First-principles Study Of TiAl/Al₂O₃Interface

γ-TiAl intermetallic alloys are the potential candidates to replace heavyNi-based alloys in certain applications due to their low density, high specificstrength, high specific stiffness, and good creep resistance at elevated temperatures.However, the practical use of γ-TiAl alloys remains limited to about750⁸00oC dueto the rapid growth of a non-protective intermixed Al₂O₃+TiO2scale formed bycompetitive oxidation of the Ti and Al at high temperatures. This mixed oxide layerprevents the formation of a continuous and dense α-alumina that would provide amore effective oxidation barrier in high temperature applications. Currently, theunderstanding of the oxidation mechanisms of TiAl alloys at atomistic level is stillin confusion. Here, we performed a series of ab initio calculations on theγ-TiAl（111）/α-Al₂O₃（0001） interface to determine the atomic structure, ideal workof adhesion （Wad）, and bonding characters aimed to clarify the oxidationmechanisms of TiAl alloys at atomistic level.Firstly, we studied the surface energy and stability of seven low index TiAlsurfaces and three Al₂O₃（0001） surfaces with different terminations. By introducingpair interaction parameters we proposed a new formula to calculate surface energyof compounds. For the γ-TiAl surfaces, the Al-terminated （110） surface is the moststable surface, while for the Al₂O₃（0001） surfaces, the single Al-terminated surfaceis the most stable surface. It was shown that the electronic structures of the atoms inthe topmost and the second layers are the key factors that determine the surfacestability and structure.Secondly, we investigated bonding characteristics of TiAl（111）/Al₂O₃（0001）interfaces. Twelve interfaces were considered. The interface constructed byTiAl（111） and O-terminated Al₂O₃（the ORII） adaptation is the most sthe O atom in Al（ta0b0l0e1o） nseu.r fIat cise so ruingdinear ttehde b[y1t1h2e] TstiAroln∥g [1in0t1er0a]cAtli2oOn3between2O3and Al atom in TiAl. While this kind interaction isweak in the Al-terminated Al₂O₃（0001）/TiAl（111） interface although a hybridizationbetween Al-p and Ti-d orbitals was observed. Finally, the influence of Al vacancy inTiAl on the interface properties was studied. It indicates that the lack of Al atoms inTiAl surface increases the interface bonding strength and enhances the activity ofinterfacial Ti atoms, and therefore promotes the formation of TiO2, which willintend to generate a non-protective intermixed Al₂O₃+TiO2scale on the surface ofTiAl.