Microstructure and phase transformations of ω$_o$-Ti$_4$Al$_3$Nb based alloys after quenching and subsequent aging at intermediate temperatures

The existence of ω$_o$ (ordered ω) phase in TiAl alloys in different compositions at intermediate temperatures has been widely reported. In this study, the phase transformation behaviors of ω$_o$-based alloys at 750–900 °C were studied by electron microscopy. It is found that fine-grained microstructures mainly composed of $γ$+ω$_o$ and a tiny amount of $α_2$ phase precipitated in the alloys quenched from high temperature during aging. Twins form in $α_2$ phase after aging, which is analogous to observations reported in deformed TiAl alloys. The $γ$ phase precipitates originate from the stacking faults of $α_2$ laths and grow up by coarsening at 750 °C. Semi-coherent interfaces between $γ$ and ω$_o$ phases are formed and a high density of stacking faults can be observed inside the $α_2$ laths. At higher temperatures, the direct nucleation of $γ$ phase from the ω$_o$/β$_o$ matrix becomes active. Various orientation relationships are found between ω$_o$, $γ$ and $α_2$ phases, leading to a number of grain clusters having smooth interfaces in between. In addition, the effects of Ta, V and Zr on the microstructure are investigated. The effect of these elements on the stability of the ω$_o$ phase is not as obvious as reported for some other elements