Microstructure and oxidation behavior of CoCrxCuFeMnNi high-entropy alloys fabricated by vacuum hot-pressing sintering

Herein, CoCrxCuFeMnNi (x = 0, 0.5, 1.0, 1.5, and 2.0, in molar ratio) high-entropy alloys (HEAs) are fabricated by vacuum hot-pressing sintering (VHPS). The effect of Cr content on the microstructure and oxidation behavior are studied. When x ≤ 1.5 mol, the phases of the four alloys were all composed of FCC2 major phase and FCC1 secondary phase, while Cr2.0 alloy consisted of a small amount of FCC1 phase and ρ phase in addition to FCC2 main phase. The elemental segregation increased with the increase of Cr content. Cr2.0 alloy exhibited the lowest oxidation rate constants in the oxidation stage and the slow oxidation stage, which were 2.29 × 10-11 and 3.46 × 10-12 g2 cm-4 s-1, respectively, showing the best oxidation resistance. The oxidation products of CoCrxCuFeMnNi HEA system were mainly Mn4O3, Mn3O2, Cr2O3 and (M,Cr)3O4-type spinel oxides. The oxidation mechanism is mainly selective oxidation, that is, the outward diffusion of metal cations and the inward diffusion of oxygen ions. The oxidation resistance of the Cr-rich FCC1 and ρ phases is better than that of the copper-rich FCC2 phase.