Effect Of Oxygen On The Martensitic Transformation Inβ-Ti₃Nb Alloy

Titanium based alloys have been widely used in industry as structural materials. The β-Ti alloys, which have body-centered-cubic crystalline structure, are a new generation Ti alloys boasting a series of unparalleled and intriguing properties. A typical β-Ti alloy has high strength, low elastic moduli, shape memory capacity, and also super-elastic properties, making it extraordinary promising in bio-medical applications.It has been revealed through extensive developments and applications that oxygen plays an important role in β-Ti alloys, improving the strength and super-elasticity, lowering the elastic moduli. Moreover, the impact of oxygen is found to be manifested in association with martensitic transformation.Using density functional theory based first-principles quantum mechanical calculations, we here in this study choose a prototype β-Ti alloy, β-Ti3Nb, to investigate the distribution of, local lattice distortion induced by, and the influence on martensitic transformation of oxygen. We have obtained the following understandings:(1) Interstitial oxygen atoms are repulsive in β-Ti3Nb. Calculations of the solution energy of oxygen at atomic number concentration of0.8%,1.5%,6%,11%,50%(TiO), and67%(TiO2) demonstrate that it decreases with increasing content. Nevertheless, at a concentration of11%, oxygen has a solution energy lower than at6%, and hence a local minimum in solution energy as a function of concentration. Such a local minimum is likely corresponding to a meta-stable Ti-Nb-O structure.(2) At a concentration of11%, oxygen causes a remarkable shuffling between neighboring (110) planes in β-Ti3Nb, leading to a lattice transformation toward the α" phase. This transformation, however, is not complete, thus we name the transformed structure as semi-α" phase. On the other hand, oxygen at low concentration induces only local lattice distortion, but not a semi-α" phase.(3) Oxygen stabilizes β-Ti3Nb and increases the β-to-ω and β-to-α" transformation barriers. Interestingly, we find that the interstitial oxygen can be grouped into different classes according to their dissimilar impact on the phase transformation. In β-to-ω transformation, there are collapsing oxygen located in collapsing{111} planes, and un-collapsing oxygen located in un-collapsing{111} planes, with the former increasing remarkably the energy barrier and the latter ones exerts no noticeable effect. And in the β-to-α" transformation, there are compressed, unaffected, and relaxed oxygen atoms. While the compressed oxygen increases the energy barrier for phase transition, the other two groups create only negligible obstacle effect.