Microstructure and phase transformations of ωo-Ti4Al3Nb based alloys after quenching and subsequent aging at intermediate temperatures

Abstract

The existence of ωo (ordered ω) phase in TiAl alloys in different compositions at intermediate temperatures has been widely reported. In this study, the phase transformation behaviors of ωo-based alloys at 750–900 °C were studied by electron microscopy. It is found that fine-grained microstructures mainly composed of γ+ωo and a tiny amount of α2 phase precipitated in the alloys quenched from high temperature during aging. Twins form in α2 phase after aging, which is analogous to observations reported in deformed TiAl alloys. The γ phase precipitates originate from the stacking faults of α2 laths and grow up by coarsening at 750 °C. Semi-coherent interfaces between γ and ωo phases are formed and a high density of stacking faults can be observed inside the α2 laths. At higher temperatures, the direct nucleation of γ phase from the ωo/βo matrix becomes active. Various orientation relationships are found between ωo, γ and α2 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 ωo phase is not as obvious as reported for some other elements.