Effect of carbon addition on microstructure and mechanical properties of a wrought Co-Cr-Mo implant alloy

The wrought Co-Cr-Mo alloys with C contents of 0.02. 0.09 and 0.18% (mass%) were fabricated by hot-forging process to study the influence of carbon contents on the microstructures and mechanical properties. The microstructures of Co-29Cr-6Mo-0.02C and Co-29Cr-6Mo-0.09C consist of equiaxed uniform grains which contain stacking faults, twins and ε martensite bands. No carbide found at inter- and intragranular region. Co-29Cr-6Mo-0.18C consists of irregular grain sizes and carbide found at inter- and intra-granular region. The carbide in Co-29Cr-6Mo-0.18C was identified as M₂₃C₆, type carbide from the XRD pattern analysis. It is found that the amount of stacking fault and ε martensite are strongly dependent upon the C content. The density of stacking faults and ε martensites observed in Co-29Cr-6Mo-0.09C decrease as compared with those observed in Co-29Cr-6Mo-0.02C. Moreover, the volume fraction of the γ phase slightly increased with C content. Within an extent of C addition that no carbide precipitation occurs, the carbon addition reduces the amounts of crystal defects such as stacking faults and twins, and ε martensites. In addition, tensile strength slightly increases with C content and ductility reaches a maximum at C content of 0.09%. though 0.2% proof strength shows no noticeable differences.