DFT study of Ni segregation at the B2-NiTi(110)/rutile-TiO2(110) interface

Abstract

NiTi alloy has important properties such as shape-memory, super-plasticity, resistance, and biocompatibility, therefore is frequently used in biomedical devices. We studied the oxidized form of this alloy and possible Ni segregation in three possible models for the TiO2/NiTi interface: i) the perfect interface; ii) the Ni enriched interface and iii) with oxygen vacancies in the TiO2 coating. We used DFT + U calculations for TiO2, because Hubbard correction encourages localization of the excess electronic charge and improves the DFT results. A standard spin polarized DFT was used for NiTi. We obtained the optimized geometries for each model and computed segregation energy, electronic structure, magnetic moment and bond order for the interface and for Ni segregation. Our calculations indicated that Ti3+ species are present at the interface, while in the bulk of the oxide layer the species is the Ti4+. Also, the electronic structure show that the metallic character of the alloy is unaffected by the interaction with oxygen. These results are consistent with experimental data from literature. Ni enriched interface containing O vacancies show a strong geometric distortion in the TiO2 layers. In all cases, Ni segregation is an a thermodynamically unfavorable process, helping to understand the biocompatibility of NiTi alloy.