Precipitation behavior of δ phase and its effect on stress rupture properties of selective laser-melted Inconel 718 superalloy

The selective laser-melted (SLM) Inconel 718(IN718) specimens were heat-treated with different solution regimes after the hot isostatic pressing (HIP). The results show that δ phase forms at grain boundary and presents needle-like pattern, and the size and volume fraction of δ phase increase with the increase of holding time and the decrease of solution temperature. The equilibrium volume fraction of needle-like δ phase in SLM-built IN718 is higher than that of short rod-like δ phase in forged-IN718 under the same heat treatment. The SLM-built IN718 specimen without δ phase at grain boundary has the best stress rupture properties, while the specimen with the largest volume fraction of δ phase at grain boundary exhibits the worst stress rupture properties. This is attributed to that the needle-like δ phase at grain boundary promotes the nucleation of void-formed cracks and accelerates the deformation damage. The analysis of the responses to the coordinated deformation capacity of grain orientation and grain size indicates that there exist the heterogeneous deformation and stress/strain concentration at grain boundary. The existence of δ phase influences the coordinated deformation capacity and promotes the stress/strain concentration around grain boundary region. Especially, the needle-like δ phase at grain boundary leads to the more detrimental effect on the rupture life and ductility of SLM-built IN718 alloy on compared with the forged-IN718 alloy.