Impact of Coarse γ′ Phase on Recrystallization Modeling in New Ni-Based Superalloy M647

Abstract

M647, a Ni-based superalloy with excellent mechanical properties and good hot deformability, was recently developed for application in airplane engine disks. In airplane engines, fine-grained superalloys are required to improve high-temperature fatigue properties. Methods to control fine grains have been extensively studied, including a refined method of applying the pinning effect of a coarse γ′ phase. However, previous reports focused on the mechanism of nucleation and abnormal grain growth, and reports on modeling to predict grain size are rare, making it difficult to optimize the forging process. The present study proposes a modeling method of M647 recrystallization with a coarse γ’ phase and compares modeling and experimental results. Recrystallization was experimentally observed in M647 with a coarse γ′ phase originating from the grain boundaries of the prior γ phase. Moreover, recrystallization is promoted by heating at a high temperature, applying a high strain, and maintaining the heat for a long duration. Grain growth is restricted by the pinning effect of the coarse γ′ phase. The area fraction of the coarse γ′ phase changed with the heating temperature, and the γ′ grain size increased with heating. Electron backscatter diffraction analysis shows that the kernel average misorientation increased with increasing forging temperature. These trends indicate that the pinning and driving forces of recrystallization fluctuate continuously during forging and reheating. The microstructure is predicted by applying Avrami-type equations, but the accuracy is insufficient because the fluctuation effects are not considered.