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

The decomposition of (α₂/γ) lamellae during service is an important microstructure degradation phenomenon that is worth studying. In this study, the decomposition mechanism of (α₂/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 α₂ →β_o phase transformation by [112¯0]α₂/[01¯1]γ semi-coherent α₂ /γ interface due to its larger misfit than that of [112¯0]α₂/[1¯10]γ interface. Ellipsoidal ω_o particles were formed at β_o /α₂ 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 α₂ /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 α₂ →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 α₂ lamellae. The α₂ /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 α₂ lamellae, while delaying the parallel decomposition of α₂ lamellae at high stress. In sum, these findings provided profound insights into the decomposition mechanism of (α₂/γ) lamellae, relevant as theoretical basis for controlling the microstructures and improving the performance of β-solidified γ-TiAl alloys.