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 (CCMHPT) 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 CCMHPT processed at εeq = 2.25 and εeq = 4.5. Low-angle crystal rotation around the  fcc direction occurs in different locations in the same elongated grain neighboring ultrafine grains, which suggests the formation of low-angle grain boundaries in CCMHPT 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 CCMHPT 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.
2016 The Japan Institute of Metals and Materials