Behaviour of Nickel-Rich Non-Equimolar High Entropy Alloys in High-Temperature Oxidizing Conditions

Abstract

This study aims to compare the behaviour of two Ni-rich non-equimolar AlCoCrFeNi high entropy alloys, i.e. Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ (at%) in isothermal high-temperature oxidizing conditions. In both cases, mass gain after 100-hr oxidation at 1173 K in synthetic air atmosphere does not exceed 1 mg/cm², indicating good resistance against the corrosive environment. Investigations on the morphology, chemical and phase composition of the alloys after the oxidation process indicate that a mixture of Al₂O₃ and Cr₂O₃ is responsible for the protective properties of the scale formed on Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅, whereas an additional chromium–iron–nickel–cobalt spinel structure was determined on the Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ sample after prolonged exposure to the above-mentioned conditions. The oxidation kinetics are slightly better in the case of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and lower amounts of the remaining constituent elements were detected in the protective scale compared to Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅. Furthermore, the Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ substrate was able to maintain its initial morphology throughout the entire alloy after the corrosion process. From all of the above, it can be concluded that Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ seems to demonstrate better oxidation properties at a high temperature than Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅.

Fig. 9 XRD patterns obtained from (a) Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and (b) Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ after oxidation at 1173 K in synthetic air for 100 h.

1. Introduction

High entropy alloys (HEAs) were initially introduced to the scientific world in 2004 by Yeh et al.¹⁾ and Cantor et al.²⁾ These pioneer works showed that metallic elements mixed together in concentrations ranging between 5 and 35 at% are capable of forming one or more solid solution phases, i.e. face centred cubic (FCC) and body centred cubic (BCC) crystalline structures.^1,2) As a result, these materials exhibit extraordinary mechanical properties. HEAs built of an FCC phase demonstrate remarkable ductility,^3–9) and those consisting of a BCC structure are very hard and have good compressive yield strength.^1,7,8,10,11) A high entropy alloy containing both phases will exhibit a compromise between the two properties; its hardness and ductility will be dependent on the respective amounts of the FCC and BCC structures constituting the material.^6–8,11) HEAs with both phases are desirable, because BCC-based high entropy alloys are often brittle,^7,8) whereas FCC-structured HEAs exhibit inferior hardness and yield strength.^3,4,6–8)

The above-mentioned combination of hardness, compressive strength and ductility has led to interest in these alloys as potential alternatives to commonly used alloys, e.g. those currently used in engines and turbines. However, certain properties of these materials must be taken into account before they can be used in high-temperature conditions. First of all, the structure of a given HEA can change during exposure to elevated temperatures, i.e. solid solution and/or intermetallic phases can form inside the alloy.^12–18) Second, the effects of high-temperature corrosion on the materials must be considered. It has been determined that Al_xCoCrFeNi high entropy alloys demonstrate promising oxidation resistance at high temperatures.^19–22) However, as-cast Al_xCoCrFeNi alloys with Al content above 18.4 at% only consist of ordered (B2) and disordered BCC structures at room temperature,^15,19,23) and form an undesired intermetallic sigma phase above 873 K.^14,15) Both an FCC phase and BCC+B2 spinodal structures, where the BCC and B2 phases demonstrate a cube-on-cube orientation relationship and the same lattice constant, can be obtained inside Al_xCoCrFeNi systems at Al concentrations between 11.1 and 18.4 at%.^15,19,23) Unfortunately, this also leads to inferior high-temperature oxidation kinetics.^19,22)

The above-mentioned lack of phase stability and necessity for lowering Al content in Al_xCoCrFeNi high entropy systems has led to research on alternative methods of improving these alloys. Studies have shown that the volume fraction of the FCC phase in AlCoCrFeNi systems increases along with Ni content¹¹⁾ and Co concentration.^24,25) Furthermore, so-called high entropy superalloys with Ni concentration exceeding the limit provided in the traditional HEA definition, i.e. >35 at%, are capable of maintaining at high temperatures a structure consisting of an FCC phase and intermetallic γ′ precipitates, which have an L1₂ structure and contribute to the alloy strength.^26–29) In this work, the decision was made to synthesize non-equimolar high entropy alloys with 35 at% Ni, as well as Al and Co content selected in amounts that should ensure the presence of both the FCC and BCC structures found in Al_xCoCrFeNi HEAs (x = 11.1–18.4 at%).

The goal of this work was to determine the phase compositions and compare the high-temperature oxidation resistance of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ (at%) high entropy alloys at 1173 K. Initial investigations on the effects of the corrosive conditions on their phase and chemical compositions, as well as their morphologies, were also carried out.

2. Materials and Methods

Elements (Al, Co, Cr, Fe, Ni) of >99.90 mass% purity were mixed together in atomic ratios selected in order to ensure obtaining alloys with the nominal compositions listed in Table 1 by means of arc melting in an induction melting furnace with a water-cooled copper plate under protective argon atmosphere. As a getter, high purity titanium was applied. To ensure proper mixing of the starting materials, the alloys were re-melted three times. Subsequently, the obtained alloys were annealed at 1273 K for 24 h in vacuum conditions in order to ensure a homogeneous distribution of elements in the metallic matrix and obtain phase equilibrium.

Table 1 Designation and nominal chemical composition of the samples investigated in this work.

A precision saw was then used to prepare samples in the shape of discs with ∼1.5 cm diameter and ∼1.5 mm thickness. These discs were then ground with SiC paper with gradations up to 1000, polished to mirror shine using diamond pastes (9, 3 and then 1 µm) and subsequently ultrasonically cleaned in ethanol. After these preparations, the samples were subjected to isothermal oxidation at 1173 K in synthetic air atmosphere for 100 h. The surface morphology, as well as the chemical and phase composition of the materials, before and after exposure to the aforementioned corrosive conditions were investigated by means of scanning electron microscopy (SEM; FEI NOVA NanoSEM200 Scanning Electron Microscope) combined with energy dispersive X-ray spectroscopy (EDS; EDAX Genesis XM 2), and X-ray diffraction (XRD; Philips PW 1410 diffractometer) using CoKα filtered radiation. The oxidized HEAs were also prepared for further analysis by mounting the samples in resin and subsequently cutting them in order to obtain cross-sections. After polishing the mounted samples using diamond pastes (9, 3 and then 1 µm), the cross-sections were observed via SEM and chemical compositions were determined by EDS point analysis.

3. Results and Discussion

The XRD patterns obtained from the studied high entropy alloys before oxidation and after 24-hr annealing at 1273 K are illustrated in Fig. 1. The peaks observed in the Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ pattern indicates that the material contains both an FCC and BCC phase. The (100) peak pertaining to a B2 structure is also noticed. The phase composition results obtained for this alloy are in accordance with those presented in literature where the Al content in an Al_xCoCrFeNi HEA is between 11.1 and 18.4 at%.^15,19,23) As for Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅, the results also confirm that FCC and BCC phases constitute this HEA without any undesired intermetallic structures. Furthermore, both the (100) and (111) planes of the B2 structure are observed for this sample, strongly indicating the presence of the phase in this alloy as well. In both cases, the results demonstrate that FCC/BCC structures can be obtained from these AlCoCrFeNi alloy compositions after proper preparation and heat treatment.

Fig. 1

XRD patterns obtained from (a) Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and (b) Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ high entropy alloys before oxidation and after heat treatment at 1273 K for 24 h in vacuum conditions.

The presence of the FCC and BCC phases result in a dendritic microstructure consisting of lighter and darker areas, as clearly seen in the case of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ (Fig. 2(a)). On the other hand, Fig. 2(b) illustrates more non-continuous interdendrite regions constituting the darker areas at certain locations in the Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ alloy. Additional EDS point analysis of both the lighter and darker regions of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ are illustrated in Fig. 2(c) and Fig. 2(d), respectively. These studies indicate that the lighter areas contains higher amounts of Cr, Fe and Co compared to the darker regions, which consist of more Al and Ni content. This is in accordance with literature, where it is stated that the B2 structure is a Ni+Al-rich phase.^15,19) However, significant Cr and Fe content was still detected at the darker area, which is characteristic for the BCC phase.^15,19)

Fig. 2

SEM image obtained from the surface of (a) Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and (b) Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ high entropy alloys, and EDS spectra obtained from (c) point 1 and (d) point 2 of Fig. 2(a).

The mass gain of the samples during oxidation at 1173 K in synthetic air is presented in Fig. 3. This figure illustrates that both samples demonstrated weight gain that did not exceed 1 mg/cm² after 100 h. This is promising when taking into consideration that the mass gains of, e.g., the AlCoCrFeNi-containing high entropy superalloys researched by Tsao et al. at that temperature were higher than the previously mentioned value after 100 h of oxidation,²⁷⁾ as was the weight change of the AlCoCrFeNi alloy at 1173 K oxidized by Zhu et al. for 100 h.²⁰⁾ This suggests that the HEAs researched in this work exhibit better long-term oxidation kinetics than those materials.

Fig. 3

Mass gain per unit area of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ high entropy alloys as a function of time during oxidation under isothermal conditions at 1173 K in synthetic air atmosphere for 100 h.

SEM images obtained from the surface of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ after oxidation at 1173 K are illustrated in Fig. 4(a) and Fig. 4(b). These microphotographs show that the surface of the scale grown during prolonged exposure to the oxidizing conditions consists of both a phase built of whiskers and a polycrystalline microstructure built of grains. EDS point analysis performed at the locations designated in Fig. 4(b) and presented in Fig. 4(c) and Fig. 4(d) indicates that both phases contain high amounts of aluminium and oxygen. However, significant amounts of the remaining elements were also detected in the grains. On the other hand, three different morphologies can be seen on the surface of oxidized Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ (Fig. 5(a)). Relatively large grains form polycrystalline round shapes create larger round formations, another oxide built of smaller grains is observed and a whisker microstructure is also visible. The chemical compositions of these different microstructures are presented in Fig. 5(b), Fig. 5(c) and Fig. 5(d), respectively. From this analysis it follows that the larger grains mostly consist of chromium and oxygen with noticeable amounts of the remaining elements (Fig. 5(b)). The smaller grains mainly contain chromium, aluminium and oxygen (Fig. 5(c)), whereas large amounts of aluminium and oxygen, along with smaller amounts of chromium, were detected in the case of the whiskers (Fig. 5(d)).

Fig. 4

(a), (b) SEM images obtained from the surface of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ at different magnifications after oxidation at 1173 K for 100 h in synthetic air, and EDS spectra obtained from (c) point 1 and (d) point 2 of Fig. 4(b).

Fig. 5

(a) SEM image obtained from the surface of Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ after oxidation at 1173 K for 100 h in synthetic air, and EDS spectra obtained from (b) point 1, (c) point 2 and (d) point 3 of Fig. 5(a).

More information on the scales grown during oxidation of the studied HEAs is provided by SEM-EDS cross-section analysis. The oxidized Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ cross-section is illustrated at different magnifications in Fig. 6(a) and Fig. 6(b), whereas Fig. 6(c) and Fig. 6(d) present the cross-section of Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ after 100-hr oxidation. The numbers on Fig. 6(b) and Fig. 6(d) designate the points at which EDS analysis was carried out, the results of which are listed in Table 2. From Fig. 6(a) and Fig. 6(b) it follows that the formed scale is mostly uniform with some amounts of another phase inside. Below that, a dark phase can be seen at the scale/substrate interface, which, in turn, is in contact with a light microstructure near the interface. The images also suggest that the metallic substrate maintained both FCC and BCC structures. On the other hand, an outer, intermediate and inner phase can be observed in the scale formed on Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅. Furthermore, a dendritic microstructure is no longer seen near the scale/substrate interface, only at greater depths inside the alloy, as seen in Fig. 6(c). Instead, only one main phase is visible, along with shapes formed from a darker phase, which can be found at certain locations.

Fig. 6

SEM cross-section images of (a), (b) Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and (c), (d) Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ at different magnifications after oxidation at 1173 K for 100 h in synthetic air. The numbers designate the locations at which EDS analysis was performed.

Table 2 Chemical compositions obtained by means of EDS point analysis performed on Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ cross-sections after oxidation at 1173 K in synthetic air for 100 h.

Table 2 demonstrates that at point 1 of Fig. 6(b) the scale primarily contains Al and O, whereas at point 2 larger amounts of Cr are detected. Point 3, on the other hand, consists of Al with practically no oxygen and only minor amounts of the other constituent elements. Points 4 and 5 confirm that the light phase contains mostly Cr, Fe and Ni, as well as relatively high Co content, similar to the chemical composition presented in Fig. 2(c). On the other hand, the darker phase in the substrate has large amounts of Ni and Al, characteristic for the B2 phase.^15,19) As for Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ (Fig. 6(d)), chemical composition analysis demonstrates that the outer oxide layer consists of chromium and oxygen, along with significant amounts of iron, nickel and cobalt, whereas the intermediate layer primarily contains only Cr and O, suggesting Cr₂O₃ growth. As for the inner layer, mostly aluminium and oxygen can be found there, indicating the formation of Al₂O₃. The main phase below the scale/substrate interface is built of the constituent HEA elements with relatively low amounts of Al. Greater Al content can be found in the previously mentioned darker shapes inside the metallic substrate.

The growth of an Al₂O₃ inner layer during high-temperature oxidation is in accordance with results obtained from Al_xCoCrFeNi high entropy alloys,¹⁹⁾ as well as some Ni-rich high entropy superalloys with similar Al content.²⁷⁾ An explanation for Al₂O₃ constituting the inner layer as opposed to being more prevalent throughout the scale in the case of Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ was provided by Tsao et al.,²⁷⁾ who determined that Cr demonstrates higher activity values in the presence of oxygen at high temperatures than Al in similar high entropy compositions, leading to the material favouring Cr₂O₃ formation over Al₂O₃ growth. Before the oxidation process, Al was one of the elements that constituted the B2 phase inside the Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ material determined in Fig. 1(b). The reaction between Al and oxygen, and the consequent loss of aluminium inside the alloy leads to the creation of an Al-depleted zone inside the metallic material below the substrate/scale interface.¹⁹⁾ As a result of the lower Al content, the B2 phase decomposes below the interface and the original microstructure is no longer seen up to a certain depth and only one main phase consisting of the remaining elements is observed. However, the remaining relatively minor amounts of Al near the interface that did not react with oxygen appear to have segregated to certain locations inside the Al-depleted zone forming, along with the other constituent elements, additional shapes inside the substrate. Conversely, the Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ sample was able to maintain its initial microstructure throughout the substrate. It should be noted that the exact diffusion processes, which take place inside these alloys during oxidation are currently unknown and further long-term investigations are required in order to completely understand the behaviour of the elements inside these materials in high-temperature oxidizing conditions.

Additional SEM combined with EDS area scanning and point analysis was performed at different locations on the oxidized Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ alloy cross-sections in order to better understand the oxidation behaviour of the materials. Figure 7 illustrates a SEM image obtained from Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ along with the areas and points selected for EDS studies. This SEM microphotograph is similar to that shown in Fig. 6(b) and illustrates a mostly uniform scale formed on a substrate built of light and darker microstructures.

Fig. 7

SEM cross-section image of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ after oxidation at 1173 K for 100 h in synthetic air. The numbers designate the selected area and the points at which EDS analysis was performed.

On the other hand, SEM images of Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ after oxidation at 1173 K along with areas and points chosen for EDS analysis are shown in Fig. 8(a) and Fig. 8(b), respectively. These pictures demonstrate that at some locations an additional light phase can be found above the inner layer of the scale. More dark formations are also observed in the substrate around the phase in contact with the inner layer.

Fig. 8

SEM cross-section images of Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ after oxidation at 1173 K for 100 h in synthetic air, on which EDS of (a) selected areas and (b) points designated in the pictures were performed.

The chemical compositions obtained from the EDS analysis performed on Fig. 7, Fig. 8(a) and Fig. 8(b) are listed in Table 3. The results of EDS area scanning confirm that the scale grown on Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ (Fig. 7) mainly consists of Al and O with lower Cr content. At point 2, large amounts of carbon were detected as the analysis was performed very close to the resin, in which the sample was mounted before cross-section preparation. Only very minor amounts of the remaining constituent elements were detected in the scale. Additionally, the results obtained at point 3 suggest that some AlN forms between the oxide scale and the light microstructure. Aluminium nitride formation in such alloys during prolonged exposure to air at high temperatures is in accordance with the results obtained for AlCoCrFeNi systems in previous works.^19,22,30) However, in those cases, aluminium and nitrogen-containing precipitates were found deeper inside the substrate. Not much is currently known concerning the exact mechanism of internal nitridation at high temperatures in such systems and the reason for AlN being located at the scale/substrate interface in the case of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ as opposed to the substrate interior is currently unknown. Further more complex long-term investigations will be necessary in order to provide a satisfactory explanation for this.

Table 3 Chemical compositions obtained by means of EDS selected area and point analysis performed on Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ cross-sections after oxidation at 1173 K in synthetic air for 100 h.

EDS analysis performed at area 1 on Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ (Fig. 8(a)) also confirms that the average chemical composition of the scale located above the light phase contains mostly chromium. Significant amounts of Co, Fe and Ni, presumably from the outer oxide layer determined from the EDS point analysis on Fig. 6(d) presented in Table 2, were also determined. Chemical composition studies of area 2, which is mostly centred around the additional light phase, indicate higher amounts of Al, Co, Fe and Ni, suggesting the presence of several of the constituent elements in this light microstructure. More precise results obtained from EDS point analysis on Fig. 8(b) reveal that this phase is similar to that of the FCC structure inside the substrate and consists of significant amounts of all the constituent elements with the exception of Al. Beneath the above-mentioned light microstructure, an Al₂O₃ layer was, once again, determined. However, the chemical composition obtained from a dark shape located at greater depths suggests that aluminium nitride precipitates form at certain places inside the substrate. These findings concerning internal AlN formation are more in accordance with the results presented in literature^19,22,30) than the previous case.

The phase compositions of the materials after 100-hr oxidation at 1173 K were determined from the XRD patterns illustrated in Fig. 9. From this figure it follows that the scale formed on Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ is primarily built of Al₂O₃ with some Cr₂O₃. The diffraction pattern presented in Fig. 9(a) also confirms the presence of FCC and B2 phases in the substrate without any additional solid solution or intermetallic phase formation. However, Fig. 9(b) demonstrates that the scale grown on Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ also contains a Cr–Fe–Co–Ni spinel structure. The presence of BCC phases could also not be detected, as the XRD analysis was performed on the sample surface and the dendrite microstructure is located deep within the substrate. From the combined SEM-EDS and XRD studies on Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅, it can be concluded that the outer layer is built of the previously-mentioned spinel structure, the intermediate layer is Cr₂O₃, and the inner layer is built of Al₂O₃. It is interesting to notice that the phases detected in Fig. 9(b) are similar to those obtained for Al_x(CoCrFeNi)_100−x (x = 9 and 12) HEAs after cyclic oxidation.²²⁾

Fig. 9

XRD patterns obtained from (a) Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and (b) Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ after oxidation at 1173 K in synthetic air for 100 h.

By comparing the results obtained for the two studied alloys, it can be seen that the HEA with higher Al content is more capable of maintaining its original microstructure during prolonged exposure to high-temperature oxidizing conditions. Furthermore, the oxidation process is more selective towards Al in the case of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ compared to Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ as lower amounts of the other constituent elements take part in growing a protective scale compared to the case of Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅, as the spinel structure is not determined in the former 20 at% Al alloy. Moreover, aluminium contributes throughout the scale in the case of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅, as opposed to Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ where Al₂O₃ constitutes the inner scale. This is interesting as Al₂O₃ was determined as the inner layer after prolonged high-temperature oxidation of equimolar AlCoCrFeNi.¹⁹⁾ As of now, a theoretical model to accurately predict the formation of different oxides in the scale of such a complex alloy system has not yet been fully developed. However, a possible explanation for the large presence of Al in the entire scale in the case of the Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ alloy is the Al concentration being above a certain critical value necessary for the formation of continuous Al₂O₃ throughout the scale. Another is the assumption that the diffusivity of that element is higher in an Ni-rich high entropy alloy than in near equimolar AlCoCrFeNi high entropy alloys, where Al₂O₃ grows only in the inner layer.¹⁹⁾ Further evidence of this can be found in the work of Kai et al.³⁰⁾ on a similar high entropy alloy composition. The exact diffusivity value has not been determined, however, the diffusivity of Al in Ni₃Al can be used as a point of reference. In the case of a binary Ni–Al system, the critical Al content can be calculated as 16.5 at%.^30–32) It is possible that the critical value in this case is similar, however, as mentioned in the Introduction section of this manuscript, these studies are only the initial investigations of the Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ high entropy alloys and more detailed long-term research is necessary to completely determine the oxidation mechanism. As for the lack of the spinel phase and less significant Cr₂O₃ growth in the case of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅, it can be assumed that the formation of a thicker continuous Al₂O₃ layer hinders the outward diffusion of the remaining elements and their subsequent reaction with oxygen. However, again, further studies are required to confirm this. It can also be noticed that, in both cases, an intermetallic phase did not form, in contrast to cases involving similar high entropy alloys exposed to elevated temperatures.^12–17) Taking into account these interesting results, further more detailed investigations on the alloy structures are planned for future studies.

4. Conclusions

From all the obtained results, the following can be concluded:

1. (1)    Both Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ high entropy alloys are capable of forming protective scales that ensure relatively low mass gain, even after long oxidation durations.
2. (2)    The scale grown on Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ consists of Al₂O₃ and Cr₂O₃, whereas a multilayer scale forms on Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ containing a chromium–iron–nickel–cobalt spinel structure, Cr₂O₃ and Al₂O₃.
3. (3)    Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ is more capable of maintaining its dendritic morphology and subsequently both FCC and BCC structures throughout the material, even after prolonged exposure to high-temperature oxidizing conditions, compared to Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅.
4. (4)    Both high entropy alloys demonstrate promising behaviour in high-temperature oxidizing conditions, however, the Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ material exhibits better oxidation properties.

Acknowledgements

Financial support for these studies was provided by the National Science Centre (NCN) in Krakow, Poland under grant agreement no. 18.18.160.08130 as part of the MINIATURA-4 project no. 2020/04/X/ST8/01238 titled “Corrosion resistance and structural properties of non-equimolar AlCoCrFeNi alloys in high-temperature oxidizing conditions”.

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