On the beta-transus and order/disorder transition temperature in superplastic super alpha(2) Ti3Al base alloy

In previous studies on the superplastic deformation mechanisms in regular a lpha(2) and super alpha(2) Ti3Al base alloys at temperature ranges of 900-1 050 degrees C, the beta grains were considered to be a disordered phase. Th e ordered alpha(2) grains were thought to act as the rigid species, and sli de and rotate more freely within the disordered beta matrix, and the disord ered beta grains are more likely to be the ones that carry accommodation th rough dislocation slip plus climb. In this study, it was found that the a g rains over the temperature range of 900-1050 degrees C were actually presen t as an ordered phase since there was no antiphase boundaries and the whole grain was of the same ordered domain. The order-disorder transition temper ature for the beta phase was determined to be around 1100 degrees C. Since the beta grains were in an ordered form at the superplastic temperature reg ime (900-1050 degrees C), the accommodation process via dislocation glide climb might have an equal chance in proceeding in the ordered beta grains as well as in ordered alpha(2) grains. Meanwhile, the beta transus temperat ure was determined to be around 1120 degrees C, based on phase characteriza tion and volume fraction extrapolation. (C) 1999 Elsevier Science S.A. All rights reserved.