The effect of Si addition on the heterogeneous grain structure and mechanical properties of CrCoNi medium entropy alloy

Abstract

In this study, CoCrNi MEAs added with different silicon contents (0, 0.1, 0.2 for molar ratio) were prepared by arc melting. The cold rolled and annealed microstructural evolution and tensile mechanical behavior of the alloys were systematically investigated. The results show that the distribution of elements in the alloys after homogenization annealing is uniform. As the rolling reduction increased from 20% to 60%, SEM-BSE characterization showed that the deformed microstructure of the alloys transformed from neatly aligned slip bands to tangled shear bands. TEM characterizations show that the alloys have typical cold rolling-induced microbands, dislocation cells, and high-density dislocation walls caused by plane slip after cold rolling for 60%. Also, these deformed microstructures are finer in size concerning silicon-alloyed MEAs compared to silicon-free MEAs. After annealing at 650 °C for 60 min, the deformed microstructure was replaced by a heterogeneous grain structure composed of ultrafine grains, micron-scale grains, high-density dislocation regions, and residual deformation regions. With the increase of Si content, the yield strength and tensile strength increased with the change of the heterogeneous grain structure. The tensile strength of the Si0.2 MEAs even exceeds 1.5 GPa, and still maintains a uniform elongation greater than 20% at 77 K. The evolution of the cold-rolled microstructure of silicon alloyed MEAs are mainly related to the reduction of stacking fault energy and a strong dependence on grain orientation and applied stress direction. The formation of nano-DTs and HCP phases resulted in the excellent combination of strength and ductility at 77 K. The typical heterogeneous grain structure realizes the strength–ductility synergy of the alloys.