Nanoarchitectured Co-Cr-Mo orthopedic implant alloys: Nitrogen-enhanced nanostructural evolution and its effect on phase stability

Our previous studies indicate that nitrogen addition suppresses the athermal gamma (face-centered cubic, fcc) -> epsilon (hexagonal close-packed, hcp) martensitic transformation of biomedical Co-Cr-Mo alloys and ultimately offers large elongation to failure while maintaining high strength. In the present study, structural evolution and dislocation slip as an elementary process in the martensitic transformation in Co-Cr-Mo alloys were investigated to reveal the origin of their enhanced gamma phase stability due to nitrogen addition. Alloy specimens with and without nitrogen addition were prepared. The N-doped alloys had a single-phase gamma matrix, whereas the N-free alloys had a gamma/epsilon duplex microstructure. Irrespective of the nitrogen content, dislocations frequently dissociated into Shockley partial dislocations with stacking faults. This indicates that nitrogen has little effect on the stability of the gamma phase, which is also predicted by thermodynamic calculations. We discovered short-range ordering (SRO) or nanoscale Cr2N precipitates in the gamma matrix of the N-containing alloy specimens, and it was revealed that both SRO and nanoprecipitates function as obstacles to the glide of partial dislocations and consequently significantly affect the kinetics of the gamma -> epsilon martensitic transformation. Since the formation of epsilon 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. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.