Published online by Cambridge University Press: 26 February 2011
Stress-assisted discontinuous precipitation was observed during creep of Ti- 25Al-12.5Nb at.% and associated with microstructures in which large primary creep strains were observed [1]. It was found that a large shift between the equilibrium βo (B2) phase composition at the heat treatment temperature and disordered β(bcc) phase at the creep temperature provided a driving force for discontinuous precipitation of disordered beta phase. Applied stress accelerated the growth of discontinuous β phase at grain boundaries perpendicular to the principal stress axis, but did not produce a significant shift in composition. The difference between beta and ordered beta phase boundaries in the Ti-Al-Nb system at 650°C and 1040°C suggests that discontinuous precipitation or related dissolution should occur in all Ti3Al-Nb alloys.