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 Co₁₈Cr₁₅Fe₅₀Ni₁₀Mo_6.5C_0.5 (at%) medium-entropy alloy fabricated by laser powder bed fusion (LPBF) to post-LPBF annealing at 900 °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 µ 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 ∼1.02 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.

Key words: Laser powder bed fusion, Medium-entropy alloy, Cell structure, Strain hardening, Precipitation, Back stress hardening