Effect of Short range order on various Physical Properties in Alloys

Abstract

Molecular dynamics simulations and first-principles orthogonalized linear combination of atomic orbitals method have been applied in the present study of alloys and ceramics. Vienna ab initio simulation package has helped to get the structures with minimum energy and maximum stability. Various other small packages were found to be helping to get typical parameters of interest.

To investigate the effects of ordering on thermal properties of Ni3Al intermetallic alloy system, we applied molecular dynamics simulations. Semi-empirical potentials, based on the embedded atom method have been employed to calculate the lattice parameter, energy per atom, and radial distribution functions for the Ni3Al intermetallic alloy system. Thermal properties like thermal coefficient of linear expansion, specific heat, and melting temperature are deduced from the calculated parameters. Despite the simplicity of the model, results are found to compare well with the experimental data. Effects of atomic short range order on these parameters have also been studied.

To study electronic structure and optical properties of the Ni3Al intermetallic alloy, we used first-principles orthogonalized linear combination of atomic orbitals method. Disordered models at different temperatures were constructed using molecular dynamics and the Vienna ab initio simulation package. It is found that the average charge transfer from Al to Ni increases steadily with temperature until the liquid phase is reached. The localization index shows the presence of relatively localized states even above the Fermi level in the disordered models. The calculated optical conductivity of the ordered phase is rich in structures and in reasonable agreement with the experimental data. The spectra of the disordered Ni3Al models show a single broadened peak at 4.96 eV in the 0 K model which shifts towards 6.62 eV at 1400 K and then down to 5.83 eV in the liquid phase. Other results on the band structure and density of states are also discussed.

The electronic structure, optical and spectroscopic properties of two ternary aluminum silicon carbide ceramics Al4SiC4 and Al4Si2C5 are studied by density functional theory (DFT) calculations based on the orthogonalized linear combination of atomic orbitals (OLCAO) method. Both crystals are shown to be small gap semiconductors with indirect band gaps of 1.05 and 1.02 eV respectively. The calculated

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hole and electron effective mass and the interband optical properties, in the form of the complex dielectric function, show a high degree of anisotropy which can be traced to the unique structures of these two crystals. The calculated refractive indices are consistent with the values proposed in the literature. Mulliken effective charge and bond order calculations show that these crystals have a high degree of covalency with considerable charge transfer from Al and Si to the C atoms. The X-ray absorption near-edge-structure (XANES) for all crystallographically nonequivalent sites (K, and L-edge) are calculated and compared with those of cubic SiC. It is shown that the site-averaged Si-K and Si-L3 edges, and also the C-K edges are slightly different and broader than those of cubic SiC. Potential applications of these new ternary ceramics are also discussed.