a period between 5 and 10 h, q substantially increased and then (10-25 h) the process sharply slowed down. In a period between 25 and 50 h, the curve showing variation in the specific weight changed its direction. When the sample cooled down, the scale partially spalled since the protective film failed under a cyclic change in temperatures because of different thermal expansion coefficients (TECs) of the scale and alloy matrix (Fig. 3).
A two-phase oxide film containing Cr2O3 and NiCr2O4 spinel formed on both alloys at 900°C for 50 h. The scales were very thin (3-5 µm thick on the Fe30Ni25Cr20Co15Al10 alloy and 7-8 µm thick on the Fe35Ni25Cr20Co20Al5 alloy) and the X-ray diffraction patterns of both alloys had also reflections from the matrix phase of the fcc solid solution (Fig. 4).
The lattice parameter of the fcc solid solutions in the Fe35Ni25Cr20Co20Al5 alloy increased compared to the starting state and became a = 0.3577 nm (since the near-surface metal layer was saturated with oxygen) and that in the Fe30Ni25Cr20Co15Al10 alloy substantially decreased and became a = 0.3585 nm ( Table 2), indicating that metal ions diffused toward the oxidation front. Some diffusion processes occurred under the scale. For example, the aluminum diffusion rate increased with temperature. Hence, the aluminum concentration toward the metal surface became higher to promote the formation of Al2O3. In turn, this indicates that the fcc phase became depleted of aluminum and its lattice parameter decreased.
The amount of the fcc phase in the 5 at.% Al alloy was 7 wt.% and that in the 10 at.% Al alloy was 41 wt.%. This substantial difference can be attributed to varying thickness of the scales; as a result, X-rays penetrate to different depths of the sample. The main phase of the scales is presented by oxides of Cr2O3 structure: their amount reaches 83 wt.% for Fe35Ni25Cr20Co20Al5 and 42 wt.% for Fe30Ni25Cr20Co15Al10.
The amount of NiCr2O4 spinel in the scale of the Fe35Ni25Cr20Co20Al5 alloy was 10 wt.% and that in the Fe30Ni25Cr20Co15Al10 alloy was 17 wt.%. Further holding changed the scale phase composition. The alloy with lower aluminum content had a scale after 100 h at 900°C containing two phases: Cr13Fe7O30 and NiCr2O4 (Fig. 4b).
The presence of Cr13Fe7O30 can be explained by the fact that iron oxidized in long-term holding to form FeO that dissolved in Cr2O3 to create the Cr13Fe7O30 phase with lattice parameters a = 0.4986 nm and c = 1.3646 nm, which is isostructural to Cr2O3.
The NiCr2O4 lattice parameter increased after oxidation at 900°C and became 0.8350 nm ( Table  2). In oxidation at 900°C for 100 h, Al2O3 formed on the Fe30Ni25Cr20Co15Al10 alloy besides Cr13Fe7O30. The X-ray diffraction pattern for the scale surface had no NiCr2O4 reflections since spinel separated from the scale surface after the thin continuous Al2O3 layer formed (being typical of the oxidation of Ni-Cr-Al alloys), so reflections from the fcc phase were predominant (Fig.  4d). After further oxidation at 1000°C for 50 h, the phase composition of the scales was similar to that of the oxide films after annealing for the same time at 900°C. In both cases, the X-ray diffraction patterns had reflections of NiCr2O4, Cr2O3, and fcc phase (Fig. 4e, f), but the amounts of phase constituents somewhat differed: the amount of Cr2O3 decreased and that of NiCr2O4 increased ( Table 2). This will further improve the high-temperature oxidation resistance of the alloys since diffusion processes slow down significantly in the layer with NiCr2O4.
Cross-sectional studies of the Fe30Ni25Cr20Co15Al10 alloy samples oxidized at 900°C + 1000°C revealed a wide continuous region of a light phase, identified by X-rays as the fcc solid solution, under the oxide film layer. This explains why there are no reflections of the bcc phase in the Xray diffraction pattern. Hence, the surface layers are structured as follows: scale, fcc solid-solution region, and matrix with bcc + fcc phases.
Since the papers (Lin et al., 2011;Kao et al., 2011;Wang et al., 2014) found differences in the phase composition of the FeNiCoCrAlx alloys (in particular, for x = 5 and 10 at.% Al) after annealing at 900, 950, and 1100°C, we conducted additional X-ray diffraction and microstructural analyses of the samples' end surfaces after complete grinding of the oxide layers to specify changes in the structure and phase composition of the alloy matrices (Fig. 5).   After annealing at 900°C for 24 h, two phases were found to form in the FeNiCoCrAl5 alloy (Lin et al., 2011): fcc and B2-ordered bcc; at 1100°C, there was only fcc phase, the phase composition of the Fe35Ni25Cr20Co20Al5 alloy remained unchanged after long-term hightemperature annealing in our case; there was only the fcc solid solution. This indicates that this alloy has high thermal stability since variations in line intensities in the X-ray diffraction pattern are not associated with structural changes but result from significant texture confirmed by the respective texture coefficient along the {200} crystallographic direction. For example, this coefficient τ was 0.2775(200) after annealing for the alloy with 5 at.% Al and 0.2553(200) for the alloy with 10 at.% Al (note that τ = 1 in the absence of texture) (Fig. 5a).

Fig. 4. X-ray diffraction patterns for the Fe35Ni25Cr20Co20Al5 (a, b, e) and Fe30Ni25Cr20Co15Al10 (c, d, f) alloys after oxidation at 900°C for 50 (a, c) and 100 h (b, d) and after oxidation at 1000°C for 50 h (e, f).
The phase composition of the Fe30Ni25Cr20Co15Al10 alloy somewhat changed (Fig. 5b): the amount of the fcc solid solution sharply decreased from 96 to 46 wt.% and the B2-ordered bcc phase constituted 54 wt.%. Compared to the starting state, the lattice parameter of the fcc phase in the FeNiCoCrAl5 alloy increased from 0.3571 to 0.3586 nm and that of the FeNiCoCrAl10 alloy conversely decreased from 0.3603 to 0.3587 nm after long-term high-temperature annealing. The lattice parameter of the bcc solid solution increased to 0.2870 nm. These changes in lattice parameters occur because of diffusion-controlled redistribution of the alloy components (Table 3) among the phase compositions. Microstructural analysis of the annealed Fe35Ni25Cr20Co20Al5 and Fe30Ni25Cr20Co15Al10 alloy matrices by scanning electron microscopy (SEM) showed that longterm annealing substantially influenced their morphology. The alloy with 5 at.% Al became homogeneous after annealing, and thus the structure of even a preliminary etched sample was not practically revealed (Fig. 6a). The size of dendrites increased and their shape changed in the Fe30Ni25Cr20Co15Al10 alloy, while a dark fine phase precipitated within light grains (Fig. 6b). The annealed alloy matrix and the distribution of elements in characteristic radiation were studied by SEM, which showed that the dendritic phase was enriched with chromium and iron, cobalt was distributed uniformly between the two phases, and nickel and aluminum were mainly in the space between dendrites and fine phase in the Fe30Ni25Cr20Co15Al10 alloy.
The mechanical characteristics of the alloys in cast state and after annealing were determined by automated indentation (Table 4). After annealing, hardness of the Fe35Ni25Cr20Co20Al5 alloy somewhat decreased to 1.8 GPa, which can be attributed to its high homogenization. The elastic modulus sharply reduced to 106 GPa since the lattice parameter of the fcc solid solution increased substantially, which in turn weakened interatomic forces and decreased E.  The hardness increased to 3.1 GPa in the Fe30Ni25Cr20Co15Al10 alloy as the B2-ordered bcc phase formed. This indicates that this alloy is a promising high-temperature oxidation-resistant material.

Summary and conclusion
A series of studies focusing on the cast and annealed alloys in the Fe40-xNiCoCrAlx system (x = 5 and 10 at.%) has established the following. The starting Fe35Ni25Cr20Co20Al5 alloy is a singlephase fcc solid solution, whose hardness is 2 GPa and elastic modulus 147 GPa. The Fe30Ni25Cr20Co15Al10 alloy is two-phase and contains fcc (96%) and bcc (4%) solid solutions. For this reason, it has higher hardness (2.5 GPa) and Young's modulus (152 GPa).
In oxidation at 900°C for 100 h, thin two-phase oxide films that contain NiCr2O4 spinel and Cr13Fe7O30 oxide form on the Fe35Ni25Cr20Co20Al5 alloy and Al2O3 and Cr13Fe7O30 oxides on the Fe30Ni25Cr20Co15Al10 alloy. Aluminum oxide in the scale promotes effective protection of the alloy against oxidation. However, when scale annealing temperature increases, Al2O3 separates and oxidation proceeds with the formation of NiCr2O4 and Cr2O3 on the surface. Despite this, the hightemperature oxidation resistance of the alloys is at the level of some hightemperature oxidationresistant alloys in the Ni-Cr-Al system. A wide continuous region of the fcc solid solution under the scale on the Fe30Ni25Cr20Co15Al10 alloy has been established to form for the first time.
Long-term high-temperature annealing substantially changes the phase composition and mechanical characteristics of the Fe30Ni25Cr20Co15Al10 alloy: the amount of the fcc solid solution sharply decreases to 46 wt.% and an ordered bcc (B2) phase forms. This substantially increases hardness: to 3.1 GPa. This indicates that the Fe30Ni25Cr20Co15Al10 alloy is a high-temperature oxidation-resistant material.