Immersed nano-sized Al dispersoids in an Al matrix; effects on the structural and mechanical properties by Molecular Dynamics simulations

We used molecular dynamics simulations based on a potential model in analogy to the Tight Binding scheme in the Second Moment Approximation to simulate the effects of aluminum icosahedral grains (dispersoids) on the structure and the mechanical properties of an aluminum matrix. First we validated our model by calculating several thermodynamic properties referring to the bulk Al case and we found good agreement with available experimental and theoretical data. Afterwards, we simulated Al systems containing Al clusters of various sizes. We found that the structure of the Al matrix is affected by the presence of the dispersoids resulting in well ordered domains of different symmetries that were identified using suitable Voronoi analysis. In addition, we found that the increase of the grain size has negative effect on the mechanical properties of the nanocomposite as manifested by the lowering of the calculated bulk moduli. The obtained results are in line with available experimental data.Comment: 15 pages, 8 figures. Submitted to J. Phys: Condens. Matte

An ab initio study of the structural and mechanical alterations of Ti-Nb alloys

This article describes a systematic theoretical investigation of the role of Nb substitution on the structural and mechanical properties of Ti-Nb alloys. The aim is to understand the origin of the low-rigidity of some of these materials. This quality makes these materials suitable for metallic implants. The mechanical stability conditions in conjunction with the calculated elastic constants of Ti-Nb predict the destabilization of α′ and ω structures, while the β-phase can be stabilized for Nb content above 10 at. %. The evaluated Young's moduli (E) follow the sequence of Eω > Eα′ > Εα″ > Εβ, revealing high Eω and Eα′ values (greater than 120 GPa), while the Eβ value converges to approximately 87 GPa. The averaged E, estimated from a weighted average of Eω, Eα′, Εα″, and Εβ ab initio values, reproduces the experimental Ti-Nb Young's modulus w-shaped curve. Young's modulus surface reveals highly anisotropic E values for all Ti-Nb phases. Eβ exhibits values under 30 GPa along the [100] direction for Nb compositions larger than 12 at. %, suggesting that the orientational growth of a Ti-Nb alloy is important for the design of low-rigidity alloys, especially at small Nb concentrations. These results can be used as a guide for the design of novel low-rigidity alloys for biomedical applications

Local atomic order, electronic structure and electron transport properties of Cu-Zr metallic glasses

We studied atomic and electronic structures of binary Cu-Zr metallic glasses (MGs) using combined experimental and computational methods including X-ray absorption fine structure spectroscopy, electrical resistivity, thermoelectric power (TEP) measurements, molecular dynamics (MD) simulations, and ab-initio calculations. The results of MD simulations and extended X-ray absorption fine structure analysis indicate that atomic order of Cu-Zr MGs and can be described in terms of interpenetrating icosahedral-like clusters involving five-fold symmetry. MD configurations were used as an input for calculations of theoretical electronic density of states (DOS) functions which exhibits good agreement with the experimental X-ray absorption near-edge spectra. We found no indication of minimum of DOS at Fermi energy predicted by Mott's nearly free electron (NFE) model for glass-forming alloys. The theoretical DOS was subsequently used to test Mott's model describing the temperature variation of electrical resistivity and thermoelectric power of transition metal-based MGs. We demonstrate that the measured temperature variations of electrical resistivity and TEP remain in a contradiction with this model. On the other hand, the experimental temperature dependence of electrical resistivity can be explained by incipient localization of conduction electrons. It is shown that weak localization model works up to relatively high temperatures when localization is destroyed by phonons. Our results indicate that electron transport properties of Cu-Zr MGs are dominated by localization effects rather than by electronic structure. We suggest that NFE model fails to explain a relatively high glass-forming ability of binary Cu-Zr alloy

Ab-initio and experimental study of phase stability of Ti-Nb alloys

A systematic theoretical and experimental study concerning the crystallographic structure and electronic properties of Ti-xNb (x 18.75 at% the β-phase is favoured against all other crystallographic structures in line with the experimental results. Interestingly, at high Nb content the α′ and ω hexagonal phases become unstable due to the electronic band filling close to the Fermi level EF, which is mainly due to Nb-p and Ti-d antibonding hybridizations. On the contrary, in the cubic β-Ti-25Nb (at%) the depletion of the occupied electronic states at the EF occurs mainly due to Nb-d and Ti-d bonding interactions, resulting in a stable β-TiNb structure. These data could enlighten the electronic origin of the Ti-Nb phase stability, thus, may contribute to the design of β stabilized low moduli Ti-based alloys suitable for load-bearing biomedical applications

Self-diffusion of adatoms, dimers, and vacancies on Cu(100)

We use ab initio static relaxation methods and semi-empirical molecular-dynamics simulations to investigate the energetics and dynamics of the diffusion of adatoms, dimers, and vacancies on Cu(100). It is found that the dynamical energy barriers for diffusion are well approximated by the static, 0 K barriers and that prefactors do not depend sensitively on the species undergoing diffusion. The ab initio barriers are observed to be significantly lower when calculated within the generalized-gradient approximation (GGA) rather than in the local-density approximation (LDA). Our calculations predict that surface diffusion should proceed primarily via the diffusion of vacancies. Adatoms are found to migrate most easily via a jump mechanism. This is the case, also, of dimers, even though the corresponding barrier is slightly larger than it is for adatoms. We observe, further, that dimers diffuse more readily than they can dissociate. Our results are discussed in the context of recent submonolayer growth experiments of Cu(100).Comment: Submitted to the Physical Review B; 15 pages including postscript figures; see also http://www.centrcn.umontreal.ca/~lewi

Cation vacancy diffusion on the NiO(001) surface by molecular dynamics simulations

We present results concerning the cation vacancy diffusion mechanisms occurring on the NiO(001) surface along with their corresponding migration energies. Using a rigid ion potential and molecular dynamics simulations we found that the cation vacancy diffusion is the main diffusion process in this surface employing the simple and two different double hopping mechanisms. Although the hopping rates of the latter are about two times lower than that of simple jumps, at high temperatures their contribution to the total vacancy diffusion coefficient is comparable with that of simple jumps. In addition, for all mechanisms saturation is found in the Arrhenius diagrams above a temperature T-s = 2850 K. We attributed this behavior to collective and correlated movements of the surface atoms that are activated from multi-phonon processes taking place above T-s. Similar behavior was found recently in the case of metals. (C) 1999 Published by Elsevier Science B.V. All rights reserved

Molecular dynamics simulation study of Ni2+ adatom diffusion on the NiO (001) surface

Using molecular dynamics simulations we studied the cation adatom diffusion on the NiO(001) surface. We found that there are two equilibrium adatom positions: one over an oxygen ion, that is raised by as much as 30% at high temperatures and another one over a flat region between the surface cations and anions. The adatom diffuses via hopping or exchange type mechanisms, in which the surface anions participate as well. The contribution of the two mechanisms to the total adatom diffusion is discussed

Cation adatom diffusion on the NiO(001) surface by molecular dynamics simulation

We present results concerning the diffusion of Ni2+ adatom on the (001) surface of NiO obtained by molecular dynamics simulations based on a rigid ion potential model. A wide temperature region was covered ranging from 0.29T(m) up to 0.85T(m), T-m being the melting point of the model system. Two possible adatom positions were found on the surface in accordance with static calculations. From the detailed analysis of the ionic trajectories it came out that the adatom diffuses on the surface via hopping and exchange mechanisms. Both processes exhibit Arrhenius behavior from where we deduced the corresponding migration energies. In addition, we found two distinct temperature regions reflecting different energetic requirements for hopping diffusion. This is due to the spontaneous creation of anionic adatom that combine with the cationic adatom at high temperatures. Moreover, we found that although the frequency rates for hopping and exchange are comparable, the exchange mechanisms participate to the diffusion coefficient more than the hopping process by as much as an order of magnitude at high temperatures. (C) 2001 Elsevier Science B.V. All rights reserved

Structure and dynamics of NiO(001) and Ni/NiO(001) surfaces by molecular dynamics simulation

Using molecular dynamics and a rigid ion potential, we studied the vibrational and structural properties of the NiO(001) surface with and without cationic adatoms. We found that the bulk phonon density of states (DOS) is altered by the presence of the surface and that the adatom introduces new frequency modes. From the relaxed positions of the surface ions, it came out that up to 0.7TT(m), the surface is contacted exhibiting rippling with outwards expansion of the cations and inwards contraction of the anions. The effect is detectable in five layers below the surface, while it persists at high temperatures. Strong contraction is found for the cationic adatom, relaxing on top of a surface anion that is lifted by more than 30% from its equilibrium position at high temperatures. (C) 2000 Elsevier Science B.V. All rights reserved