Enhanced tensile properties by heterogeneous grain structures and coherent precipitates in a CoCrNi-based medium entropy alloy

A dual heterogeneous structure with both heterogeneous grain structure and coherent L12 nanoprecipitates was obtained in a CoCrNi-based medium entropy alloy (MEA) with chemical composition of Co34.5Cr32Ni27.5Al3Ti3 (in at%). The volume fraction of L12 phase is observed to become higher and the corresponding interspacing becomes smaller after aging, resulting in a more severe heterogeneity. The unaged samples are found to have better tensile properties as compared to those for the CoCrNi MEA with homogeneous and heterogeneous grain structures. The aged samples display an even better synergy of strength and ductility than the corresponding unaged samples. The hetero-deformation-induced hardening plays a more important role in the aged samples than in the unaged samples, especially at the elasto-plastic transition stage, producing higher density of geometrically necessary dislocations for better tensile properties. Multiple deformation twins, stacking faults and Lomer-Cottrell locks are the dominant deformation mechanisms for the unaged samples, while interactions between these defects and L12 nanoprecipitates play important roles in the aged sample, the shearing hardening mechanism is observed for L12 nano-particles.