Titanium aluminides based alloys have been considered to be the most potential high temperature structural materials for their combined properties of low density, high specific strength and relatively good properties at elevated temperatures. However, low oxidation resistance above 800 ℃ is the main obstacle to their widely application in industry. Due to the restrictions of experimentally observational techniques and the varieties of oxidation conditions, the mechanism at atomistic level is not clearly understood yet. The first-principles calculation, a powerful tool to theoretically explore the surface oxidation, was used in this thesis to identify a number of consecutive atomic surface structures through which γ-TiAl proceeds during the initial oxidation process, in order to provide theoretical evidence to understand the mechanism and improve the resistance.
Firstly, γ-TiAl surface structures and stabilities were calculated, followed by oxygen adsorption on these surfaces. It is shown that Ti terminated surfaces are chemically active due to a relative high surface energy, leading to strong oxygen adsorption on these surfaces. For stoichiometric surfaces the surface energy is lower and oxygen adsorption is weaker. As for Al terminated surfaces, they are more stable under high Al chemical potential ranges, thus oxygen adsorption is the weakest. Compared with the clean γ-TiAl(111) surface, oxygen covered surface structures are more stable, and high oxygen coverage is accomplished very soon for surface adsorption. Additionally, preference of O binding with surface Ti rather than Al is revealed on γ-TiAl(111) surface.
Secondly, how oxygen incorporate in γ-TiAl(111) surface was investigated. It is found that on-surface Ti2Al neighboring sites are occupied first. Sub-surface interstitial sites have lower binding energies and start to dissolve at high surface oxygen load, forming the protectiveless mixed-layer structure instead of aluminides or titanide oxide structures, but it can be relaxed to an ordered-layer structure with fewer vacancies near the surface.
Thirdly, the influence of Nb addition on surface adsorption and bonding was calculated. It is noticed that Nb doping does not reverse the preference of the surface adsorption site; instead it lowers the binding energy of oxygen adsorption. However, this is localized effect and the sub-layer Nb addition can only take effect after oxidation of surface layer. The energy barrier for oxygen diffusion into sub-surface interstitial site is increased by Nb addition, which thus results in enhancement of the oxidation resistance of the alloy.
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