Microstructural Evolution of a High W Content Ni-Based Superalloy at γ′ Sub-Solvus Temperatures

The evolution of γ′ phase and γ/γ′ eutectic during short-time heat treatment of the Ni-based superalloy with high W content, K416B are investigated. In this study, high temperature Differential Scanning Calorimetry (DSC) is employed to design the heat treatment experiments and the precipitate temperature of secondary γ′ is found in the cooling curve. The standard coarsening model is applied to figure out the coarsening rate constant of γ′ precipitate. On this basis, a novel method is proposed to predict the globularization time of γ phase in γ/γ′ eutectic during the heat treatment process. As holding temperature increases, more γ lamellae convert to platelets before reaching thermodynamic equilibrium. Electron microscopy observations exhibit the boundary splitting and necking formation on the γ plate surface. Moreover, the dissolution kinetics of γ phase is quantitatively assessed by JMAK model. The volume fraction descends rapidly in the initial stage, and then reach to a threshold. Above proposed approaches are validated via various heat treatment experiments.