Phases, microstructure and mechanical properties of Al_xCoCrFeNi high-entropy alloys at elevated temperatures

摘要

The Al_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_0-Al_(0.3): FCC structure; Al_(0.5)-Al_(0.7): mixed structure (FCC + spinodal A2 + B2) → FCC + B2 structure; Al_(0.9)-Al_(1.2): spinodal A2 + B2 structure (<873 K) → FCC + σ + B2 structure (≥873 K) → FCC + B2 structure (≥ 1235 K); and Al_(1.5)-Al_(1.8): spinodal A2 + B2 structure → B2 structure. The hot hardness transition temperature (T_T) range of this alloy system was at 810-930 K. The Al_(0.5) alloy exhibited the highest T_T/ T_m value. Above T_T, the Al_0 and Al_(0.3) alloys possessed the highest softening coefficient and the Al_(0.9) and Al_(1.0) alloys exhibited the maximum softening coefficient amongst the Al_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.