The nucleation of microcracks under tensile stress in multi-phase high Nb-containing TiAl alloys

A series of bending tests was conducted on specimens of different TiAl alloys to investigate the nucleation of microcracks under tensile stress. The distribution of microcracks and the deformation traces around them were characterized to study the role of plastic deformation intersecting with microstructural features in the microcrack nucleation. Results show that the microcrack nucleation plays an important role in the fracture of multi-phase high Nb-containing TiAl alloys. The microcracks tend to nucleate at colony boundaries in near fully lamellar and fully lamellar alloys, while they prefer B2/γ phase boundaries in (B2+γ) alloy. The heterogeneous deformation in different lamellar colonies was caused by plastic anisotropy of the lamellar structure. It leads to the nucleation of microcracks at colony boundaries. Meanwhile, due to the stress concentration at lamellar interfaces in the transversal deformation mode, a few microcracks nucleate inside lamellar colonies which have an orientation angle Φ near to 90°. The existence of brittle B2 phase leads to strain incompatibility and facilitates microcrack nucleation at B2/γ boundaries.