On the fracture behavior and toughness of TA15 titanium alloy with tri-modal microstructure

In this work, the micro-scale damage development and crack propagation features of TA15 titanium alloy with tri-modal microstructure consisting of equiaxed alpha (alpha(p)), lamellar alpha (alpha(l)) and beta transformed matrix alpha (beta(t)) were firstly investigated. On that basis, the dependences of fracture toughness of tri-modal microstructure on microstructural parameters were discussed. The results indicate that in the damage and fracture of tri-modal microstructure, voids mainly nucleate and grow up at the triple junctions and the interfaces of alpha(p)/alpha(p), alpha(p)/beta(t) and single-alpha(l)/beta(t), due to the micro-scale strain incompatibility at these locations. By internal necking or local shearing connection with these voids, the main crack achieves an increment. It leads to two main categories of crack paths, i.e., the connection-type path caused by the connection behavior and the natural-extension-type path progressing through voids after the connection with voids. The connection-type path exerts greater influence on the fracture toughness than the natural-extension-type path. Moreover, as for the connection-type path, it can also be classified into two sub-categories, i.e., the internal necking connection-type path and the local shearing connection-type path. As far as the energy consumption is considered, more internal necking connection-type path is favorable for improving the fracture toughness, while more local shearing connection-type path is detrimental. Besides, the above void evolution and crack propagation behavior will affect the tortuosity of crack path to some extent, which will also change the overall energy consumption and final fracture toughness. As for the dependence of fracture toughness on microstructural parameters, it is found that with the increase of alpha(p) content, the proportion of low-energy-consumed crack paths (local shearing across alpha(p) and colony-alpha(l)) increase and the crack path tortuosity decreases, both of which reduce the energy consumption and result in the continuous reduction of fracture toughness. As the alpha(l) content increases, the disorderly distributed single-alpha(l) significantly increases at lower alpha(l) content, which improves the crack path tortuosity and energy consumption. However, at a higher level of alpha(l) content, single-alpha(l) decreases dramatically and the content of colony-alpha(l) becomes high, which reduces the crack path tortuosity and energy consumption remarkably. So, the fracture toughness first increases and then decreases with the increase of alpha(l) content.