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-T beta (about 1023 K)) after solution treatment, alpha precipitation occurs first at beta grain boundaries, followed by intragranular alpha precipitation. Intragranular alpha precipitates are lath-shape. At lower temperatures (573-673 K), alpha aggregates which are composed of fine alpha precipitates, are formed uniformly within beta grains by nucleating at precursory isothermal omega particles. In the aging of cold-deformed supersaturated beta, the competition of recovery/recrystallization of beta matrix and cc precipitation takes place. Cold rolling after solution treatment introduces coarse and planar slip bands in beta. Subsequent aging at temperatures lower than around 973 K, results in the preferential nucleation of alpha laths on the dislocations in such slip bands. Variants of alpha are strongly restricted by the nature of dislocations, Short time annealing above the beta transus after cold rolling produces the fully recovered, fine beta subgrain structure. When the recovered specimens are aged, alpha precipitates nucleate at beta subgrain boundaries. More numbers of oc 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 omega precipitation at 523 K and the subsequent a precipitation at temperatures higher than 573 K, results in uniform and finer distribution of alpha precipitates. (C) 2001 Elsevier Science B.V. All rights reserved.