Effect of intermetallic compounds on the high-temperature oxidation resistance of Al0.25CoCrFeNiCuAu (x = 0, 0.1, 0.3) high-entropy alloys

In this work, the effect of gold alloying on the microstructure, phase composition, and high-temperature oxidation resistance of Al0.25CoCrFeNiCu high-entropy alloy (HEA) was studied. It is shown that gold is partly incorporated in the dendritic face-centered cubic (fcc) phase, and it is mainly precipitated as intermetallic compounds (ICs) in the interdendritic space. Al2Au and AuCu3 ICs could be precipitated depending on the concentration of gold. The formation of Al2Au ICs increased the parabolic oxidation rate constant (kp) from 378.5 × 10–13 (g2/cm4s) for Al0.25CoCrFeNiCu to 1079.5 × 10–13 (g2/cm4s) for Al0.25CoCrFeNiCuAu0.1 HEA, while the precipitation of AuCu3 in Al0.25CoCrFeNiCuAu0.3 HEA reduced this index to kp = 94.9 × 10–13 (g2/cm4s). Microstructural analysis of the oxidized samples revealed that binding of aluminum into Al2Au IC decreases the amount of available aluminum in the solid solution phase to form a protective Al2O3 oxide layer on the surface of Al0.25CoCrFeNiCuAu0.1 alloy during high-temperature oxidation. On the contrary, the binding of copper with gold into AuCu3 IC effectively prevents Cu from oxidation and provides routes for the formation of a dense layer of Al2O3 oxide with superior protective properties.