On microstructural homogenization and mechanical properties optimization of biomedical Co-Cr-Mo alloy additively manufactured by using electron beam melting

The electron beam melting (EBM), a layer-by-layer additive manufacturing (AM)technique, has been recently utilized for fabricating metallic components with complex shape and geometry. However, the inhomogeneity in microstructures and mechanical properties are the main drawbacks constraining the serviceability of the EBM-built parts. In the present study, we found remarkable microstructural inhomogeneity along build direction in the EBM-built Co-based alloy, owing to the competitive grain growth and subsequent isothermal γ-fcc → ε-hcp phase transformation, which affects the corresponding tensile properties significantly. Then, we succeeded in eliminating the inhomogeneities, modifying the phase structures and refining grain sizes via comprehensive post-production heat treatment regimes, which provides a valuable implication for improving the reliabilities of AM-built metals and alloys. The Co-based alloy can be selectively transformed into predominant ε or predominant γ phase by the regime, and the grains were refined to 1/10 of the initial sizes by repeated heat treatment. Finally, we investigated the tensile properties and fracture behaviors of the alloy before and after each heat treatment. The γ → ε strain-induced martensitic transformation is the major deformation mode of the γ phase, meanwhile the formation of stripped ε phase at {111}_γ habit planes contributed to a good combination of strength and ductility. Nevertheless, the ε phase was deformed mainly by (0001)_ε <11 2¯0 >_ε basal and {1 1¯00}_ε <11 2¯0 >_ε prismatic slip systems, exhibiting very limited ductility and strength. In addition, the ε grains act as secondary hardening factor in the samples consisting of dual γ/ε phase, leading to a non-uniform deformation behavior.