Decomposition and phase transformation mechanisms of α2 lamellae in β-solidified γ-TiAl alloys

The decomposition of (α ₂ /γ) lamellae during service is an important microstructure degradation phenomenon that is worth studying. In this study, the decomposition mechanism of (α ₂ /γ) lamellae was systematically investigated by transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) techniques. The results showed facilitated α ₂ →β _o phase transformation by [11 2 ¯ 0]α ₂/[0 1 ¯ 1]γ semi-coherent α ₂ /γ interface due to its larger misfit than that of [11 2 ¯ 0]α ₂/[ 1 ¯ 10]γ interface. Ellipsoidal ω _o particles were formed at β _o /α ₂ interface and nucleated at the interface dislocations. The energy-dispersive X-ray spectroscopy (EDS) results demonstrated feasible depletion of Mo and enrichment of Ti at α ₂ /β _o interface to form ω _o phase. In addition, lath-shaped ω _o precipitates were observed between two precipitated large β _o grains formed by direct α ₂ →ω _o phase transformation. The formation mechanism of the lath ω _o phase was attributed to both element segregation and strain accommodation caused by β _o precipitate in α ₂ lamellae. The α ₂ /β _o interface served as preferential nucleation sites for ω _o phase due to the atomic structure of the interface, which crystallographically was closely associated with the structure of ω _o phase. Moreover, β _o precipitates at low stress introduced stacking faults in α ₂ lamellae, while delaying the parallel decomposition of α ₂ lamellae at high stress. In sum, these findings provided profound insights into the decomposition mechanism of (α ₂/γ) lamellae, relevant as theoretical basis for controlling the microstructures and improving the performance of β-solidified γ-TiAl alloys.