Formation Mechanisms of Coprecipitates in Inconel 718 Superalloys

Recent experimental observations have shown various gamma '/ gamma '' coprecipitate microstructures in Inconel 718-based alloys. Although their growth and coarsening be-haviors have been investigated recently, the heterogeneous nucleation mechanisms of gamma '' precipitates on pre-existing gamma ' precipitates that lead to the formation of these coprecipitates are yet to be understood. In this study, we use primarily phase field simulations to analyze the individual and combined effects of concentration and coherency stress fields associated with a growing gamma ' precipitate at different sizes on heterogeneous nucleation of gamma '' on the surfaces of the gamma ' particle and the coprecipitate configurations formed. The chemical driving force for nucleation is obtained from a pseudo-ternary database calibrated against the latest CALPHAD databases for Ni-base superalloys. At the same time, contributions from elastic interaction and anisotropic interfacial energy of the gamma '/ gamma '' interface are quantified. Subsequently, a statistical analysis based on the classical nucleation theory is combined with the phase field simulations to understand the nucleation events leading to various coprecipitate configurations. Our analyses, for the first time, indicate that the choice between the formation of compact and sandwich-like coprecipitates is determined by the size of gamma ' before the first nucleation event of gamma '', interfacial energies between the gamma, gamma ' and gamma '' phases, and the solute depletion zone around a growing gamma ' precipitate.