α2 phase precipitation behavior and tensile properties at room temperature and 650 °C in an (α + β) titanium alloy

The microstructure, room temperature and 650 °C tensile properties of an (α + β) titanium alloy were investigated after aging over a temperature range of 600–750 °C following solution treatment. The results exhibit that both aging temperature and aging time influence the precipitation behavior of α2 phase, and the size of α2 phase affects the tensile properties. The growth speed of α2 phase gets quicker with the aging temperature increasing under the same aging time. For a given aging temperature, the size of α2 phase gets larger when prolonging the aging time. There are significant improvements in yield and ultimate tensile strength at room temperature and 650 °C after aging at 600 °C for 2 h, but these values decrease with the size of α2 phase increasing. It is found that α2 phase, less than 7 nm, does not impair the room-temperature ductility. However, when the size of α2 phase grows up from 7 to 15 nm, ductility decreases drastically. During 650 °C tensile process, with the particle size increasing from 3 to 15 nm, the interaction between moving dislocations and coherent particles changes from cutting mechanism to bypass mechanism. With the α2 particle size increasing from 3 to 15 nm, the interaction between moving dislocations and coherent particles changes from cutting mechanism to bypass mechanism at 650 °C, and 6 nm is in the range of critical radius, which corresponds to the maximum strengthening effect of α2 phase at 650 °C