Size Effects of Fracture Toughness and Dependence of Microstructure in Ti-Alloys

摘要

The effects of microstructure and constraint on the process of micro-void coalescence type of fracture and toughness of alpha/beta titanium alloys with equiaxed a phase were investigated. Materials tested were 3 types of titanium alloys (Ti-6Al-4V, Ti-4.5Al-3V-2Fe-2Mo, and Ti-10V-2Fe-3Al alloy) and two or three different heat treatments were conducted for these three alloys in order to obtain different alpha grain size. Fracture toughness testing was performed and J_(IC) and T_(mat) values were evaluated. Correlation between the toughness and the ductility under the high triaxial stress state was investigated to clarify the dominant factors constructing the fracture toughness. It was shown that the refinement of microstructure leads to the reduction of ductility and fracture toughness because the most of micro-voids in alpha/beta titanium alloys were nucleated at alpha/beta interface. However, near-beta titanium alloys with ultra-fine particle of alpha (< 1 mu m) have exceptionally a different tendency from other alpha/beta titanium alloys. About the size effects, the difference of specimen thickness hardly affects on J_(IC) value but T_(mat) in paricular material was significantly influenced by the specimen thickness, corresponding to the dependence of triaxiality on the ductility.