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 alpha phase increases with the increasing deformation, and the amount of Mo in alpha phase decreases with globularization. The fracture toughness exhibits a decreasing trend as the increasing volume fraction of globularized alpha phases. It is noteworthy that the fracture toughness of basket-weave structure has a significant decrease even though a small amount of alpha phases are globularized. The mechanism of this phenomenon could be explained by the different crack propagation mode between the microstructure with weaving tightly lamellar alpha phases and that with partially or fully globularized alpha 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 alpha 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 alpha phase. The model has a little error for prediction of fracture toughness with basket-weave structure due to underestimation of the secondary cracks.