Oxygen-induced lattice distortion in β-Ti3Nb and its suppression effect on β to α″ transformation

We report a first-principles density functional theory study on the interaction of interstitial O atoms in body-centered cubic β-Ti3Nb alloy, the lattice distortion induced by O and its effect on the β to α″ transformation. Our calculations demonstrate strong repulsion between O atoms in general, but also reveal a weak attraction at an O–O separation of around 5.63 Å (3a), which corresponds to a Ti6Nb2O structure. At low concentration (Ti96Nb32O), O induces a local α″-like structure in β-Ti3Nb lattice, albeit to the extent of the nearest- and next-nearest neighbors, and exerts either a remarkable (6–8 meV atom−1) or a negligible (0–1 meV atom−1) energy barrier to the β to α″ transformation, dependent on the orientation of its elastic dipole with respect to the external stress. At high concentration (Ti6Nb2O), interstitial O atoms give rise to the shuffling of neighboring {1 1 0} planes along the 〈11¯0〉 direction and generate a semi-α″ structure, which is believed to be the nanodomain structure observed in experiment by Miyazaki and co-workers. In both cases, O brings up suppression effect on the β to α″ transformation.