Formation of the NiNb δ-phase in stress-relieved Inconel 625 produced via laser powder-bed fusion additive manufacturing.

The microstructural evolution of laser powder-bed additively manufactured Inconel 625 during a post-build stress-relief anneal of 1 h at 1143 K (870 °C) is investigated. It is found that this industry-recommended heat treatment promotes the formation of a significant fraction of the orthorhombic D0 NiNb δ-phase. This phase is known to have a deleterious influence on fracture toughness, ductility, and other mechanical properties in conventional, wrought Inconel 625; and is generally considered detrimental to materials' performance in service. The δ-phase platelets are found to precipitate within the inter-dendritic regions of the as-built solidification microstructure. These regions are enriched in solute elements, particularly Nb and Mo, due to the micro-segregation that occurs during solidification. The precipitation of δ-phase at 1073 K (800 °C) is found to require up to 4 h. This indicates a potential alternative stress-relief processing window that mitigates δ-phase formation in this alloy. Ultimately, a homogenization heat treatment is recommended for additively manufactured Inconel 625 because the increased susceptibility to δ-phase precipitation increases the possibility for significant degradation of materials properties in service.