20 research outputs found
The Boson peak of model glass systems and its relation to atomic structure
Abstract.: Bulk metallic glasses (BMGs) exhibit a rich variety of vibrational properties resulting from significant atomic scale disorder. The Boson peak, which reflects an enhancement of states in the low frequency regime of the vibrational density of states (VDOS), is one such experimental signature of amorphous materials that has gained much interest in recent times. However, the precise nature of these low frequency modes and how they are influenced by local atomic structure remains unclear. Past simulation work has demonstrated that such modes consist of a mixture of propagating and localized components, and have been referred to as quasi-localized modes. Using standard harmonic analysis, the present work investigates the structural origin of such modes by diagonalising the Hessian of atomistic BMG structures derived from molecular dynamics simulations using a binary Lennard Jones pair potential. It is found that the quasi-localized vibrational modes responsible for the low frequency enhancement of the VDOS exist in a structural environment characterized primarily by low elastic shear moduli, but also increased free volume, a hydrostatic pressure that is tensile, and low bulk moduli. These findings are found to arise from the long-range attractive nature of the pair-wise interaction potential, which manifests itself in the corresponding Hessian as long-range off-diagonal disorder characterized by a distribution of negative effective spring constant
Bonding in MgSi and AlMgSi Compounds Relevant to AlMgSi Alloys
The bonding and stability of MgSi and AlMgSi compounds relevant to AlMgSi
alloys is investigated with the use of (L)APW+(lo) DFT calculations. We show
that the and phases found in the precipitation sequence are
characterised by the presence of covalent bonds between Si-Si nearest neighbour
pairs and covalent/ionic bonds between Mg-Si nearest neighbour pairs. We then
investigate the stability of two recently discovered precipitate phases, U1 and
U2, both containing Al in addition to Mg and Si. We show that both phases are
characterised by tightly bound Al-Si networks, made possible by a transfer of
charge from the Mg atoms.Comment: 11 pages, 30 figures, submitted to Phys. Rev.
Vibrational Properties of Nanoscale Materials: From Nanoparticles to Nanocrystalline Materials
The vibrational density of states (VDOS) of nanoclusters and nanocrystalline
materials are derived from molecular-dynamics simulations using empirical
tight-binding potentials. The results show that the VDOS inside nanoclusters
can be understood as that of the corresponding bulk system compressed by the
capillary pressure. At the surface of the nanoparticles the VDOS exhibits a
strong enhancement at low energies and shows structures similar to that found
near flat crystalline surfaces. For the nanocrystalline materials an increased
VDOS is found at high and low phonon energies, in agreement with experimental
findings. The individual VDOS contributions from the grain centers, grain
boundaries, and internal surfaces show that, in the nanocrystalline materials,
the VDOS enhancements are mainly caused by the grain-boundary contributions and
that surface atoms play only a minor role. Although capillary pressures are
also present inside the grains of nanocrystalline materials, their effect on
the VDOS is different than in the cluster case which is probably due to the
inter-grain coupling of the modes via the grain-boundaries.Comment: 10 pages, 7 figures, accepted for publication in Phys. Rev.
The Boson peak of model glass systems and its relation to atomic structure
ISSN:1434-6028ISSN:1434-603
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