The effects of microstructure evolution on the fracture toughness of BT-25 titanium alloy during isothermal forging and subsequent heat treatment

Isothermal compression with four different height reductions (0, 30%, 50% and 80%) and subsequent heat treatment was applied to BT25 alloy. Microstructure evolution following deformation and heat treatment was studied, meanwhile, the effect of various microstructure on fracture toughness of BT-25 alloy was analyzed in this paper. The microstructure observation and chemical analysis shows that volume fraction of globularized α phase increases with the increasing deformation, and the amount of Mo in α phase decreases with globularization. The fracture toughness exhibits a decreasing trend as the increasing volume fraction of globularized α phases. It is noteworthy that the fracture toughness of basket-weave structure has a significant decrease even though a small amount of α phases are globularized. The mechanism of this phenomenon could be explained by the different crack propagation mode between the microstructure with weaving tightly lamellar α phases and that with partially or fully globularized α phases. Fractography shows that fracture surface of undeformed material is characterized by deep secondary cracks and big fracture steps, and the surface becomes flatter with increasing volume fraction of globularized α phases. In addition, a model that can predict K_IC using tensile properties is utilized. It can provide a relatively reliable prediction for fracture toughness of BT-25 alloy with globularized α phase. The model has a little error for prediction of fracture toughness with basket-weave structure due to underestimation of the secondary cracks.