Effect of phase transformation on the mechanical properties of the Co-Cr-Mo alloy fabricated by selective laser melting

In this study, we examined the effect of stress relief annealing (SRA) temperatures on the microstructure and mechanical behavior of a Co-Cr-Mo alloy fabricated by selective laser melting (SLM). For the SRA, the as-built SLM samples were annealed at 750 °C and 1050 °C. In the as built SLM sample, the γ phase was mainly formed, but some ε phases were present together. The percentage of the ε phase increased significantly after SRA at 750 °C, and a microstructure with a 100% γ phase formed after SRA at 1050 °C. Changes in mechanical properties were characterized by an increase in both hardness and yield strength, while the elongation decreased after SRA at 750 °C. This was caused by the formation of the ε phase, which possesses high strength and hardness but brittleness. After SRA at 1050 °C, the hardness and yield strength decreased, and the elongation increased compared to the as-built SLM sample because of the decrease in the residual stress. In the γ phase of the Co-Cr-Mo alloy, strain-induced γ to ε martensitic transformation occurring during deformation, and it was more marked in the as-built SLM sample with high residual stress. Therefore, when the as-built sample was deformed, the brittle ε phase formed more rapidly than in the sample after SRA at 1050 °C, resulting in low elongation.