The nucleation and growth of recrystallized grains in the dendritic structure of an as-cast Ni-based single-crystal superalloy during sub-solvus annealing

The hot deformation generated during the cooling stage of directional solidification may trigger recrystallization in single-crystal (SX) superalloy blades when subjected to subsequent solid solution treatment. To investigate the static nucleation and growth of the recrystallized grains and retard the recrystallization kinetics, an as-cast SX superalloy was compressed to a plastic strain of ∼9% at 1200 °C and annealed at sub-solvus temperatures of 1250 or 1300 °C, followed by microstructural, thermodynamic and kinetic analyses. It was shown that the distribution of the driving force for recrystallization was closely related to the dendritic structure, and the spatial variation of Zener drag on grain boundaries within the dendritic structure exhibited temperature dependence. Nevertheless, the preferential nucleation site for recrystallization was in the interdendritic region at all temperatures due to its significantly higher driving force and local misorientation compared to other regions. Furthermore, it was confirmed that high-angle grain boundaries (HAGBs) were formed near carbides and eutectics through discontinuous subgrain coarsening or annealing twinning, and subsequently migrated within the interdendritic region by promoting the dissolution of γ′ precipitates. Some of these HAGBs then swept through the γ′-depleted zones, leading to the formation of dendrite-like recrystallized grains. These results suggested that the recrystallization nucleation can be effectively suppressed by minimizing plastic deformation in the interdendritic region. This work will deepen the understanding of recrystallization in SX superalloys and be helpful for optimizing the heat treatment for inhibiting recrystallization.