The cyclic deterioration mechanisms of Al_xCoCrFeNi high-entropy alloys

This study delves into the cyclic deterioration mechanisms of Al_xCoCrFeNi high-entropy alloys (HEAs) under cyclic nanoindentation by examining the evolution of mechanical properties, surface morphology and microstructures. With the increase of aluminum content, different deformation mechanisms are found responsible for the cyclic softening deterioration behavior and the hardness enhancement in these HEAs. For Al_0.1CoCrFeNi and Al_0.3CoCrFeNi HEAs, the cyclic softening behavior is attributed to dislocation planar slipping, stacking faults, deformation twinning and phase transformations from FCC phase to BCC and HCP phases. Al_0.5CoCrFeNi HEA exhibits compound regional deterioration, with the BCC-phase region dominated by dislocation-dominated plasticity and the FCC-phase region dominated by the synergistic effects of stacking faults and deformation twinning, which undergoes phase transformations from FCC and BCC phases to HCP phase. The cyclic deterioration in AlCoCrFeNi HEA involves dislocation-dominated plasticity, stacking faults and phase transformation from BCC phase to HCP phase. Moreover, slip lines and pile-ups are observed only in Al_0.1CoCrFeNi HEA due to the low hardness and Al_0.5CoCrFeNi HEA due to incompatible deformation caused by dual-phase microstructure. The critical role of aluminum content in governing phase composition, plastic deformation mechanisms and mechanical properties is highlighted, with profound implications for designing high-performance HEAs.