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 (SHT) was studied. Various samples were extracted from isothermally forged turbine disks and submitted to SHTs in a laboratory furnace. Grain evolution occurs quickly during the heating step to the solution temperature and is likely governed by static recrystallization, with the final grain size distribution defined by the density of recrystallized grain nuclei. Microstructures with limited work hardening can exhibit heterogeneous grain structures, where some grains can exceed 200 μm in size, adversely affecting the material's fatigue resistance. Once the residual work hardening is consumed, the grain structure stabilizes due to the pinning pressure from nanometric oxide and carbide particles on grain boundaries. A slower heating rate in slightly work-hardened samples enhances the formation of excessively large grains and a heterogeneous grain size distribution. Additional interrupted SHTs, microstructural investigations, and calculations of driving pressures exemplify that such phenomena may arise from multiple factors during the temperature ramp to SHT, such as the dissolution of precipitates and limited statically recrystallized nuclei.