Influences of thermal exposure on the microstructural evolution and subsequent mechanical properties of a near-α high temperature titanium alloy

The microstructural evolution with respect to the precipitation of silicides and ordered α₂ phase in near-α TA29 Ti alloy during thermal exposure at 650 °C for different time has been investigated in this work, and the subsequent mechanical properties have been evaluated by tensile and fracture toughness tests. The results indicated that the (Ti, Zr)₆Si₃ silicides firstly precipitated in residual β films, and then began to precipitate within α plates. A large number of fine α₂ precipitates showing sphercial shape concurrently formed in the α plates at a short time. The ordered α₂ precipitates asymmetrically grew up along different crystallographic directions with increasing exposure time, leading to their spherical shape change to ellipsoidal shape and finally olive shape, in which the long axis was perpendicular to [0001]_α as viewed along [2‾110]_α direction. The number density of ordered α₂ was obviously decreased after 1000 h due to Ostwald ripening. The precipitation of silicides and ordered α₂ phase improved the tensile strength but deteriorated the ductility and fracture toughness, whereas the strength was decreased and the room temperature elongation was recovered slightly after long-term exposure because of α₂ ripening.