Strain-induced martensitic transformation near twin boundaries in a biomedical Co-Cr-Mo alloy with negative stacking fault energy

Biomedical Co-Cr-Mo (CCM) alloys have been commonly used for artificial hip and knee joint prostheses, but a need to improve their biomedical inertness and wear resistance has become widely recognized. The mechanical behavior of CCM alloys is dominated by strain-induced martensitic transformation (SIMT), which causes crack initiation during plastic deformation but dramatically enhances the wear resistance in practical use. To develop more reliable CCM alloys it is essential to clarify the factors affecting the occurrence of SIMT. In the present study we have focused on the effect of annealing twin boundaries (ATBs) on SIMT behavior. We have analyzed in detail the substructures near a parallel pair of ATBs after deformation under a stress preferential for slip parallel to the ATBs. Preferential formation of Z-hexagonal close-packed (HCP) phase at ATBs in metastable γ-face-centered cubic (FCC) phase was found by both scanning electron microscopy with electron backscattered diffraction (EBSD) analysis and transmission electron microscopy (TEM). High resolution TEM images indicated that thickening of the Z-HCP phase does not proceed regularly on every second atomic plane, which would form perfect Z-phase HCP structure, but irregularly leaving a high density of stacking faults. Furthermore, the thickness of the Z-HCP phase was found to be different at ATBs on the two sides of the twin. The difference was attributed to the internal stress due to strain incompatibility at the ATBs on the basis of residual stress analysis by the EBSD-Wilkinson method and phase-field simulation of solute segregation at ATBs.