Discontinuous Dynamic Recrystallization Mechanism and Twinning Evolution during Hot Deformation of Incoloy 825

Discontinuous dynamic recrystallization (DDRX), which involves multiple nucleation and grain growth processes, plays a crucial role in grain refinement; however, the underlying mechanism and the significant role of twin boundaries (TBs) remain poorly understood. Here, the evolution of characteristic microstructures and the fraction and density of TBs under different deformation conditions (i.e., 1050–1200 °C, 0.001–1 s⁻¹) in Incoloy 825 was investigated through a thermomechanical simulator, electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM). The first strand of recrystallized grains nucleated along the original grain boundaries (GBs), separated by the newly formed random high-angle grain boundaries (HAGBs), which were transformed from low-angle grain boundaries (LAGBs) and pre-existing TBs. Subsequently, straight TBs forming in the new grains increased the misorientation angle, promoting the migration of stagnated grain boundaries, and parts of newly generated twinning chains at the front of the recrystallization zone converted into random HAGBs, providing sites for the following layer of nucleation. Moreover, triple junctions between the recrystallized and deformed grains also served as potential nucleation sites when the LAGBs in the large misorientation gradient were transformed into random HAGBs. Quantitative relationships between the recrystallized grain size and TB density were obtained.