Anisotropy-induced nonuniform deformation behavior in polysynthetically twinned TiAl bicrystals at 900 °C

Polysynthetically twinned (PST) TiAl crystals exhibit remarkable high-temperature mechanical properties, and their well-aligned lamellar orientations via directional technology enable a potential increase in service temperature to 900 °C. To address the strong deformation anisotropy of PST single crystals, this study explores deformation coordination between two PST crystals to achieve the strength-ductility trade-off via tailored orientation composition. Focusing on two bicrystals B1 (65°, 8°) and B2 (20°, 15°) with distinct orientation compositions, EBSD examined their orientations, and the misorientation Δθ₁₂ is confirmed to be similar, as 61° and 66° respectively, while introducing a single crystal of S1 (23°) as a reference. SEM and EBSD analyses elucidate deformation behaviors, including crack propagation, dislocation slip, and potential twinning, and the further interfacial failure mechanism and dislocation mechanism were discussed via TEM observation. Results demonstrate that the deformation of bicrystals always initiates at the relatively softer grain and propagates to harder counterparts. Bicrystals' deformation compatibility determines whether the nonuniform deformation occurs and whether the work hardening effect or the strain coordination effect is the dominant strengthening mode. Elevated temperatures facilitate γ-phase twinning (γ variant-dependent) and α₂-phase dislocation slip (related to α₂ cylindrical slip tendency). Further discussion on the dislocation bahaviors indicates that ordinary dislocations significantly affect the initial deformation capacity, while the ability to activate newborn twins and superlattice dislocations affects the interfacial failure tolerance capacity and subsequent deformation coordination. A further understanding of bicrystals' deformation anisotropy facilitates achieving the strength-ductility trade-off by designing appropriate lamellae composition.