Microstructure control by thermomechanical processing in β-Ti-15-3 alloy

The microstructure development in Ti-15V-3Cr-3Sn-3Al alloy by various thermomechanical processings is reviewed. In the aging at higher temperatures (673K-Tβ (about 1023K)) after solution treatment, α precipitation occurs first at β grain boundaries, followed by intragranular α precipitation. Intragranular α precipitates are lath-shape. At lower temperatures (573-673K), α aggregates which are composed of fine α precipitates, are formed uniformly within β grains by nucleating at precursory isothermal ω particles. In the aging of cold-deformed supersaturated β, the competition of recovery/recrystallization of β matrix and α precipitation takes place. Cold rolling after solution treatment introduces coarse and planar slip bands in β. Subsequent aging at temperatures lower than around 973K, results in the preferential nucleation of α laths on the dislocations in such slip bands. Variants of α are strongly restricted by the nature of dislocations. Short time annealing above the β transus after cold rolling produces the fully recovered, fine β subgrain structure. When the recovered specimens are aged, α precipitates nucleate at β subgrain boundaries. More numbers of α variants than in the cold rolling and aging are formed locally, leading to the improvement of strength-ductility balance. The low-temperature-high-temperature two-step aging treatment, which composed of the ω precipitation at 523K and the subsequent α precipitation at temperatures higher than 573K, results in uniform and finer distribution of α precipitates.