Velocity‐Iterative Directional Annealing Enables Ultra‐Elongated Columnar Grains and Exceptional Ductility in TiAl Alloys

ABSTRACT This study introduces a novel velocity‐iterative directional annealing (IDA) strategy to overcome stagnation challenges in columnar grain growth of Ti‐48Al‐2Cr‐2Nb alloys. By dynamically adjusting hot zone migration rates across iterative cycles, the IDA protocol achieved 92.2% synchronization between anisotropic α‐grain boundary (GB) migration and programmed thermal displacement producing ultra‐elongated columnar grains (41.5 mm length, 8 mm width) with refined α 2 /γ interlamellar spacing. High‐temperature tensile tests at 1173 K revealed exceptional ductility (106.4% elongation) and strength in IDA‐processed specimens attributed to synergistic deformation mechanisms in γ lamellae (γ L ): (1) hierarchical dislocation slip involving 1/2 < 110] ordinary dislocations and superdislocations with jog‐induced multiplication; (2) stress‐induced γ→α 2 phase transformation; (3) stress‐induced twins. Competitive dynamic recrystallization (DRX)‐twinning regimes were governed by γ L /loading‐axis orientation ( ϕ ) where soft‐oriented lamellae ( ϕ = 65.6°) exhibited 71.9% DRX fraction versus 22.3% twin density in hard‐oriented domains ( ϕ = 2.9°). The IDA‐driven columnar microstructure reduced transverse grain boundaries while establishing orientation‐optimized lamellar colonies, enabling coordinated plasticity through multiscale interface engineering. This work provides a paradigm for microstructure–property optimization in advanced TiAl alloys for extreme temperature applications.