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 (γ″ phase, γ′ phase, needle-shaped δ phase, and granular δ phase). Based on microstructural characterization and mechanical testing, the effects of these precipitated phases on the mechanical properties and fracture mechanisms were investigated. The results showed that a double-aging treatment process was the key to ensuring the complete precipitation and growth of the γ″ and γ′ phases. A small amount of δ phase (∼2.5%) precipitating in the solution treatment stage did not affect the precipitation of the γ″ and γ′ phases in the subsequent aging treatment stage. An Inconel 718 alloy without δ 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 δ 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 δ phase was transformed from needle-shaped to granular-shaped. According to the theory of the dihedral angle (2θ) for precipitated phase morphologies, a quantitative relationship between the interfacial energies of the needle-shaped δ phase and granular δ phase was determined. The interfacial energy of the needle-shaped δ phase was only ∼76% of that of the granular δ phase, meaning that the needle-shaped δ phase was likely to debond readily from the matrix, resulting in the formation of intergranular cracks.