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

Abstract The grain coarsening behavior of a powder metallurgy polycrystalline γ – γ′ nickel-based superalloy under γ '-supersolvus solution heat treatment (SHT) was studied. Various samples were extracted from isothermally forged turbine disks and submitted to SHTs in a laboratory furnace. γ grain evolution take place quickly during the heating step to the solution temperature and is most likely governed by static recrystallization, where the final grain size distribution is defined by the density of recrystallized grain nuclei whose boundaries migrate to consume the residual work hardened grains in the microstructure. Microstructure with limited work hardening ( i.e., determined as having a 0.16 and 0.38 deg grain-surface averaged grain averaged misorientation and grain orientation spread respectively) can form heterogenous grain structures composed of some grains of much larger size than their neighbors (> 200 µ m), which decreases the material’s fatigue resistance. Once the residual work hardening is consumed, the grain structure shortly remains stable due to the pinning pressure exerted by nanometric (Hf, Zr)O 2 oxide and (Nb, Ti)C carbide particles on grain boundaries. On slightly work-hardened sample material, a slower heating rate to the solution temperature increases the propensity to form grains of excessive size (> 200 µ m) and rather heterogeneous grain size distribution. Additional interrupted SHTs, microstructural investigations and rough calculations of the driving pressures exerted on the grain boundaries illustrated that such a phenomenon could be favored by several concomitant factors during the 2 °C/min temperature ramp up to the SHT temperature, such as ripening of the γ ′ precipitates population during their dissolution, limited statically recrystallized nuclei and exposure to a slightly positive grain boundary driving pressure balance over a prolonged period of time.