Grain Structure Behavior of Powder Metallurgy Nickel Base c–c¢ Superalloys Under c¢-Supersolvus Solution Heat-Treatment

The grain coarsening behavior of a powder metallurgy polycrystalline c–c¢ nickel-based superalloy under c'-supersolvus solution heat treatment was investigated. This study aimed to understand the microstructural changes that occur during the heating process and their effects on the mechanical properties of the material. Various samples from isothermally forged turbine disks were subjected to solution heat treatments in a laboratory setting. It was found that the grain evolution occurs rapidly during the heating phase, influenced primarily by static recrystallization. The final grain size distribution predominantly depends on the density of recrystallized grain nuclei that migrate to consume the residual work-hardened grains. The research revealed that microstructures with limited work hardening develop heterogeneous grain sizes, where some grains exceed 200 μm, adversely affecting fatigue resistance. Moreover, the heating rate also plays a significant role; a slower rate correlated with increased formation of excessively large grains. Additional investigations into interrupted SHTs and the driving pressures on grain boundaries indicated that several simultaneous factors, including precipitate ripening and grain boundary driving pressure balance, contribute to grain structure behavior during the heating ramp to solution temperature.