Effect of prior α2 phase on precipitation kinetics of O-phase in advanced Ti2AlNb alloy

To study the influence of prior α2 phase on O-phase formation kinetics in β matrix, different isothermal heat treatments were performed at 876 and 780 °C using an advanced Ti2AlNb alloy with varying fraction of α2 phase. The kinetics of phase transformations was followed by electric resistivity measurements coupled to multi-scale microstructure characterization using scanning and transmission electron microscopy, energy dispersive X-ray spectrometry (EDS) and quantitative X-ray diffraction. It is revealed that the effect of α2 phase on the formation of O-phase depends on transformation temperature: while at 876 °C prior α2 precipitates lead to significantly slower formation of O-phase, as compared to α2 free alloy, the kinetics of O-phase formation is faster at 780 °C in the presence of α2 phase. Using EDS data as well as accurate measurements of lattice parameter of β phase, the diffusion character of O-phase formation in β matrix with and without prior α2 is shown for 876 °C while at 780 °C the diffusionless character of the O-phase formation is revealed for the beginning of the transformation followed by the partitioning of the alloying elements for prolonged treatment durations.