The Effect of δ Phase on the Mechanical Properties of an Inconel 718 Superalloy

The relationship between the delta phase content and the mechanical properties of Inconel 718 superalloy is still uncertain in the scientific literature. This study investigated the effects of the amount of delta phase and of the grain size on the mechanical properties of an aged gamma-matrix with gamma' and gamma aEuro(3) precipitates. The material in as-received condition in the form of a forged bar was solution-treated in different conditions and aged according to UNS7718 standard. The microstructures were characterized using optical, scanning, and transmission electron microscopy. Hardness and tensile tests were also conducted. After solution treatment, gamma' and gamma aEuro(3) phases are dissolved and delta phase volume fraction is reduced to a minimum amount only observed by TEM, resulting in an increase of the grain size and a decrease of hardness and strength. After aging, the precipitation of gamma' and gamma aEuro(3) occurs and the amount of delta phase increases. The volume fraction of delta phase varying from 0.30 to 1.38% and the grain size varying from ASTM 7 to 5 do not have a significant effect on the tensile properties and hardness.