Competitive dislocation nucleation quantifies γ-TiAl ductility

The twin boundaries (TBs) play an important role in improving the strength or ductility of γ-TiAl, yet their underlying mechanism remains incompletely elucidated. In this work, we elaborate the ductile mechanism of nanotwinned γ-TiAl and establish a theoretical model to clarify the competitive relationship between slip systems during TB migration-induced ductility under different TB orientations. The atomic mechanism is attributed to the deformation induced TB migration which is accompanied by breaking and re-forming Ti–Ti and Al–Al bonds in the region of easy slip system ( 111 ) [ 11 2 ¯ ] . The TB migration capacity is closely related to the proportion ( ρ ) of the ( 111 ) [ 11 2 ¯ ] orientation in the nanotwinned structure, which results in the linear trend between the ultimate strain and the ρ in γ-TiAl. These theoretical predictions are directly validated by in situ shear tests of polysynthetic twinned TiAl crystals, where post-deformation microstructural analysis confirms TB migration as the operative ductility mechanism and underscores the universality of interface-dominated deformation in γ-TiAl alloys. Our model further reveals that TB migration is enabled when the dislocation nucleates and slips along the TBs; this activation occurs because the critical stress along the ( 111 ) [ 11 2 ¯ ] direction being reached earlier than that along other directions. Consequently, nanotwinned γ-TiAl exhibits ductility only within specific TB orientation ranges (e.g., 0-26°, 72-108°, and 160-180° in true nanotwinned γ-TiAl).