ABSTRACT
Oxidation behavior of the two-phase Nb/Nb(5)Si(3) composite is of significant importance for the potential applications of the composite at high-temperature conditions. We investigate the atomic-scale oxidation mechanism of the Nb/Nb(5)Si(3) composite with first-principles density-functional theory and thermodynamics analysis. In particular, the effects of energetics, thermodynamics, segregation, and interfaces are identified. The clean composite surface is found to be composed of both Nb(110) and Si-terminated Nb(5)Si(3)(001). Energetics and thermodynamics calculations show that, during the oxidation process, the Nb(110) surface is oxidized first, followed by the segregation of niobium of the Nb(5)Si(3)(001) surface and subsequent oxidation of the Nb element of Nb(5)Si(3). High coverage of oxygen results in dissolved oxygen in bulk Nb through the diffusion of oxygen in the surface and at the interface. The theoretical investigation also provides an explanation, at the atomic-scale, for the experimental observation that the oxidation layer is essentially composed of niobium oxide and almost free of silicon. Furthermore, the methodology of this work can be applied to investigations of the oxidation behavior of other two-phase and multi-phase composites.
ABSTRACT
Ab initio density-functional theory and thermodynamics calculations are combined to establish a microscopic mechanism for the oxidation of the α(2)-Ti(3)Al(0001) surface. The surface energies as functions of the chemical potentials, as well as structural relaxations and electronic densities of states, are determined. The surface phase diagram (SPD) of the α(2)-Ti(3)Al(0001) systems with different defects and at various oxygen coverages is constructed. It is found that the Al antisite defect prefers to segregate on the α(2)-Ti(3)Al(0001) surface and oxygen adsorption enhances the segregation with the formation of the surface with three Al antisites per unit surface cell (i.e. the top surface layer is full of Al atoms) at the initial stage of oxidation, accounting for the aluminum selective oxidation observed experimentally. After the initial stage of oxidation, the O-α(2)-Ti(3)Al(0001) system manifests itself with a non-uniform double-phase SPD, suggesting the competition between oxidations of the Al and Ti elements in the oxidation process. This result explains the experimentally observed second regime of oxidation in which both metal elements are oxidized.
