Interaction between γ′ precipitate distribution and microstructure homogeneity during hot deformation in a Ni-based superalloy

The inhomogeneous microstructure of the Ni-based superalloys used for turbine disks was an intolerable defect for the mechanical performance. The effects of the distribution of γ′ precipitate, forging temperature, and strain level on the microstructure evolution of GH4730 alloy were investigated by EBSD during hot deformation. The results showed that the heterogeneous factor peaked at the transition temperature from the single-phase to the double-phase region. The coupling effect of heat and stress led to the heterogeneous precipitation and distribution of γ′ phase during the transition region, which was the main reason for the formation of inhomogeneous microstructures. The coherent γ′ phases of approximately 0.3 μm were diffusely distributed inside the large grains, increasing the grain strength, making recrystallization refinement difficult, and thus forming large unrecrystallized grains. The incoherent γ′ phases with a size of approximately 1.1 μm located at the grain boundaries and pinned the grain boundaries, and thus the accumulated strain at the grain boundaries caused the occurrence of discontinuous dynamic recrystallization and promoted continuous refinement of the grains. The microstructure evolution of new Ni-based superalloys during hot forging was focused, and the formation mechanism of inhomogeneous microstructure and control measures was explained. A theoretical basis for improving the microstructure homogeneity of the new cast and wrought superalloys was provided.