Restructuring the cell network of non-equiatomic CoCrFeNiMoC medium-entropy alloy fabricated by laser powder bed fusion

Abstract

Metal additive manufacturing (MAM) enables near-net shape production of components with minimized waste and excellent mechanical performance based on multi-scale microstructural heterogeneity. Especially, the dislocation cell network that often bears elemental segregation or precipitation of a secondary phase contributes to enhancing the strength of additively manufactured materials. The cell boundaries can also act as active nucleation sites for the formation of precipitates under post-MAM heat treatment, as the chemical heterogeneity and profuse dislocations generate a driving force for precipitation. In this work, we subjected a Co18Cr15Fe50Ni10Mo6.5C0.5 (at%) medium-entropy alloy fabricated by laser powder bed fusion (LPBF) to post-LPBF annealing at 900 degrees C for 10 min. Microstructural investigation revealed that the cell boundaries of the as-built sample, which were decorated by Mo segregation, are replaced by mu phase and M6C type carbide precipitates during annealing while the grain structure and size remain unaffected, indicating that the post-LPBF annealing delivered the proper amount of heat input to alter only the cell structure. The yield strength slightly decreased with annealing due to a reduction in the strengthening effect by the cell boundaries despite an increased precipitation strengthening effect. However, the post-LPBF annealing improved the strain hardenability and the ultimate tensile strength was enhanced from similar to 1.02 to similar to 1.15 GPa owing to reinforced back stress hardening by the increased dislocation pile-up at the precipitates. Our results suggest that the cell structure with chemical heterogeneity can be successfully controlled by careful post-MAM heat treatment to tailor the mechanical performance, while also providing insight into alloy design for additive manufacturing. (c) 2024 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.