Microstructural evolution and mechanical properties of biomedical Co-Cr-Mo alloy subjected to high-pressure torsion

The effects of severe plastic deformation through high-pressure torsion (HPT) on the microstructure and tensile properties of a biomedical Co-Cr-Mo (CCM) alloy were investigated. The microstructure was examined as a function of torsional rotation number, N and equivalent strain, ε_eq in the HPT processing. Electron backscatter diffraction analysis (EBSD) shows that a strain-induced martensitic transformation occurs by the HPT processing. Grain diameter decreases with increasing ε_eq, and the HPT-processed alloy (CCM_HPT) for ε_eq=45 exhibits an average grain diameter of 47nm, compared to 70μm for the CCM alloy before HPT processing. Blurred and wavy grain boundaries with low-angle of misorientation in the CCM_HPT sample for ε_eq<45 become better-defined grain boundaries with high-angle of misorientation after HPT processing for ε_eq=45. Kernel average misorientation (KAM) maps from EBSD indicate that KAM inside grains increases with ε_eq for ε_eq<45, and then decreases for ε_eq=45. The volume fraction of the ε (hcp) phase in the CCM_HPT samples slightly increases at ε_eq=9, and decreases at ε_eq=45. In addition, the strength of the CCM_HPT samples increases at ε_eq=9, and then decrease at ε_eq=45. The decrease in the strength is attributed to the decrease in the volume fraction of ε phase, annihilation of dislocations, and decrease in strain in the CCM_HPT sample processed at ε_eq=45 by HPT.