Fracture toughness of the bimodal size lamellar O phase microstructures in Ti-22Al-25Nb (at.%) orthorhombic alloy

Bimodal size lamellar O phase microstructures containing coarse lamellar O phases and fine acicular O phases were obtained in Ti-22Al-25Nb orthorhombic alloy. The fracture toughness of such microstructure was studied. It is found that a higher volume fraction of B2 matrix and finer B2 grains are both favorable to increase the fracture toughness by inhibiting the propagation of the principal cracks. The microstructure with a higher volume fraction of B2 matrix has fewer lamellar O phases and fewer O/O boundaries. In front of the principal crack, O/O boundaries often provide initiation points of microcracks. The microcracks can promote crack growth by merging with the principal crack. Fewer O/O boundaries provide fewer initiation points of microcracks. In addition, more B2 matrix can blunt the crack more effectively. Thus more B2 matrix also helps prevent the principal crack merging with microcracks. As for the effect of the B2 grain size, finer B2 grains offer more B2 grain boundaries, which provide more obstacles to crack propagation. It is observed that the crack trended to cleavage along some crystal planes of B2 grains. B2 grain boundaries can deflect the cleavage crack and significantly increase the tortuosity of the crack path. Consequently, microstructure with a high volume fraction of B2 matrix and fine B2 grains possesses a high K_IC value up to 35.9 MPam. A model that can predict K_IC using tensile properties is utilized. The predicted K_IC shows a good agreement with the experimentally measured fracture toughness of the bimodal size lamellar O phase microstructures.