Surface segregation of Si and its effect on oxygen adsorption on a γ-TiAl(111) surface from first principles.

We perform first-principles calculations based on the density-functional theory to study the surface segregation of Si and its effect on the oxygen adsorption on a γ-TiAl(111) surface for a range of oxygen coverage 0<Θ≤1.0 monolayer (ML). The calculated results show that the alloying Si atoms prefer occupying surface Ti sites to the sites in the bulk of γ-TiAl, which suggests the occurrence of Si surface segregation. When oxygen atoms adsorb on a pure γ-TiAl(111) surface, the most favorable sites are the adsorption sites with more Ti atoms as their nearest neighbors in the surface layer at all the calculated coverages and the interactions between adsorbed oxygen atoms are repulsive. However, when oxygen atoms adsorb on an Si-alloyed γ-TiAl(111) surface, the interactions between the adsorbed oxygen atoms are attractive at oxygen coverage 0<Θ≤1.0 ML. Meanwhile, the interactions between O and Al atoms become stronger whereas those between O and Ti atoms become weaker relative to oxygen adsorbed on a pure γ-TiAl(111) surface. The atomic geometry and density of state are analyzed. The results show that the surface ripple of the top metal layer for oxygen on a pure γ-TiAl(111) surface is Ti upwards, while that for oxygen on an Si-alloyed γ-TiAl(111) surface is Al upwards at high oxygen coverage (Θ≥0.50 ML). This effect of Si is of benefit to the nucleation of alumina, which is attributed to Si surface segregation and an increase of the surface Al:Ti ratio. This can help to explain why alloying the γ-TiAl(111) surface with Si could favor the formation of the Al(2)O(3) scale at the first stage and result in good oxidation resistance in experiments.