Precipitation kinetics of niobium carbide (NbC) during homogenization heat treatment of additively manufactured inconel 718 superalloy

Abstract

High cooling rate of the laser powder bed fusion (L-PBF) additive manufacturing can suppress the formation of niobium carbide (NbC) phase in the fabricated Inconel 718 nickel-based superalloy parts. However, it might precipitate during elevated temperature exposure. Accordingly, in the present work, the precipitation kinetics of the NbC carbide was investigated during homogenization heat treatment at temperatures in the range of 1000–1150 °C for soaking times of 15 min to 10 h. It was observed that increasing the homogenization temperature leads to the faster kinetics of NbC formation. The precipitation kinetics was modeled by the general phase transformation kinetics formula (based on the transformed fraction of 0.5 or 50%, t0.5) and Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis, resulting in the precipitation activation energies of 159.2 and 157.4 kJ/mol, respectively. It was concluded that the diffusion of both C and Nb atoms in the Ni matrix are the underlying mechanisms for the NbC carbide formation during homogenization heat treatment. The relationship of 1/t0.5∝exp(0.0097T) was also obtained for the precipitation rate as a function of temperature.