Nitrogen-enhanced nanostructural evolution and its effect on phase stability in biomedical Co-Cr-Mo alloys

Nitrogen addition is known to effectively suppress the athermal γ (fcc) → ε (hcp) martensitic transformation of biomedical Co-Cr-Mo alloys and ultimately provides a combination of high strength and good ductility. In this work, the nanostructural evolution and its influence on dislocation slip as an elementary process in the martensitic transformation were investigated to reveal the origin of their enhanced γ phase stability due to nitrogen addition. The biomedical Co-29Cr-6Mo (wt.%) alloys containing nitrogen in different concentrations (0-0.24 wt.%) were prepared. A single phase γ matrix was attained by adding nitrogen contents higher than 0.1 wt.%. We discovered nanosized Cr₂N precipitates that form on the {111}γ planes in the N-containing alloy specimens. It was revealed that the nanoscale inhomogeneities function as obstacles to the glide of partial dislocations and consequently significantly retard the γ → ε martensitic transformation. Since the formation of ε martensite plays a crucial role in plastic deformation and wear behavior, the developed nanostructural modification associated with nitrogen addition must be a promising strategy for highly durable orthopedic implants. Technology (MEXT), Japan.