First-principles investigation of site preference and bonding properties of alloying element in TiAl with O impurity

We have investigated site preference and bonding properties of alloying elements including Nb, Mo, Ni and Ag in TiAl with O impurity using a first-principles method based on the density functional theory. We found that the preferable sites for O are the Ti-rich octahedral interstitial ones, while those for the alloying elements are the substitutional ones. Among these elements which are beneficial to improve the mechanical properties of TiAl, Ni and Ag occupy the Al sites, while Nb and Mo occupy the Ti sites. We demonstrate that the presence of O alters the site preference of these alloying elements in TiAl, making these elements prefer to substitute Al that is the first nearest neighbor of O, because O prefers to bond with Ti rather than Al. We suggest that, according to the local density of states results, O can be deleterious to the ductility of TiAl with Nb and Mo, but has little effect on that of TiAl with Ni and Ag.