Tuning anisotropic grain growth and high-temperature deformation in TiAl alloys via directional annealing: role of hot zone migration rate and 3D twin networks

Directional solidification (DS) enables fabrication of polysynthetically twinned (PST) TiAl alloys with superior mechanical properties; yet persistent challenges in industrial scalability and process stability hinder practical implementation. This work demonstrates crucible-free processing of columnar-grained/single-crystal TiAl alloys through solid-state directional heat treatment, proposing a novel pathway for industrial production of high-performance intermetallics. Through advanced multi-scale microstructural characterization and in situ grain boundary (GB) tracking, we reveal that reduced hot zone (HZ) migration rates ($V_{\text{thermal}}$=2 μms^-1) enhance α-GB mobility via curvature-driven dynamics, with the disparity in migration rates across the double GB junctions being compensated by the formation of GB coupling steps, producing columnar structures with 38.8 mm length and aspect ratios exceeding unity. Conversely, accelerated $V_{\text{thermal}}$ (8 μms^-1) refine α₂/γ lamellar spacing by 90.8 %, elevating 900 °C tensile strength by 87.4 MPa but compromising ductility by 48.1 %. The remarkable ductility observed at 2 μms^-1 is attributed to a three-dimensional twin network within the γ lamellae (γ_L), which is maintained by secondary twinning at stress-concentrated interfaces. These findings establish quantitative relationships between thermal processing parameters, GB kinetics, and deformation mechanisms, offering critical insights for designing high-performance TiAl components with balanced strength-ductility synergies.