Influence of the initial cooling rate from γ′ supersolvus temperatures on microstructure and phase compositions in a nickel superalloy

Different cooling paths from a supersolvus temperature have been applied to FGH96, a polycrystalline nickel base superalloy for turbine disc applications, in order to simulate the different microstructures that exist through the thickness of a disc following an industrial heat treatment. Secondary and tertiary γ′ precipitate size distributions and morphology have been analysed and compared for the different heat treatments using SEM and atom probe tomography (APT). Detailed compositional data for both γ′ precipitate and γ matrix are presented, and compared to equilibrium compositions calculated by Thermo-Calc. For the heat-treatments studied, the secondary γ′ composition indicates a shell of differing composition to that towards the precipitate core. From sequential equilibria compositional calculations, it is suggested that the 'shell' forms at a lower temperature than the precipitate core. The fine tertiary precipitates do not show the core-shell compositional differences on continuous cooling. W peaks are noted at the γ/γ′ interfacial region, which is of significance for retarding coarsening. A γ′ depletion zone surrounds the secondary precipitates, within which the γ matrix composition differs significantly to the γ far-field values, Finally, a precipitate nucleation and growth mechanistic model is suggested based on the experimental data and Thermo-Calc calculations.