Achieving serrated grain boundaries within gradient microstructures of nickel-based superalloys for dual-property turbine disks in aeroengines

Gradient microstructures strengthened by serrated Grain Boundaries (GBs) were achieved through a combination of Gradient Strain Deformation (GSD) and Serration Heat Treatment (SHT), with particular focus on microstructural evolution, underlying mechanisms, and the critical influencing factors. Dynamic recrystallization governed the microstructural evolution in the fine-grained and transition regions during GSD, where multiple nucleation mechanisms were active. Plastic deformation facilitated the dissolution of γ′ phase in fine-grained regions, ultimately resulting in its morphological transformation. During the subsequent SHT, serrated GBs formed within the gradient microstructures produced by prior GSD without disrupting the grain size gradient, thereby enhancing creep resistance. Two distinct mechanisms associated with γ′_gb particles governed the formation of the serrations at GBs. Owing to the stronger dragging effect of grain boundary junctions in fine-grained regions, the amplitude and wavelength of serrations in these regions were smaller than those in coarse-grained regions. Moreover, the formation of serrations exhibited a strong dependence on the inherent properties of the GBs. The random high-angle grain boundaries (HAGBs) with misorientation angles in the range of 30–59° tended to become serrated more easily during SHT due to their high mobility and the accelerated precipitation of γ′_gb particles at them. Low-∑ HAGBs and low-angle GBs were not prone to form serrations. In particular, serration formation was completely inhibited at ∑3 twin boundaries due to their extremely low mobility and the absence of γ′_gb particles.