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

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 expo-sure. 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 degrees 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 ki-netics 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/t0.5fexp(0.0097T) was also obtained for the precipitation rate as a function of temperature. (c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

The high cooling rate in laser powder bed fusion (L-PBF) additive manufacturing can suppress the formation of niobium carbide (NbC) phase in Inconel 718 nickel-based superalloy parts. However, the NbC phase may precipitate during elevated temperature exposure. This study investigates the precipitation kinetics of NbC carbide during homogenization heat treatment and finds that increasing the homogenization temperature accelerates its formation. The diffusion of C and Nb atoms in the Ni matrix is identified as the underlying mechanism for NbC carbide formation during homogenization heat treatment.