Influence of alpha/beta processing on fracture toughness for a two-phase titanium alloy

Effect of alpha/beta processing on fracture toughness of Ti-17 alloy is investigated. For this purpose, Ti-17 alloy is deformed to 0, 30% and 50% height reductions at 820 degrees C, subsequently heat treated for 820 degrees C/4h/WQ (water quenching) + 630 degrees C/8h/AC (air cooling). The interrelationship of fracture toughness and microstructure is evaluated, and the fracture mechanism is analyzed. The results show that beta particles are stretched along the metal flow direction as alpha/beta deformation, and the aspect ratios for microstructures with 0, 30% and 50% deformations are 1, 2 and 4, respectively. Alpha phase presents an almost perfect basket-weave structure for undeformed material, whereas the globularized structure can reach 25% and 60% for material of 30% and 50% deformation. Fracture toughness exhibits a decreasing trend as the increasing deformation degree, which is opposite with change law of the globularization fraction. The lamellar alpha can increase the tortuosity of the crack propagation path. For undeformed material, the fracture surface is characterized by large amplitude of ravines with steep ups and downs, and a long crack propagation path is created. In this case, the highest fracture toughness is obtained. As alpha/beta processing, the fracture surface is smoother, the steep ups and downs disappear, and a shorter crack path is created. This results in decreased fracture toughness of material. The tortuosity ratios L(epsilon)/L-0(epsilon) are about 1.45, 1.23 and 1.15 for 0, 30% and 50% deformations, respectively. In addition, the prediction model of fracture toughness considering tensile properties is established based on the Griffith-Orowan-Irwin relation, and it can provide a relatively reliable prediction for fracture toughness of Ti-17 alloy during alpha/beta processing.