Role of defects on microstructure development of beta titanium alloys

Microstructures of a precipitation from a deformed beta matrix by thermomechanical processing were studied in a representative beta titanium alloy, i.e. Ti-15V-3Cr-3Sn-3A1 alloy. In the aged specimens, after solution treatment, the grain boundary is the most preferential nucleation site in the alpha precipitation. Localized slip occurs in the beta 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 a phase preferentially forms on those defects in the beta 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 beta transus (beta recovery treatment) after cold rolling produces the fine beta subgrain. By subsequent aging, a precipitates form preferentially on beta subgrain boundaries and also within beta subgrains. A great number of a variants are observed locally in the recovered and aged specimens than those in the cold rolled and aged specimens. beta recovery treatment prior to aging improves the strength-ductility balance mostly because of the increase of local elongation.