Post-dynamic recrystallization in a heavily alloyed Ni-based superalloy
The work is devoted to a study of post-dynamic recrystallization in a recently developed nickel-based superalloy SDZhS-15 manufactured by ingot metallurgy and intended for disc applications. The as-cast superalloy was subjected to long-term homogenization annealing and canned forging at subsolvus temperatures (T=Ts-(40-70), where Ts is the γ′ solvus temperature) under quasi-isothermal conditions. This led to the formation of a refined recrystallized fine-grained structure with individual non-recrystallized γ grains in the central part of the forged workpiece. However, in the near-surface layers of the forged workpiece, the volume fraction of non-recrystallized grains was appreciably higher. To further recrystallize the as-forged microstructure the recrystallization annealing was performed at a temperature (T=Ts-70) for different times to obtain a homogeneous recrystallized and fine-grained structure (dγ<10 μm) without causing a strong grain growth. The produced microstructures were examined by scanning electron microscopy including the electron backscattered diffraction technique. It was established that the increase of the annealing time led to the refinement of coarse non-recrystallized γ grains and insignificant coarsening of fine-grained recrystallized structure. The fraction of high-angle grain/interphase boundaries including twin boundaries increased. It was revealed that the use of the recrystallization annealing resulted in a slower γ grain growth during subsequent solid solution treatment in contrast to the conditions obtained via forging followed by solid solution treatment. Based on the performed work, the processing route of the heavily alloyed ingot-metallurgy SDZhS-15 superalloy was specified. It should include homogenization annealing, canned hot forging at subsolvus temperatures, post-forging recrystallization annealing, solid solution treatment, and ageing.