Effects of hyperthermal exposure on microstructures and mechanical properties of casting TiAl-4822 alloys

This work investigates the alterations in the microstructure and mechanical properties of the Ti-48Al-2Cr-2Nb alloy (referred to as 4822 alloy) following simulated short-term superheated service at 800-950 ℃. The research focuses on the effects of thermal exposure temperature and holding time on the microstructure, room-temperature and high-temperature tensile properties, and high-temperature endurance properties of 4822 alloy. The results reveal that after exposure at 800-950 ℃, the α₂ phase within the α₂/γ lamellar colonies dissolves, forming coarse γ lamellae. As the thermal exposure time increases, the dissolution of the α₂ phase becomes more pronounced, the ends of the α₂/γ lamellar structures coarsen, and the lamellae grow into adjacent colonies. After treatment at 900 ℃, spherical B2 precipitates form at the boundaries between lamellar colonies and equiaxed γ grains. At 950 ℃, fine B2 precipitates appear within the coarse γ lamellae inside the colonies, and the number of B2 precipitates increases. Prolonging the exposure time leads to the precipitation of extremely fine γ lamellae within blocky α₂ phases, with their quantity increasing; equiaxed γ grains at colony boundaries gradually grow, and the number of equiaxed γ grains within the colonies also rises. After thermal exposure at 850 ℃/10 min, 850 ℃/120 min, and 900 ℃/10 min, both the room and high-temperature yield strengths of the alloy increase, while the room and high-temperature elongations decrease. However, after thermal exposure at 950 ℃/10 min, the room-temperature elongation of the alloy is (2.4±0.55)%, representing an increase of 5.73% compared to its original state. After thermal exposure at 850 ℃/10 min, 850 ℃/120 min, and 900 ℃/10 min, the stress rupture lives of the alloy under both 650 ℃/400 MPa and 700 ℃/350 MPa conditions are extended. However, after thermal exposure at 850 ℃/120 min, the stress rupture life under 650 ℃/400 MPa is (14.85±3.55) h, which decreased by 94.02% compared to its original state.