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23-08-2017 | Issue 11/2017

Metallurgical and Materials Transactions A 11/2017

Formation of the Ni3Nb δ-Phase in Stress-Relieved Inconel 625 Produced via Laser Powder-Bed Fusion Additive Manufacturing

Journal:
Metallurgical and Materials Transactions A > Issue 11/2017
Authors:
Eric A. Lass, Mark R. Stoudt, Maureen E. Williams, Michael B. Katz, Lyle E. Levine, Thien Q. Phan, Thomas H. Gnaeupel-Herold, Daniel S. Ng
Important notes
Manuscript submitted July 6, 2017.

Abstract

The microstructural evolution of laser powder-bed additively manufactured Inconel 625 during a post-build stress-relief anneal of 1 hour 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 D0a Ni3Nb 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 hours. 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.

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