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Titanium alloys are ideally suited for use as lightweight structural materials, but their use at high temperature is severely restricted by oxidation. Niobium is known to confer oxidation-resistance, and here we disprove the normal explanation, that Nb<sup>5+</sup> ions trap oxygen vacancies. Using density functional theory calculation, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) we show that Nb is insoluble in TiO<inf>2</inf>. In fact, the Ti-Nb surface has three-layer structure: the oxide itself, an additional Nb-depleted zone below the oxide and a deeper sublayer of enhanced Nb. Microfocussed X-ray diffraction also demonstrates recrystallization in the Nb-depleted zone. We interpret this using a dynamical model: slow Nb-diffusion leads to the build up of a Nb-rich sublayer, which in turn blocks oxygen diffusion. Nb effects contrast with vanadium, where faster diffusion prevents the build up of equivalent structures.
- Computer simulations
- Gas-solid reactions
- Scanning electron microscopy SEM
- Synchrotron radiation
- Transition metal alloys and compounds
Ackland, G., Siemers, C., Tegner, B. E., Saksl, K., Brunke, F. & Kohnke, M., 2015, Ti-2015 conference procedings. 5 p.
Research output: Chapter in Book/Report/Conference proceeding › Conference contributionOpen AccessFile