Phase decomposition in biomedical Co-29Cr-6Mo-0.2N alloy during isothermal heat treatment at 1073 K

The precipitation behavior and phase transformation, from a face-centered cubic γ phase to a hexagonal close-packed ε phase, in the biomedical Co-29Cr-6Mo-0.2 N (wt.%) alloy during isothermal heat treatment at 1073 K were investigated to obtain a sound understanding of the microstructural evolution occurring in this alloy system. The alloy initially had a single-phase γ matrix. The isothermal γ → ε martensitic transformation was identified in the specimens at the early stage of the heat treatment. As the heat treatment progressed, the microstructures were almost completely replaced with the lamellae consisting of the ε and Cr₂N phases. The ε + Cr₂N lamellar structures formed from the γ grain boundaries via a eutectoid reaction. The formation of a very fine σ phase was also identified. The present study indicates that adding nitrogen to the alloys significantly influences the phase transformation that directly affects their mechanical properties, wear resistance, and corrosion behavior.