Influence of Hot Deformation Temperature on Grain Size and γ′ Phase in U720Li Alloy After Sub-Solvus Heat Treatment

Precise control of forging and heat treatment parameters is essential to achieve microstructural homogeneity in turbine disks, ensuring optimal mechanical performance for aerospace applications. This study examines the influence of the hot deformation temperatures on the grain size and γ′ phase characteristics of U720Li alloy following subsequent heat treatments. Samples extracted from a hot-extruded U720Li billet were subjected to isothermal compression within the temperature range of 1100–1130 °C, followed by holding at 1100 °C and 1120 °C for 4 h and air cooling. The results demonstrate that increasing the deformation temperature from 1100 °C to 1120 °C reduces the γ′ phase volume fraction at grain boundaries from 13% to 5%, weakens pinning effects, promotes grain growth during deformation, elevates grain boundary energy, and diminishes stored deformation energy, despite maintaining an equivalent degree of dynamic recrystallization. When the sub-solvus heat treatment temperature was 20 °C below the effective deformation temperature, Ostwald ripening dominated, resulting in a multimodal γ′ phase distribution after cooling. Conversely, when the sub-solvus heat treatment temperature 20 °C exceeded the effective deformation temperature, a significant portion of the intergranular γ′ phase dissolved, leaving a bimodal distribution comprising both large- and small-sized particles.