Grain refinement mechanism and evolution of dislocation structure of Co-Cr-Mo alloy subjected to high-pressure torsion

Ultrafine-grained materials often possess superior mechanical properties owing to their small grain size. The high-pressure torsion (HPT) process is a severe plastic deformation method used to induce ultra-large strain and produce ultrafine grains. In this study, the grain refinement mechanisms in the Co-28Cr-6Mo (CCM) alloy, evolution of dislocation density as a result of HPT and its effects on mechanical properties were investigated. The dislocation density and subgrain diameter were also calculated by X-ray line profile analysis. The microstructure of the CCM alloy subjected to HPT processing (CCM_HPT) was evaluated as a function of torsional rotation number, N and equivalent strain, ϵeq. Strain-induced γ→ϵ transformation in neighboring ultrafine grains is observed in CCM_HPT processed at ϵeq= 2.25 and ϵeq= 4.5. Low-angle crystal rotation around the [110] fcc direction occurs in different locations in the same elongated grain neighboring ultrafine grains, which suggests the formation of low-angle grain boundaries in CCM_HPT processed at ϵeq= 2.25 and ϵeq= 4.5. Two possible grain refinement mechanisms are proposed. The maximum dislocation densities, which are 2.8 × 10¹⁶ m^-2 in γ phase and 3.8 × 10¹⁶ m^-2 in ϵ phase, and maximum subgrain diameters, which are 21.2 nm in γ phase and 36 nm in ϵ phase, are achieved in CCM_HPT processed at ϵeq= 9. HPT processing causes a substantial increase in the tensile strength and hardness owing to the grain refinement and a significant increase in the volume fraction of ϵ phase and dislocation density.