The evolution mechanisms of large-sized primary γ′ phases during isothermal compression in GH4175 superalloy

In this paper, a dual-phase fine-grained microstructure of γ (1.75 ± 0.31 μm) + γ′ (0.99 ± 0.32 μm) with excellent high temperature plasticity is achieved through grain refinement and twinning formation of the primary γ′ phases during isothermal compression in GH4175 superalloy. The clustered and large-sized primary γ′ phases are refined through the synergistic effect of dynamic recrystallization (DRX) and dissolution. It is concluded that the primary γ′ phases are uniformly distributed, with the grain size refined to 0.99 μm at a deformation temperature of 1100℃, strain rate of 1 s⁻¹, and strain of 0.9. High deformation temperature, low strain rate, and high strain facilitate the occurrence of continuous-DRX (CDRX) and discontinuous-DRX (DDRX) in the primary γ′ phases; under such conditions, the fraction of DRX reaches up to 87.7 %. Besides Cr diffusion through the γ/γ′ phase interfaces dominates the dissolution process, the high angle grain boundaries (HAGBs) that originally exist and those newly generated by DRX promote the dissolution of the clustered and large-sized primary γ′ phases, thereby accelerating grain refinement. At the same time, the value of the Schmidt factor (m) increases from 0.29 to 0.49 through the formation of Σ3-type twinning in the primary γ′ phases, thereby improving plasticity.