A novel strategy for ingot cogging without homogenization : dynamical recrystallization and nucleation mechanisms associated with as-cast dendrites of nickel-based superalloys

Abstract

Since the as-cast microstructure benefits dynamic recrystallization (DRX) nucleation, the present research is focused on the microstructure evolution associated with the dendrites and precipitates during the thermal deformation of an ingot without homogenization treatment aiming at exploring a new efficient strategy of ingot cogging for superalloys. The as-cast samples were deformed at the sub-solvus temperature, and the DRX evolution from dendritic arms (DAs) to inter-dendritic regions (IDRs) was discussed based on the observation of the fishnet-like DRX microstructures and the gradient of DRX grain size at IDRs. The difference in the precipitates at DAs and IDRs played an essential role during the deformation and DRX process, which finally resulted in very different microstructures in the two areas. A selective strain-induced grain boundary bulging (SIGBB) mechanism was found to function well and dominate the DRX nucleation at DAs. The grain boundary was able to migrate and bulge to nucleate on the condition that the boundary was located at DAs and had a great difference in dislocation density between its opposite sides at the same time. As for DRX nucleation at IDRs, the particle-stimulated nucleation (PSN) mechanism played a leading role, and the progressive subgrain rotation (PSR) and geometric DRX were two important supplementary mechanisms. The dislocation accumulation around the coarse precipitates at IDR resulted in progressive orientation rotation, which would generate DRX nuclei once the maximum misorientation there was sufficient to form a high-angle boundary with the matrix. The PSR or geometric DRX functioned at the severely elongated IDRs at the later stage of deformation, depending on the thickness of the elongated IDRs. The uniform microstructure was obtained by the deformation without homogenization and the subsequent annealing treatment. The smaller strain, the lower annealing temperature, and the much shorter soaking time requested in the above process lead to a smaller risk of cracking and a lower consumption of energy during the ingot-cogging process.