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.

Previous Particle Boundary (PPB), as the detrimental structure in Powder Metallurgy (PM) components, should be eliminated by subsequent hot process to improve the mechanical properties. The objective is to investigate the Dynamic Recrystallization (DRX) nucleation mechanisms and grain growth behavior of the 3rd-generation PM superalloy with PPB structure. Microstructure observation reveals that PPB decorated with (Ti, Ta, Nb)C carbides belongs to the discontinuous chain-like structure. The elimination of PPB networks can be achieved effectively via hot deformation due to the occurrence of DRX. Four different DRX nucleation mechanisms were proposed and discussed in detail according to the special microstructure characteristics of the PM superalloy. Firstly, local lattice rotations can be detected in the vicinity of (Ti, Ta, Nb)C carbides during hot deformation and thus PPB structure serves as the preferential nucleation sites for DRX grains via Particle-Stimulated Nucleation (PSN). Then, Discontinuous-DRX (DDRX) characterized by grain boundary bulging dominates the microstructure refinement and Continuous-DRX (CDRX) operated by subgrain rotation can be regarded as an important assistant mechanism. At last, the initial Σ3 boundaries would lose their twin characteristics owing to the crystal rotation and then transform into the general High Angle Grain Boundaries (HAGBs). The distorted twins provide additional DRX nucleation sites, viz., twin-assisted nucleation. Particular attention was focused on the grain growth behavior of the PM superalloy in subsequent annealing process. The recrystallization temperature was determined to be about 1110 °C and 1140 °C can be considered as the critical temperature for grain growth. The findings would provide theoretical support for microstructure refinement of the 3rd-generation PM superalloy, which is of pivotal significance for improving the mechanical properties of aviation components.