Effects of microstructure and stress triaxiality on ductility in alpha/beta titanium alloys

About the alpha/beta titanium alloys with equiaxed alpha grain, the effects of the microstructure and the stress triaxiality on the process of a micro-void coalescence type of fracture were investigated. Materials tested were 8 types of the titanium alloys with different alpha grain size. It was shown that the most of micro-voids in the alpha/beta titanium alloys were nucleated at the alpha/beta interfaces, and the microstructural parameter defined as the average distance between the alpha/beta interfaces was closely related with the dimple size observed on the fracture surface. The refinement of the microstructure leads to the deterioration in the ductility, though the near-beta titanium alloys with the ultra-fine particle of alpha (less than 1 mum) have exceptionally a different tendency from other alpha/beta titanium alloys. The dependency of the stress triaxiality on the fracture strain obtained from the notched round bar specimen were also investigated. The significant deterioration in the fracture strain with the increase of the stress triaxiality is observed, and it is shown that the void growth strain in the higher strength materials is relatively small. In addition, the correlation between the toughness and the ductility under the high triaxial stress state was investigated to clarify the dominant factors constructing the fracture toughness.