Microstructure evolution and underlying mechanisms during the hot deformation of as-homogenized Ni-Co-Cr superalloys with millimeter-grade coarse grains

The as-homogenized microstructure, characterized by millimeter-grade coarse grains, should be refined during the cogging process to enhance the mechanical properties. In this work, an as-homogenized Ni-Co-Cr superalloy was hot-mechanically processed under conditions of 1150°C/0.1s -1 with varying strain levels (0.11-0.92). The microstructure at different strains was characterized by optical microscopy (OM) and electron backscatter diffraction (EBSD) to elucidate the underlying mechanisms of microstructural evolution during hot deformation. The results indicate that the yield drop phenomenon occurred due to the absence of mobile dislocations in the as-homogenized Ni-Co-Cr superalloy and their accelerated multiplication at the initial stage of deformation. The fragmentation of the initial coarse grains was primarily driven by differential lattice rotation and local lattice rotation within the individual grains. During the hot deformation process of the as-homogenized Ni-Co-Cr superalloy, the dynamic recrystallization (DRX) process was first observed to nucleate at the original grain boundaries (GBs) and the distorted twin boundaries via grain boundary bulging. Subsequently, the transition bands served as the preferential nucleation sites for DRX grains through secondary discontinuous dynamic recrystallization (DDRX) and subgrain rotation. It is evident that, under the synergistic effects of these mechanisms, the original grain boundaries, twin boundaries, and transition bands were gradually replaced by DRX grains, ultimately forming a band-like structure consisting of numerous fine DRX grains. The findings facilitate comprehension of the flow behavior and microstructural evolution mechanisms of as-homogenized superalloy with millimeter-grade coarse grains during hot deformation, thereby providing a foundation for the precise control of microstructures during the ingot-cogging process.