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 (673 K–Tβ (about 1023 K)) 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 (573–673 K), 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 973 K, 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 523 K and the subsequent α precipitation at temperatures higher than 573 K, results in uniform and finer distribution of α precipitates.