Controlling the grain boundary morphology and secondary γ′ precipitate size distribution in Ni-base superalloys

Abstract The size, morphology and fraction of each of the γ′ precipitate populations in polycrystalline γ- γ′ nickel-base superalloys are highly sensitive to the cooling rate from the solution temperature. As a result, thermal processing parameters are carefully selected to control precipitate populations in an effort to modify specific microstructural features that can impact resulting properties. The present investigation reports on how a stepped cooling rate was able to modify the microstructure of two experimental Ni-base superalloys. Cooling rates of 0.1 °C/s and 0.01 °C/s were maintained through the γ′ solvus temperature to a transition temperature ∼10 °C to 15 °C below the solvus. Following the transition temperature, a controlled cooling rate of 1 °C/s was applied to control the size and distribution of the secondary γ′ precipitates. For both experimental alloys, slow cooling rates through the γ′ solvus temperature resulted in the formation of coarse grain boundary precipitates while secondary γ′ precipitates formed upon the subsequent faster cooling rate of 1 °C/s. Coarse grain boundary γ′ precipitates resulted in higher amplitudes and wavelengths of serration along the boundary. In addition to cooling rate, the grain boundary misorientation was also found to influence the precipitation of grain boundary γ′ with larger precipitate populations found along high angle grain boundaries. Results from this study appear to support the concept that meso-scale engineering of grain boundary structures may potentially be used to enhance and optimize the high temperature properties of Ni-base superalloys.