Effect of high-temperature heat treatment on microstructure and mechanical properties of AlxCoCrFeNi (0.5≤x≤0.8) high-entropy alloys

Abstract

AlxCoCrFeNi (0.5≤x≤0.8) high-entropy alloys were prepared by arc-melting method and the effect of 1100 ℃ heat treatment on the microstructure and mechanical properties of the alloys was investigated. The results show that the as-cast AlxCoCrFeNi (0.5≤x≤0.8) high-entropy alloys present FCC dendrite (x=0.5 and 0.6), "eutectic-like" structure (x=0.7) and BCC/B2 dendrite (x=0.8) morphologies successively as Al content increases. Correspondingly, the yield strength and tensile strength of the alloys increase from 291 MPa and 733 MPa (x=0.5) to 1004 MPa and 1423 MPa (x=0.7), respectively, and the elongation decreases from 39.7% (x=0.5) to 6.8% (x=0.7). After heat treatment at 1100 ℃, a large number of rod-like B2 phases are precipitated from the FCC dendrite region, which can improve the strength of heat-treated alloys, while the BCC/B2 spinodal structure transforms into FCC and B2 dual-phase structure, which can enhance the plasticity of heat-treated alloys. Therefore, the yield strength and tensile strength of heat-treated Al0.5CoCrFeNi alloy with FCC dendrite morphology increase to 370 MPa and 866 MPa, respectively, and the elongation decreases to 30.1%. However, both phase transition behaviors have great influence on the microstructure and property of Al0.6CoCrFeNi alloy due to the increasing fraction of spinodal structure. Therefore, the mechanical properties of heat-treated Al0.6CoCrFeNi alloy are basically unchanged in comparison to that of the as-cast alloy. The as-cast Al0.7CoCrFeNi and Al0.8CoCrFeNi alloys contain higher fractions of spinodal structure, while the heat-treated alloys present typical FCC and B2 dual-phase structure, resulting in an increase of plasticity but a decrease of strength. Correspondingly, the elongation of heat-treated Al0.7CoCrFeNi alloy increases to 14.2%, and the yield strength and tensile strength decrease to 586 MPa and 1092 MPa, respectively.

Keywords

• high-entropy alloy
• heat treatment
• phase transition
• microstructure
• mechanical property