Obtaining ultra-fine grains by hot deformation without inducing secondary phase pinning grain boundaries in Ni-based superalloy

To overcome the limitations of grain refinement via η-phase precipitation in Ni-based superalloys, this study establishes a precipitation-free hot deformation strategy within the γ single-phase region of Allvac 718Plus. By systematically varying temperature (1000–1150 °C) and strain rate (0.001–1 s^-1), we decouple their effects and reveal an interplay between dislocation-driven dynamic recovery (DRV), dynamic recrystallization (DRX), and deformation twinning. DRX nucleation is primarily controlled by dislocation density, promoted under high strain rates, while grain growth is enhanced at elevated temperatures and low strain rates. Twinning plays a crucial role in twin-induced DRX (TDRX) and microstructural stabilization. A mechanism map is proposed to illustrate the transitions among DRV, DRX, and twinning across deformation regimes. The optimized condition (1150 °C, 0.1 s^-1) achieves a uniform, ultra-fine (∼10 μm) grain structure with high hardness and no secondary phase formation. This framework offers a transferable approach to grain refinement in Ni-based and other FCC alloys without relying on precipitation control.