Role of defects on microstructure development of beta titanium alloys

Microstructures of α precipitation from a deformed β matrix by thermomechanical processing were studied in a representative β titanium alloy, i.e. Ti-15V-3Cr-3Sn-3Al alloy. In the aged specimens, after solution treatment, the grain boundary is the most preferential nucleation site in the α precipitation. Localized slip occurs in the β matrix by cold rolling after solution treatment whereas {332}<113> deformation twins are formed by subzero rolling at 77 K. After subsequent aging of the rolled specimens, the α phase preferentially forms on those defects in the β matrix with a low energy orientation relationship (Burgers relationship). There is strong variant restriction in heterogeneous nucleation on such defects. Controlled short time annealing above the β transus (β recovery treatment) after cold rolling produces the fine β subgrain. By subsequent aging, α precipitates form preferentially on β subgrain boundaries and also within β subgrains. A great number of α variants are observed locally in the recovered and aged specimens than those in the cold rolled and aged specimens. β recovery treatment prior to aging improves the strength-ductility balance mostly because of the increase of local elongation.