Effect of Hot Deformation Parameters on Grain Refinement of an As-Cast Ni-Based Superalloy

The grain refinement behavior in superalloys has been identified for a long time, while its exact underlying mechanism remains to be explored. In this study, transmission electron microscope and electron backscatter diffraction technique were employed to investigate the fundamental mechanism governing the process of grain refinement for as-cast superalloys GH4720LI. The results show that grain refinement highly depends on the distributions of γ′ precipitates. In γ + γ′ two-phase region, γ′ precipitates could promote the dislocation multiplication, hinder the dislocation motion and lead to the formation of high-density dislocation substructure. Hence, the sub-boundaries and high-angle grain boundaries (HABs) are formed resulting from the gradual transformation of the dislocation substructures. In this process, the new grains are developed by quasi-continuous dynamic recrystallization (quasi-CDRX) and discontinuous dynamic recrystallization (DDRX). In addition, strain rate has little influence on grain size and DRX behavior at high strain rate and low temperature. While in γ single-phase region, the dislocation within grains is partly consumed through continuous original boundary migration (COBM) at 0.01 s⁻¹. Moreover, in some deformed grains which are expected to possess high dislocation density, new grains can be formed by DDRX. With the strain rate increasing, the process of COBM was suppressed while DDRX was promoted. Therefore, the main dynamic softening mechanism of alloy is DDRX under the high temperature and high strain rate.