*3.1. Microstructure and Phase Analysis*

Figure 1 shows the XRD scans of the Al0.3CoFeCrNi (A1) and Al0.3CuFeCrNi2 (A2) HEAs. It is evident there is one set of fundamental FCC reflections that verifies an FCC solid solution crystal structure without the presence of secondary phases. Using the indexed (111) reflections, the lattice constants (*a*) for A1 and A2 alloys are calculated to be 3.591 Å and 3.589 Å, respectively. Based on the resolution of the x-ray diffractometer, these values are within experimental error, and thus there are no lattice parameter differences between the two single FCC-phase HEAs. These HEAs retain a single phase at higher temperatures when cooled rapidly due to the entropy of too many alloying elements, along with sluggish diffusion kinetics.

Figure 2 and Table 1 show representative SEM images and corresponding EDS chemical analysis, respectively, of the A1 and A2 alloys. The alloys exhibit microcrystalline structures with elongated grains (sizes in the range of 300–600 μm) and have no compositional segregation. Wang et al. [8] also reported that as-cast Al0.3CoCrFeNi HEA has an FCC single phase with a columnar microstructure. Similarly, Kao et al. [17] determined that Al0.37CoCrFeNi has an FCC single-phase crystal structure, also shown by Guo et al. [18] for AlxCrCuFeNi2 HEAs. The latter authors also determined that Al0.5CrCuFeNi2 alloy exhibits a dendritic microstructure. The SEM images in Figure 2 show evidence of possible interdendritic and dendritic regions retaining an FCC phase with high closeness in the lattice constants, also reported in [19,20]. Ng et al. [21] showed that a Al0.5CrCuFeNi2 alloy

exhibits an FCC single phase, but due to XRD peak overlaps, they surmised there were two disordered FCC phases with very close lattice parameters at ~3.59 Å, which is equivalent to the above calculated lattice parameters for A1 and A2 alloys. Gwalani et al. [22] determined with transmission electron microscopy and selected area diffraction patterns that Al0.3CuFeCrNi2 (with the same composition as A2 alloy) has an FCC-type solid solution at ambient temperature. They further stated the high mixing entropy and sluggish effects that decrease the Gibbs energy develop a solid solution rather than intermetallic compounds, and as a result were surmised to be the reasons for the formation of a single solid solution instead of intermetallic compounds [22]. From a thermodynamic point of view, the mixing enthalpy overcomes and leads to decomposition in the matrix to form two phases.

**Figure 1.** XRD patterns of A1 and A2 alloys showing FCC reflections.

**Figure 2.** SEM images of the A1 (**left**) and A2 (**right**) alloys.


**Table 1.** Chemical composition (in at%) of A1 and A2 alloys.

#### *3.2. Microhardness*

It is crucial to assess the hardness of the material in order to understand the likely underlying wear process, since material hardness is often correlated to wear resistance [23]. Table 2 lists the averaged microhardness values for the A1 and A2 alloys at room temperature (RT) and 300 ◦C (acquired after the sliding wear tests and outside the wear tracks). Both alloys exhibit similar RT hardness values. However, at 300 ◦C, there is thermal softening in A1 and hardening in A2 that could be due to Ni content differences, resulting in the likely formation of a harder NiO scale on the A2 alloy. In addition, Qiu et al. [24] determined that adding more Ni to Al2CrFeCoCuTiNix HEAs resulted in increasing RT microhardness and strength. They attributed this increase to the Ni content that increased the content of the BCC crystal structure in the alloys. In contrast, López Ríos et al. [25] reported that because of Cr and Fe precipitates dissolving in the nickel-rich matrix and forming a stable solid solution, the hardness values for AlCrFeCoNi decreased to 562 HV, 455 HV, and 316 HV, with reduction in nickel concentration. Kuo et al. [26] determined that a CuFeTiZrNi0.1 alloy exhibits a microhardness of 935 HV, wherein increasing the FCC phase in the alloy is correlated to an increase in the Ni content. Therefore, they concluded that the low hardness of the FCC phase causes the alloy's hardness to gradually decrease with increasing Ni content; however, the hardness was not measured at elevated temperatures, e.g., at 300 ◦C, where the effects of oxide scales and thermal softening can influence the microhardness values.

**Table 2.** Average microhardness (HV1) values and standard deviations of A1 and A2 alloys at RT and 300 ◦C.

