Dynamic recovery and recrystallization of an as-cast SX superalloy during hot deformation

The plastic deformation introduced during the cooling stage (above 1000°C) of directional solidification is one of the primary reasons for the recrystallization of Ni-based single-crystal (SX) turbine blades in aero-engines during subsequent heat treatment. An as-cast SX superalloy DD33 was compressed at 1200°C with a Gleeble thermo-mechanical simulator to mimic such deformation. The microstructural evolution, dynamic recovery, and dynamic recrystallization nucleation of the as-cast SX superalloy during hot deformation are investigated. The results show that the highest stored energy occurs in the vicinity of the eutectics, and its energy in the interdendritic regions is higher than that in the dendrite cores/arms. The formation of deformation bands and related transition bands near the eutectics are the primary characteristics of microstructural evolution during hot deformation. The dynamic recovery in the eutectic regions includes the entanglement and annihilation of dislocations at eutectic/matrix interface, within nearby γ matrix or within the eutectic γ′ phase, as well as the formation of dense dislocation networks in these sites. Subsequently, the low-angle grain boundaries in the transition bands migrate, merge, and finally transform into high-angle grain boundaries. In other words, the recrystallized grains nucleate near the eutectics via subgrain growth. In contrast, the dislocations only tangle and annihilate at the γ/γ′ interfaces in other interdendritic regions and the dendrite cores/arms without initiating recrystallization under moderate plastic deformation (ε_p = 11.9%). This study will be helpful for understanding the local microstructural evolution of SX superalloys during directional solidification, as well as the recovery and recrystallization nucleation during the subsequent annealing.