Effect of nitrogen content on microstructure of hot-pressed Co-Cr-Mo alloy compacts for biomedical applications

The gas atomized Co-29Cr-6Mo alloy powder was hot-pressed in Ar, Ar and N₂ (Ar + N₂), and N₂ atmosphere at 1273 K and 80 MPa for 7.2 ks. Nitrogen content in the compacts can be controlled by the mixture ratio of Ar and N₂ gases and the compacts contains nitrogen as high as 0.81 mass%. Microstructure and constituent phase of the compacts depends on the nitrogen content. In the compacts containing nitrogen less than 0.2%, the γ(fcc) ε (hcp) and σ phases coexist in the compacts. The volume fraction of σ phase decreases with increasing nitrogen content up to 0.2%. The γ phase is dominantly confirmed in the compacts with nitrogen content of approximately 0.2%. In the compacts containing nitrogen over 0.2%, Cr₂N phases in addition to the γ and ε phases are detected in the compacts. This result suggests nitrogen suppress the σ phases formation and stabilize γ phase in the Co-29Cr-6Mo alloy. When the Co-29Cr-6Mo alloy compacts are hot-pressed with Cr₂N powder as a nitrogen source, the σ and nitrides coexist and the microstructure is inhomogeneous. Therefore, it is found that controlling the mixture ratio of Ar and N₂ gases during hot-pressing is effective methods to distribute nitrogen uniformly in the Co-Cr-Mo alloy compacts.