Evolution of microstructure and δ phase in an aging-treated nickel-based superalloy during hot compression

Abstract

The evolution of microstructure and δ phase (Ni3Nb) in an aging-treated nickel-based superalloy is studied by hot compression tests. Combining with the detailed microstructure observation experiments, the influences of deformation parameters on the dynamic recrystallization (DRX), fracture behaviors and dissolution mechanism of δ phase are characterized and discussed. The results indicate that the DRX fraction increases with the strain rising or the strain rate declining. The discontinuous DRX is the main nucleation mechanism, and the continuous DRX nucleation occurs under the low strain rate. Furthermore, the promoting effect of DRX on the fracture behaviors of δ phase presents in two mechanisms. On the one hand, the DRXed grains grow towards the fractured δ phase and extrude δ phase, during which the fracture of δ phase is further promoted. On the other hand, the high-density dislocation regions first transform into subgrains and then into DRXed grains, and δ phase is twisted by the torsional forces of subgrain rotation and DRXed grain rotation, consequently expediting the fracture of δ phase. In addition, the poor Nb regions are found in the γ matrix near δ phase under the low strain. But with the further straining, the poor Nb regions around δ phase disappear, and the distribution of Nb element tends to be uniform. This is because that the dislocations around the phase boundary between γ matrix and δ phase act as the diffusion channels for Nb atoms, which benefits the diffusion of Nb atoms and therefore facilitates the dissolution of δ phase.