Optimization of microstructure and mechanical properties of Co-Cr-Mo alloys by high-pressure torsion and subsequent short annealing

The main target of this study is to optimize the microstructure and to achieve an optimization for the mechanical properties in a biomedical Co-Cr-Mo (CCM) alloy with the nominal composition of Co-28Cr-6Mo (mass%) subjected to high-pressure torsion (HPT) and subsequent short annealing. The γ → ε phase transformation and grain refinement occur in the CCM alloy subjected to HPT processing at an equivalent strain (ε_eq) of 2.25 (CCM_HPT). The HPT processing causes a decrease in the elongation due to the formation of an excessive amount of ε phase. For removal of the excessive amount of ε phases, the CCM_HPT was subjected to a short annealing (CCM_HPTA). The effect of the short annealing temperature (1073 K, 1273 K, and 1473 K; annealing time was fixed at 0.3 ks) on CCM_HPT was investigated. In addition, the effect of the length of duration for the short annealing (0.06 ks, 0.3 ks, and 0.6 ks;) for a fixed annealing temperature of 1273 K on CCM_HPT was studied. CCM_{HPTA(1273 K)} annealed for 0.3 ks shows a good optimization of mechanical properties that include high strength and large elongation owing to its ultrafine-grained microstructure, and removal of excessive ε phases.