Phases, microstructure and mechanical properties of Al sub(x)CoCrFeNi high-entropy alloys at elevated temperatures

The Al sub(x)-Co-Cr-Fe-Ni high-entropy alloy system (x = 0-1.8 in molar ratio) was prepared by vacuum arc melting and casting method. Variations of temperature on crystal structure, microstructure and mechanical properties were investigated. The evolution of structure with temperature can be classified into four types: Al sub(0)-Al sub(0.3): FCC structure; Al sub(0.5)-Al sub(0.7): mixed structure (FCC + spinodal A2 + B2) arrow right FCC + B2 structure; Al sub(0.9)-Al sub(1.2): spinodal A2 + B2 structure (=873 K) arrow right FCC + B2 structure (>=1235 K); and Al sub(1.5)-Al sub(1.8): spinodal A2 + B2 structure arrow right B2 structure. The hot hardness transition temperature (T sub(T)) range of this alloy system was at 810-930 K. The Al sub(0.5) alloy exhibited the highest T sub(T)/T sub(m) value. Above T sub(T), the Al sub(0) and Al sub(0.3) alloys possessed the highest softening coefficient and the Al sub(0.9) and Al sub(1.0) alloys exhibited the maximum softening coefficient amongst the Al sub(x) alloys. Differences of constituent phases, phase distribution and morphology could account for the softening difference. The mechanism for high softening resistance was also discussed.