The phase transformation behavior between γ lamellae and massive γ in a Ta containing TiAl-based alloy

The phase transformation behavior between massive γ (γ_m) phase and γ lamellae was studied through interrupted quenching from single α-phase region and α+γ two-phase region in a Ti–48Al–3Nb-0.5Ta alloy. Massive α→γ_m phase transformation temperature T₀ is confirmed to be 1400 °C (in α+γ two-phase region). Combining with the thermodynamic analysis, it indicates that the formation of γ_m phase is gradually inhibited with the decrease of quenching temperature, instead fine γ lamellae are preferred. In this case EBSD (Electron back scattered diffraction) and HRTEM (high resolution transmission electron microscope) have shown that besides nucleating on α grain boundary, γ_m phase can nucleate on defects in the interior of an α grain with Blackburn orientation relationship or nucleate on fine γ lamellae along close-packed {111}γ with orientation relationship of [121‾] γ_lamellae//[1‾11]γ_m1// [101‾1‾]α₂ at their interface. Furthermore, the ledge growth mechanism of γ_m phase is identified.