Development of γ′ morphology in P/M rotor disc alloys during heat treatment

The microstructural evolution of γ′ has been investigated in a series of advanced nickel-base superalloys, produced via the powder metallurgy route. The size, morphology and distribution of γ′ within the microstructure has been found to be highly sensitive to the rate of cooling from the supersolvus solutioning temperature. Precipitate morphologies and the resulting γ′ particle size distributions, were correlated to the various cooling rates, ranging from 6 to 600°C/min. Cooling rates below about 60°C/min resulted in a bimodal distribution of γ′, while rates above 60°C/min were able to suppress the formation of tertiary γ′ and yield a unimodal distribution of γ′ in this set of alloys. Following the supersolvus heat treatment, the effect of ageing these various microstructures at 800°C was assessed. While characteristic precipitate coarsening was observed in microstructures containing a bimodal distribution of γ′, anomalous coarsening to a critical size followed by the splitting of the γ′ precipitates, was observed in specimens initially containing a unimodal distribution of γ′.