Microstructure transition and mechanical property variation of Al_0.6CoCrFeNi high-entropy alloys prepared under different cooling rates

Cooling rate is attracting considerable attention because of its significant effect on the phase stability, switchable properties, and elements' diffusion mechanism in alloys, especially in high-entropy alloys (HEAs) with metastable structure. In this study, the microstructure transition and mechanical property variation of Al_0.6CoCrFeNi HEAs with a change in the cooling rate were investigated in detail. Five cooling rates of ∼2.5 K s^-1 (I_0.6 alloy), 40 K s^-1 (C_0.6-10 alloy), 2.5 × 10² K s^-1 (C_0.6-4 alloy), 10³ K s^-1 (C_0.6-2 alloy) and ∼10⁵ K s^-1 (R_0.6 alloy) were obtained by adjusting the prepared methods and alloy sizes. The structure of Al_0.6CoCrFeNi HEAs changed from an FCC + BCC dual-phase (I_0.6 alloy) to a single BCC phase (R_0.6 alloy) with an increase in the cooling rate, and the microstructure of alloys evolved from a columnar dendrite (I_0.6 alloy) to a lamellar morphology (C_0.6-2 alloy), and finally to an equiaxed grain microstructure (R_0.6 alloy). Both the hardness and yield strength of alloys exhibited first a minor and then a considerable increase with an increase in the cooling rate, which is mainly ascribed to microstructure and/or phase structure transitions. The R_0.6 alloy displayed the highest hardness (501 HV) and yield strength (1530 MPa) in five alloys and had the least plasticity. Our findings may help to explore HEAs with excellent mechanical properties and provide support for HEAs' industrialization.