Evolution of cold-rolled microstructures of biomedical Co-Cr-Mo alloys with and without N doping

The effects of nitrogen doping on microstructural evolution during cold rolling of Ni free Co-Cr-Mo alloys have been investigated. Nitrogen doping improved the cold workability of this alloy system, although initiation of edge cracks was observed for a cold rolling reduction of 30% in a Co-29Cr-6Mo-0.17N (in mass%) alloy, which has the highest nitrogen content of the alloys used in the present study. Nitrogen addition of 0.17% sufficiently stabilizes the γ phase (fcc structure) at room temperature, suppressing the athermal martensitic γ → ɛ transformation during cooling after solution treating, while the primary deformation mechanism is still the strain-induced martensitic transformation (SIMT). The SIMT is responsible for the limited cold workability of Co-Cr-Mo alloys with and without N addition. The development of γ matrix –ɛ martensite lamellae in the initial stages of cold rolling and subsequent shear band (SB) formation in the vicinities of cracks was observed by transmission electron microscopy. Fine grains, which elongate along the shear direction, were observed inside SBs; this is similar to other materials with low stacking fault energies. Such a SB evolution at relatively low strain is thought to originate from the lamellar microstructure that consists of strain-induced ɛ martensites, which leads to crack initiation and propagation at and along γ matrix –ɛ martensite boundaries where stress concentrations readily occur.