Effects of ω-phase precipitation on β→α, α′′ transformations in a metastable β titanium alloy

Effects of ω phase precipitation on β→α, α′′ transformations in a metastable β titanium alloy have been studied by means of transmission electron microscopy. Isothermal holding at 300°C for 100 min after the solution-treatment at 1000°C yielded homogeneously distributed spherical ω phase particles of ∼10 nm in diameter. With raising the holding temperature, ω phase particles of the same orientation variant precipitated as groups. At temperatures above 350°C, α phase laths nucleated at the ω/β interfaces and grew into both the matrix and ω particles with consuming ω phase particles. Thus, the α/ω Takemoto and the ω/β Burgers relationships were held simultaneously, and the orientation of α laths nucleating at an ω/β interface was determined uniquely. In the specimens quenched directly from β single-phase region, α′′ martensite plates nucleated preferentially at β grain boundaries. The α′′ formation, however, was accelerated by a short time holding at temperatures between 600 and 300°C, but was largely retarded by the progress of ω phase precipitation.