On the β-transus and order/disorder transition temperature in superplastic super α2 Ti3Al base alloy

In previous studies on the superplastic deformation mechanisms in regular α2 and super α2 Ti3Al base alloys at temperature ranges of 900–1050°C, the β grains were considered to be a disordered phase. The ordered α2 grains were thought to act as the rigid species, and slide and rotate more freely within the disordered β matrix, and the disordered β grains are more likely to be the ones that carry accommodation through dislocation slip plus climb. In this study, it was found that the β grains over the temperature range of 900–1050°C were actually present as an ordered phase since there was no antiphase boundaries and the whole grain was of the same ordered domain. The order–disorder transition temperature for the β phase was determined to be around 1100°C. Since the β grains were in an ordered form at the superplastic temperature regime (900–1050°C), the accommodation process via dislocation glide+climb might have an equal chance in proceeding in the ordered β grains as well as in ordered α2 grains. Meanwhile, the β transus temperature was determined to be around 1120°C, based on phase characterization and volume fraction extrapolation.