Optimization and controlling of the microstructure and mechanical properties of high-Nb TiAl alloy by extrusion temperature

Hot extrusion is an effective route to tailor the microstructure and mechanical performance of TiAl intermetallic alloys. In the present work, a Ti-45Al-8Nb-0.2W-0.1B alloy was multi-pass extruded at five temperatures spanning the (α + γ) two-phase region to the α single-phase region. The influences of extrusion temperature on microstructural evolution, room-temperature and high-temperature tensile properties, as well as fracture and deformation mechanisms were systematically investigated via SEM, EBSD and TEM characterizations. With increasing extrusion temperature, the volume fraction and average diameter of lamellar colonies continuously increase, and the microstructure gradually evolves from near-gamma to fully lamellar. The duplex microstructure exhibits a favorable combination of strength and ductility at both room and elevated temperatures. Fracture mode shifts from inter-granular fracture for near-gamma microstructure to inter-lamellar/trans-lamellar fracture for near/fully lamellar microstructure. Dislocation slip and twinning are the dominant deformation mechanisms at both room temperature and elevated temperature. However, at elevated temperature, a limited amount of dynamic recrystallization of γ grains and a stress-induced α 2 → ω o phase transformation are also observed, which locally contribute to stress relaxation and improved high-temperature plasticity. This work provides experimental guidance for microstructure optimization of high-Nb TiAl alloys by regulating extrusion temperature.