Influence of Heat Treatment Conditions on the Evolution of the Precipitate Population in VDM Alloy 780

In order to understand and model microstructural evolutions during hot deformation processes, it is necessary to understand the precipitation kinetics and the evolution of the precipitates at temperatures near the solvus. The goal of the present work is to investigate the influence of thermal paths prior to the isothermal heat treatment on the nucleation and growth of precipitates in nickel-based superalloy VDM Alloy 780. Two different heat treatment paths have been suggested for studying the influence of cooling and reheating conditions on the final precipitation distribution. By comparing the precipitate populations generated after each heat treatment using SEM, EBSD, and ECCI techniques, it can be determined that cooling path affects the precipitate morphology, nucleation density, and overall kinetics of the precipitation process. Precipitation after reheating from room temperature shows rather homogeneous, spherically shaped precipitates while precipitation after continuous cooling is dendritically shaped and heterogeneously spaced in sparse clusters of dense precipitation. The differences in morphology between both heat treatments may be explained by a difference in the nucleation rate before holding. Clusters seem to form from infinitesimal amounts of residual stored energy that persist after solubilization, generating preferential sites for precipitation. Discontinuous precipitation is considered to be a consequence of the heterogeneous state of the observed precipitation.