DRX mechanisms of a Ni-Co-W type superalloy with typical columnar grains during hot compression

Abstract

Columnar evolution and DRX mechanisms of a Ni-Co-W type superalloy with typical columnar grains during hot deformation were investigated processed at the γ′ sub/super-solvus temperatures. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) was employed to characterize the nucleation at columnar boundary. The results show that columnar grains gradually replaced by new dynamic recrystallization (DRX) grains and nucleation was prioritized at original columnar boundary or/and folding position. Meanwhile, DRX mechanisms of the columnar structure associated with γ′ phase were identified deeply. Discontinuous dynamic recrystallization (DDRX) was the main mechanism when deformation processed at γ′ sub/super-solvus temperatures. Bulging columnar grain boundaries are first as the nucleation sites of the new DDRX grains under various conditions. High angle grain boundaries (HAGBs: θ > 15°) bends to the side with γ′ phase, and then the DRX grains formed at the bulging grain boundary. In addition, high dislocation density zone occurred around the large size γ′ phases (more than 1 µm), leading to the misorientation generated rapidly and then the HAGBs formed. The formation of DRX grains by this way is also called particle stimulated nucleation (PSN) mechanism. Continuous dynamic recrystallization (CDRX) occurs when hot deformation processed at γ′ sub-solvus temperature with high strain level, which is accompanied by dislocation accumulation and an obviously increase in the fraction of grain boundaries with 10–15° misorientation angle. The findings help to understand different DRX mechanisms of Ni-based superalloy with columnar structure during microstructure evolution under hot forming, and are also beneficial for optimizing the microstructure to control the required properties of the Ni-based superalloys for manufacturing turbine disc.