Enhancement of strength and ductility in non-equiatomic CoCrNi medium-entropy alloy at room temperature via transformation-induced plasticity

Introducing the transformation-induced plasticity (TRIP) effects into the CoCrNi medium entropy alloy (MEA) to improve the strength and ductile of the alloy is a hot issue in MEA. Herein, we developed two novels non-equiatomic CoCrNi (with Co content of 40 and 50 at. %) MEAs with TRIP effects by increasing the content of Co to lowering the stacking fault energy. It was found that the HCP phase is too tiny to be neglected in Co40 alloy, while in Co50 alloy the 2% HCP phase mainly precipitating at the interface between the annealing twin and the matrix can be found. The tensile test results show that Co40 alloy exhibits a simultaneously improved yield strength (YS) and ultimate tensile strength (UTS) without sacrificing plasticity, while for Co50 alloy, the sample shows significantly increased total elongation (TE) and UTS. The microstructure evolution of Co50 alloy during the deformation process was systematically studied by interrupting the tensile test under different strains. This work proposed that non-equiatomic MEA with more desirable mechanical properties than the equiatomic one can be obtained via introducing TRIP effects.