Effect of precipitated phases on the mechanical properties and fracture mechanisms of Inconel 718 alloy

In this study, hot-rolled Inconel 718 alloys were subjected to three different heat treatment schedules to develop various precipitated phases (gamma '' phase, gamma ' phase, needle-shaped delta phase, and granular delta phase). Based on microstructural characterization and mechanical testing, the effects of these precipitated phases on the me-chanical properties and fracture mechanisms were investigated. The results showed that a double-aging treat-ment process was the key to ensuring the complete precipitation and growth of the gamma '' and gamma ' phases. A small amount of delta phase (-2.5%) precipitating in the solution treatment stage did not affect the precipitation of the gamma '' and gamma ' phases in the subsequent aging treatment stage. An Inconel 718 alloy without delta phase precipitation had excellent impact toughness, and the fracture mode was ductile fracture caused by dislocation slip separation. However, if it was present, the morphology of delta phase determined the effect on the impact toughness. The fracture mechanism changed from a mixed fracture mode of intergranular and transgranular fracture to quasi -cleavage fracture when the morphology of the precipitated delta phase was transformed from needle-shaped to granular-shaped. According to the theory of the dihedral angle (2 theta) for precipitated phase morphologies, a quantitative relationship between the interfacial energies of the needle-shaped delta phase and granular delta phase was determined. The interfacial energy of the needle-shaped delta phase was only-76% of that of the granular delta phase, meaning that the needle-shaped delta phase was likely to debond readily from the matrix, resulting in the formation of intergranular cracks.