We have investigated site occupancy and interfacial energetics of a TiAl-Ti(3)Al binary-phase system with O using a first-principles method. Oxygen is shown to energetically occupy the Ti-rich octahedral interstitial site, because O prefers to bond with Ti rather than Al. The occupancy tendency of O in TiAl alloy from high to low is α(2)-Ti(3)Al to the γ-α(2) interface and γ-TiAl. We demonstrate that O can largely affect the mechanical properties of the TiAl-Ti(3)Al system. Oxygen at the TiAl-Ti(3)Al interface reduces both the cleavage energy and the interface energy, and thus weakens the interface strength but strongly stabilizes the TiAl/Ti(3)Al interface with the O(2) molecule as a reference. Consequently, the mechanical property variation of TiAl alloy due to the presence of O not only depends on the number of TiAl/Ti(3)Al interfaces but also is related to the O concentration in the alloy.

No abstract

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.

We employed a first-principles method based on the density functional theory to investigate the effect of impurity O on the site preference and elastic properties of α2- Ti 3 Al . We found that the O atom prefers to occupy the Ti -rich octahedral interstitial site in α2- Ti 3 Al . We calculated the elastic constants of α2- Ti 3 Al with single O atom, which demonstrate that the O presence has no large effect on α2- Ti 3 Al according to the empirical criterions. Other factors such as O cluster should be taken into account to understand the deleterious effect of O on α2- Ti 3 Al . Our results provide a useful reference to further study the mechanical properties of TiAl alloys.

No abstract