Micro-mechanisms and precipitation kinetics of delta (δ) phase in Inconel 718 superalloy during aging

The formation of delta phase in Inconel 718 superalloy during aging was quantitatively studied based on improved analyzing techniques to unambiguously determine the mechanism of delta phase precipitation and its kinetics. Based on the general transformation rate expressed in terms of the time corresponding to the transformed fraction of 0.5 or 50% and also the modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, the average activation energy for the formation of delta phase was determined as 212.4 kJ/ mol and 197.5 kJ/mol, respectively. These values are comparable to the activation energy for diffusion of Nb in Ni (202.59 +/- 4.71 kJ/mol). Therefore, the micro-mechanism for the formation of delta phase was characterized as the diffusion of Nb in the matrix. Based on the kinetic studies, the temperature of 1055.7 K (782.7 degrees C) was found as the lower bound for the formation of delta phase, which was consistent with the precipitation-temperature-time (PTT) plot developed for this material. Finally, a formula was obtained to quantitatively show the effects of aging temperature and cold rolling reduction on the transformation rate for the precipitation of delta phase. (C) 2019 Elsevier B.V. All rights reserved.