Thermal modulation impacts on α-phase restructuring in directionally annealed TiAl: γL deformation mechanism at 900°C

Abstract

This study examines the impact of varied hot zone temperatures on the microstructural evolution of γ-TiAl alloy during directional annealing and its tensile properties at 900°C. Using the scanning electron microscope, electron backscatter diffraction, and transmission electron microscopy, the research focuses on the deformation mechanisms of γ_L at 900°C. Key findings include a positive correlation between the aspect ratio of α columnar crystals and hot zone temperature, with optimal conditions under non-isothermal continuous heating (e.g., 1350°C → 1410°C). The reduction of transverse grain boundaries in α columnar crystals enhances the ductility of the TiAl alloy at 900°C. The presence of active micron-scale γ_M within the near-lamellar structure is found to be detrimental to high-temperature ductility. Discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization are identified as the primary softening mechanisms during high-temperature deformation, with dynamic recrystallization grains potentially multiplying according to an interface relationship of <110>/70°. Cross-slip of numerous 1/2[110] ordinary dislocations facilitate DDRX and certain twin behaviors within DRX grains. These insights contribute to understanding the anisotropic migration capabilities of α grain boundaries influenced by varying (α+γ)/α hot zone temperatures and the deformation mechanisms of γ-TiAl alloy at 900°C.