PPB structure elimination, DRX nucleation mechanisms and grain growth behavior of the 3rd-generation PM superalloy for manufacturing aviation components
Baoyun Zhang, Yongquan Ning, Zhaotian WANG, Qiaomu LIU
Abstract
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.