Evolution of grain size and grain shape during thermomechanical processing in a powder metallurgical nickel-based superalloy

With a strain rate range of 0.01–10 s⁻¹ and a deformation temperature range of 1110–1200 °C, the isothermal compression test was performed on one powder metallurgy superalloy which is macroscopic segregation free. Using electron backscatter diffraction, the effect of strain rate and deformation temperature on grain shape and grain size of superalloys during thermal deformation was studied. The results established that exquisite and equiaxed dynamic recrystallization (DRX) grains are procured at supernal deformation temperature and high strain rate because of the high dislocation density. At the same time, the interaction between high DRX nucleation rate and low grain growth rate at high strain rate is favorable in making finer DRX grains. The equivalent medial grain size expanded with lowering strain rate and elevating proof temperature. Moreover, the grain shape was researched by the effective method of aspect ratio. Most aspect ratio of original grains is 0.61, and the aspect ratio has important implications for DRX and grain growth process. The average aspect ratio increases slightly when deformation temperature rises from 1110 to 1140 °C, while the average aspect ratio increases memorably as the deformation temperature is higher than 1140 °C.