Formation of the Ni3Nb delta-phase in stress-relieved Inconel 625 produced via powder-bed laser fusion additive manufacturing
Eric Lass, Mark R. Stoudt, Maureen E. Williams, Michael B. Katz, Thien Q. Phan, Lyle E. Levine, Thomas H. Gnaeupel-Herold
The microstructural evolution of laser powder-bed additively manufactured Inconel 625 during a post-build stress-relief anneal of 1 h at 870 °C is investigated. It is found that this industry-recommended heat treatment promotes the formation of a significant fraction of the orthorhombic D0a Ni3Nb delta-phase. This phase is well known to have a deleterious influence on fracture toughness, ductility, and other mechanical properties in conventional, wrought Inconel 625. The delta-phase platelets are found to precipitate within the interdendritic regions of the as-built solidification microstructure. These regions are enriched in solute elements, particularly Nb and Mo, due to the microsegregation that occurs during solidification. The precipitation of delta-phase at 800 °C is found to require up to 4 h. This indicates a potential alternative stress-relief processing window that mitigates delta- phase formation in this alloy. Ultimately, a homogenization heat treatment is recommended for additively manufactured Inconel 625 because the increased susceptibility to delta-phase precipitation increases the possibility for significant degradation of materials properties in service.
Metallurgical Transactions A-Physical Metallurgy and Materials Science