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

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 D0 a Ni 3 Nb d-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 d-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 d-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 d-phase formation in this alloy. Ultimately, a homogenization heat treatment is recommended for additively manufactured Inconel 625 because the increased susceptibility to d-phase precipitation increases the possibility for significant degradation of materials' properties in service.

Elastic Residual Strain and Stress Measurements and Corresponding Part Deflections of 3D Additive Manufacturing Builds of IN625 AM-Bench Artifacts Using Neutron Diffraction, Synchrotron X-Ray Diffraction, and Contour Method

“…Neutron diffraction measurements were performed at the NCNR using the BT8 diffractometer. The measurement procedures are similar to those reported in previous works [8,9]. Measurements were performed in monochromatic mode, using a neutron beam with a wavelength of approximately λ= 0.1537 nm.…”

Effect of Nb on δ Phase Precipitation and the Tensile Properties in Cast Alloy IN625

“…Although there are many studies in the literature on alloy IN625, most of the reports are about wrought alloy IN625 [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. So, studies on cast alloy IN625, such as the one presented in this paper, is needed to investigate its potential application in A-USC power plants.…”

“…According the studies on wrought alloy IN625, the δ phase is observed to precipitate easily after long-term aging at 700-760 • C (the service temperature of A-USC power plants) [9][10][11][12][13], although it is initially designed as a solid solution hardening alloy [14]. The δ phase is an equilibrium phase corresponding to the γ phase with a composition of Ni 3 Nb and an orthorhombic (D0 a ) crystal structure [15][16][17][18].…”

Influence of Niobium or Molybdenum Addition on Microstructure and Tensile Properties of Nickel-Chromium Alloys

“…Consequently, the total strain of the Ni-24Cr-8Nb alloy was reduced from 6.5% to 1.8% in samples of as-cast Ni-20Cr-12Nb alloy whereas their elastic module was increased between 41 GPa and 70 GPa, respectively. It is well known that the Ni 3 Nb platelets has a detrimental effect on ductility and other mechanical properties that are related to precipitate within the inter-dendritic regions of the as-cast microstructures [29]. In this regard, the development of multicomponent alloys containing the Mo and Nb additions could produce both the high ductility and mechanical strength for structural applications.…”