First-principle study of the of oxygen behaviour in γ-TiAl
Résumé
New TiAl-based alloys, as for instance the γ-TiAl intermetallic, are adopted for the last generation turbomachines in aerospace industry and their use is likely to increase in the future1. In this evolution, the environmental conditions will move to higher temperature and pressure conditions. The oxidation behaviour is therefore essential for such applications and it has been studied in the past2. However, fundamental data, as the diffusion coefficients of oxygen in these materials, are missing. The aim of this work is to derive these quantities from ab initio methods. γ-TiAl crystallizes in the P4/mmm space group and it can be seen as built by two structural units, constituted by octahedra, respectively made of 4Al+2Ti and 4Ti+2Al atoms. These sites are herein labelled as [Ti2Al4] and [Ti4Al2]. The accommodation of O atoms in the centre of these octahedra has been investigated by DFT calculations performed with the VASP3 code, using the projector augmented wave pseudopotential and the GGA approximation. Vibrational properties and interatomic forces constants were obtained by the finite displacement method, using the phonopy4 code. The results indicate that oxygen is not stable in the octahedral sites and that, when the structure is fully relaxed, it moves toward two other interstitial sites (Figure 1): the first, named α, corresponds to the Al-Al corners in [Ti2Al4]; the second, named β, is obtained by a shift along the z direction, toward the top or the bottom of the [Ti4Al2] octahedral centre. The latter is the site with lowest energy.
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