An in-situ study on deformation and cracking initiation in oxygen-doped commercial purity titanium

Oxygen solid solution has been long known as a double-edge sword to Ti alloys, where, on the one hand, it has a strong strengthening effect while, on the other hand, it also embrittles the alloys. In the present work, in-situ observations using SEM and EBSD were carried out under tension to explore the mechanism that controls those effects. The results suggest that, by increasing oxygen content, the 〈a+c〉 type slip becomes more favorable than the 〈a〉 type, other than suppressing deformation twinning. As a result, the crack nucleation sites transit from grain boundaries (especially at triple junctions) into grain interior with a planar shape, which finally leads to easy cleavage. Comprehensive analysis and discussion suggest that the transition of fracture mode is due to the presence of oxygen solutes that modify the local stress state of geometrically necessary dislocation (GND) pile-ups, which then interfere with the condition for {112¯2¯} twin nucleation and turn to the activation of {112¯2}〈a+c〉 slips.