Formation mechanism of γ twins in β-solidified γ-TiAl alloys

Defects, such as stacking faults (SF) and twins, play a crucial role in stabilizing the order phase and improving the mechanical properties for engineering alloys. The formation mechanism of the γ twins from α₂-platelets within β₀ areas was analyzed at atomic-scale by atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HADDF-STEM). The growth faults (ABAB˙CBC) were formed in α₂-platelets, yielding a local D0₁₉→L1₀ (α₂ → γ) phase transformation. During the annealing process, the growth faults can be rearranged into γ (ABCABC) and γ^T (ACBACB) platelets via the slipping of Shockley partial dislocations 1/3[1¯010] and 1/3[101¯0], respectively. The γ and γ^T can merge into γ twins by accident during growing. The interface steps between α₂ and γ platelets implied that this transformation is a diffusion-controlled defect migration process. The present work provides profound insight towards the formation mechanism of γ twins, contributing to microstructure controlling and performance improving for TiAl alloys.