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 L1(2) nanoprecipitates was obtained in a CoCrNi-based medium entropy alloy (MEA) with chemical composition of Co34.5Cr32-Ni27.5Al3Ti3 (in at%). The volume fraction of L1(2) 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 het-erogeneous 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 in-teractions between these defects and L1(2) nanoprecipitates play important roles in the aged sample, the shearing hardening mechanism is observed for L1(2) nano-particles.

Automated summary

A dual heterogeneous structure consisting of both heterogeneous grain structure and coherent L1(2) nanoprecipitates is achieved in a CoCrNi-based medium entropy alloy. Aging leads to an increase in the volume fraction of L1(2) phase and a decrease in the corresponding interspacing, resulting in enhanced heterogeneity. Unaged samples exhibit better tensile properties compared to alloys with homogeneous and heterogeneous grain structures. Aged samples demonstrate improved strength and ductility, with hetero-deformation-induced hardening playing a crucial role, particularly at the elasto-plastic transition stage.