First-principles Study Of Oxygen Diffusion On TiAl Surface And TiAl/TiO₂Interface

One obstacle for TiAl-based alloys used as high temperature structuralmaterials is its low oxidation resistance above850°C. The present thesis studied theinterfacial property between the stoichiometric γ-TiAl and its oxidation productTiO2(rutile) including the adsorption of oxygen atom at the (111) and (110) surfacesof TiAl, the diffusion of oxygen atom on TiAl surfaces and on TiAl/TiO2interfacesunder the framework of first-principles total energy calculations. The mechanism ofTiO2film influences on the oxidation of TiAl was clarified from the electronicstructure analysis.For the free surfaces of TiAl, calculations indicate that the (111) surface is themost stable one, while the Al terminated (110) surface (shorted as Al-(110) in thistheses) is less stable than the (111) surface by~0.3J/m2higher surface energy.Study on the adsorption of oxygen on the free surfaces of TiAl shows that the Oatom prefers to adsorb at the Ti-rich environment: the fcc-Al site on the (111)surface and the b1/oct-Ti sites on/under the Al-(110) surface. Movement of oxygenatom was restricted by the interactions between O and superficial atoms introducedby the adsorption of oxygen. On the dense (111) surface, there is a competitionbetween O-Al and O-Ti interactions and the former is however stronger than thelater. It is difficult for oxygen atom to diffuse in the surface atomic layer. For theAl-(110) surface, the diffusion of O on the surface layer is limited by the O-Alinteraction with the energy barrier of~1.37eV from the most stable site to thesubaltern stable site.The research shows that the diffusion of O from the TiAl free surface intobulk is mainly controlled by the O-Al and O-Ti interactions, but weakly depends onto the environment (either the surface or bulk). The diffusion of oxygen from theAl-(110) surface into the TiAl bulk requires~2.08eV energy to overcome theconstraint from the surface Al atomic layer. The energy of oxygen diffusion from thesurface into the bulk along the minimum energy pathway varies nearly periodicallyas atomic layer alternatively between Al and Ti layers. The diffusion is obstructedby the atomic layer, especially Al atomic layer, and therefore the diffusion of oxygenfrom the Al atomic layer to the Ti atomic layer is more difficult than that in theopposite direction. If the interactions between Al and Ti were weakened then oxygencan diffuse into the bulk and therefore oxidize TiAl further.During the formation of Al-(110)/TiO2(110) interface, oxygen prefers to detachfrom TiO2and diffuses into TiAl. A complex defect composed of an O vacancy (inTiO2side) and an O interstitial (in TiAl side), which is labeled as OV-I, is formed. The diffusion barrier of oxygen from TiO2to TiAl is~1.20eV regardless whetherthe interface contains an OV-Idefect or is an O-rich interface, which lower by~1.40eV comparing to the corresponding process of Al-(110). The above results indicatethat the main diffusion barrier is Al atomic layer origins by strong O-Al interaction.Once O detaches from TiO2and overcomes the O-Al interaction, it will diffuse intoTiAl and participate in oxidation. That indicates the TiO2film can’t prevent thematrix from oxidation.