Phase transformation and microstructure evolution in near β Ti-B19 titanium alloy during aging treatment

The phase transformation and microstructure evolution of near β titanium alloy Ti-B19 during solution treatment and aging treatment has been studied systematically using OM, SEM and TEM. The results showed that Ti-B19 solution treated at 900°C for 1 h, air cooled, presented an equiaxed β structure consisting of metastable β phase and silicide phase. When the specimens were aged between 300°C and 350°C, the globular ω phase formed firstly in β matrix, and then gradually transformed to equilibrium particle α phase with prolonging of aging time, with increasing the aging temperature to 400°C-450°C, acicular α could precipitate directly from metastable β phase besides particle α precipitation from β phase through transition of co phase which is similar to that of 300°C to 350°C. Upon increasing the aging temperature to 500°C-650°C, acicular α phase precipitated directly in β grains or along β grain boundaries. Linear α phase consisting of particle α phase was found at the ageing temperature above 500°C. Aging at 700°C to 750°C, rod-like α phase precipitated along β grain boundaries or in β grains because of lower rate of nucleation and rapid rate of growth of α phase. Bulk-like α was found when aged at 700°C for 10 min. Usually, most of α phase precipitated from β phases obeyed the Burger's orientation relationship, however, some non-burger's α phase, e. g. 2α phase was observed in the specimens aged at 400°C-500°C for about 10 hours.