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 degrees C and 1050 degrees C. In the as built SLM sample, the gamma phase was mainly formed, but some epsilon phases were present together. The percentage of the epsilon phase increased significantly after SRA at 750 degrees C, and a microstructure with a 100% gamma phase formed after SRA at 1050 degrees C. Changes in mechanical properties were characterized by an increase in both hardness and yield strength, while the elongation decreased after SRA at 750 degrees C. This was caused by the formation of the epsilon phase, which possesses high strength and hardness but brittleness. After SRA at 1050 degrees 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 gamma phase of the Co-Cr-Mo alloy, strain-induced gamma to epsilon 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 epsilon phase formed more rapidly than in the sample after SRA at 1050 degrees C, resulting in low elongation.

Automated summary

This study investigated the impact of stress relief annealing temperatures on the microstructure and mechanical behavior of a Co-Cr-Mo alloy fabricated by selective laser melting. The results showed that annealing at 750 degrees C led to a significant increase in the percentage of the epsilon phase, while annealing at 1050 degrees C resulted in a microstructure with 100% gamma phase. The mechanical properties exhibited increased hardness and yield strength, but decreased elongation after annealing at 750 degrees C. However, annealing at 1050 degrees C led to a decrease in hardness and yield strength, and an increase in elongation compared to the as-built sample, due to the reduction in residual stress. The formation of the brittle epsilon phase during deformation was more pronounced in the as-built sample with high residual stress, leading to lower elongation.