Discontinuous dynamic recrystallization and nucleation mechanisms associated with 2-, 3- and 4-grain junctions of polycrystalline nickel-based superalloys

Despite the important role of grain junctions to the dynamic recrystallization (DRX) of nickel-based superalloys, the nucleation mechanisms operating there and the influence factors besides deformation condition on nucleation priority are still unclear. The microstructure evolution considering grain junction effects has not been explored in depth yet. In this paper, DRX nucleation at the 2-, 3- and 4-grain junctions of a nickel-based superalloy was investigated. A proposed passive grain boundary bulging (PGBB) mechanism operated well at 2-grain junctions, and accounted for the continuity of necklace structure during the early stage of hot deformation. For the 3-grain junctions, a two-step strain-induced bulging of grain boundary fragments, which closely adjoined the junctions, was found to dominate the DRX nucleation. In addition, an increasing nucleation priority from 2-, 3- to 4-grain junctions was confirmed by the established thermodynamic model, while the nucleation priority differences of the same kind grain junctions were quantitatively analyzed by using the Taylor factors of their component grains. Finally, the DRX microstructure evolution of a 4-grain stacking unit during hot deformation was described. The understanding of DRX associated with 2-, 3- to 4-grain junctions made it more effective to tailor the microstructure of nickel-based superalloy forgings.