Dissolution of the Laves phase and δ-precipitate formation mechanism in additively manufactured Inconel 718 during post printing heat treatments

Abstract

The microstructures of additively manufactured Inconel 718 consist of the primary phase (dendritic) and the secondary phases (Laves and carbides). Among the secondary phases, the Laves phase, in particular, adversely affects the mechanical performance of the material. A post-processing heat treatment is necessary to eliminate undesirable secondary phases. The objective of the present study is to eliminate long-chain Laves using heat treatment below the δ – solvus temperature and thereby establish the real-time dissolution mechanism of Laves phase. The specimens are produced using the laser powder bed fusion (LPBF) process, followed by two different heat treatment procedures based on the temperatures determined using Scheil calculation, and those below the δ – solvus temperature. While the Laves phase is invariably found in the as-printed stage, this study demonstrates the successful elimination of a long singular chain of Laves phase when heat treated below the δ – solvus temperature. Additionally, the real-time transformation and growth mechanisms of δ – phase during annealing, are effectively captured through in situ heat treatment in a Transmission Electron Microscope (TEM). A re-distribution of the solute from the Laves phase to the γ – matrix is observed, with the δ – phase in the form of a needle on either side of the Laves at 900 °C. The in situ observations have shed important insights on the transformation mechanism of Laves phase to δ – phase in additively manufactured Inconel 718 samples.