First-principles Investigationon On The Oxidation Behavior Of Ti₂AlNb Alloy

As a candidate material for the replacement of Ni-based superalloys,Ti2AINb-based alloys have a broad application prospect in aerospace field due to its excellent mechanics such as high specific strength,good high temperature fracture toughness,strong creep resistance,and so on.However,their practical applications are hindered by the serious oxidation at elevated temperatures.First principles calculations were performed in the present thesis to investigate the oxidation behaviors of Ti2AlNb alloys mainly the low index surfaces of O phase and the O/B2 interface.Adsorption behaviors of oxygen on the {001} surfaces of the O phase of Ti2AlNb alloy were studied to clarify the interaction mechanisms between oxygen and Ti2AlNb.The calculated surface energies indicate that the stoichiometric(010)surface is the most stable surface among the {001} surfaces.Various adsorption sites of oxygen atom on these surfaces were therefore considered to search the most stable adsorption configurations.The calculated adsorption energies and the electronic structures illustrate that O atoms tend to bond with transition metals in the surface and in the subsurface layers.The hybridization between O p,Nb d and Ti d orbitals was observed,and the charge transferring from nearest neighboring Ti and Nb atoms to the adsorbed oxygen occurred indicating that at the initial stage of the oxidation of Ti2AlNb alloy in high temperatures,oxygen atoms tend to form O-Ti and O-Nb oxides,a mixed oxide product,instead of a protective alumina.Dissociation and adsorption processes of oxygen molecules on the(010)surface were further studied.Oyxgen molecules spontaneously dissociate first,and the dissociated O atoms then adsorb on the surface and make bounding with metal atoms in the surface demonstrating that chemical reactions occurred instead of physics actions.Migration of O atom in(or toward)the surface(or the bulk)was calculated via the NEB method.The initial oxidation of the(010)surface is mainly controlled by the diffusion of oxygen atom from free surface into bulk,which required to over 2eV to overcome the diffusion barrier.The second part of this work studied the oxidation of an interface between the O phase and the B2 phase.In term of the crystal structures,the connection between 0(1 1 0)and B2(21 1)slabs owns the minimum lattice mismatch,which was therefore built up as a O/B2 interface.Through a serious examination including the influences of atomic arrangement,terminal elements,and relative positions on the stability of the interface,the HCP stacking,that the interfacial atoms of O phase are located at the top of the atoms in the second layer of B2 phase,was clarified as the most coherent interface between O(1 1 0)and B2(21 1)slabs.Adsorption of oxygen atom in the interface was studied and the estimated adsorption energies show that process is thermodynamiclly stable but with slightly less stable than that the adsorption of O atom on the O(010)surface.First-principle molecular dynamics simulations were performed at 300 K,1000K,and 1300 K,the nucleation of oxides at the O/B2 interface region was also simulated.At room temperature,we observe mixed oxides stacking together,including TiO2 and Al2O3 with varied crystalline structures were observed.With increasing of temperature,the formation of oxides turns from compact into porous making oxygen atoms easily diffuse through,which reconfirmed the oxidation resistance of Ti2AlNb alloys deteriorates at elevated temperatures.In summary,the inherent nature of Ti2AlNb determines that it can easily coordinate to oxygen,and oxide film containing a variety of elements may forms during the oxidation and keeps stable at moderate temperature.The protectiveness may lose in the environment with high temperature for the oxygen diffusion and the formation of loose oxides.