G.A. Evangelakis

Shear band melting and serrated flow in metallic glasses

Scanning electron microscopy observations of shear steps on Zr-based bulk metallic glasses show direct evidence of shear band melting due to heat generated by elastic energy release. The estimated range of attained temperatures and the observed morphologies are consistent with shear steps forming at a subsonic speed limited by a required redistribution of local microscopic stresses. The calculations indicate that a 0.2μm layer melts in the vicinity of a shear band forming a 1μm shear step. The plastic part of the stress strain curve is serrated but a majority of shear events are not associated to serrations.

Adatom self-diffusion processes on (001) copper surface by molecular dynamics

Abstract Using molecular dynamics and an n -body potential adapted to copper we have studied the self-diffusion of Cu adatoms on Cu(001) surface. The simulations covered the temperature range between 700 and 1100 K. Besides the simple hopping and the exchange mechanism, the detailed trajectory analysis revealed multiple hopping events and complicated multi-particle exchange processes, involving several atoms that do not necessarily belong to the same nearest-neighbor row. These processes exhibit Arrhenius behavior from which we derived the migration energies associated with each process. It is found that the hopping mechanism requires an energy of 0.43 eV, in very good agreement with available experimental data, while the energy associated with the exchange mechanism is 0.70 eV. These results are in qualitative agreement with recent ab-initio calculations. In addition, we found that all mechanisms, even the most complicated, require about the same migration energy with the simple exchange and that for temperatures above 900 K they contribute almost equally to the total diffusion. Furthermore, the activation barriers for the hopping and the exchange mechanism deduced from energy minimizations, at T = 0 K, compare well with the simulation values.

Second-moment interatomic potential for Cu-Au alloys based on total-energy calculations and its application to molecular-dynamics simulations

We have evaluated interatomic potentials of Cu, Au and Cu-Au ordered alloys in the framework of the second-moment approximation to the tight-binding theory by fitting to the volume dependence of the total energy of these materials computed by first-principles augmented-plane-wave calculations. We have applied this scheme to calculate the bulk modulus and elastic constants of the pure elements and alloys and we have obtained a good agreement with experiment. We also have performed molecular-dynamics simulations at various temperatures, deducing the temperature dependence of the lattice constants and the atomic mean square displacements, as well as the phonon density of states and the phonon-dispersion curves of the ordered alloys. A satisfactory accuracy was obtained, comparable to previous works based on the same approximation, but resulting from fitting to various experimental quantities.

Second-moment interatomic potential for Al, Ni and Ni–Al alloys, and molecular dynamics application

Abstract We present an interatomic potential for Al, Ni and Ni–Al ordered alloys within the second-moment approximation of the tight-binding theory. The potential was obtained by fitting to the total energy of these materials computed by first-principles augmented-plane-wave calculations as a function of the volume. The scheme was validated by calculating the bulk modulus and the elastic constants of the pure metals and alloys that were found to be in fair agreement with the experimental measurements. In addition, we performed molecular-dynamics simulations and we obtained the thermal expansion coefficient, the temperature dependence of the atomic mean-square displacements and the phonon density of states of the compounds. Despite the simplicity of the model, we found satisfactory agreement with the available experimental data.

Molecular dynamics study of gold adatom diffusion on low-index copper surfaces

Abstract The transport properties of single Au adatoms on low-index Cu surfaces have been studied using molecular dynamics. We found that on the Cu(001) face, the adatom substitutes a surface Cu atom via the exchange diffusion mechanism, while this process is absent on the Cu(111) surface. On the Cu(110) face, hopping and exchange diffusion processes coexist with almost equal probability. The migration energies associated with the various diffusion processes have been deduced. It turns out that the diffusion of Au adatoms differs significantly from Cu self-diffusion on the low-index Cu surfaces. In addition, the calculated adatom binding energies, as well as the temperature dependence of the mean-square-displacements and the relaxed positions in the normal to the surfaces direction are compatible with the diffusion results.

On the deposition mechanisms and the formation of glassy Cu–Zr thin films

We report on molecular dynamics (MD) simulations and physical vapor deposition experimental results concerning the development of glassy and nanocrystalline Cu–Zr thin films. MD has revealed that when Cu and Zr are deposited sequentially, a thin film overlayer is formed that consists of nanocrystalline a-Zr and t-Zr2Cu, while if Cu and Zr are simultaneously deposited, amorphous CuZr thin film emerges, due to the formation of icosahedral-like clusters that impede nucleation. Thin films grown by pulsed laser deposition and magnetron sputtering techniques were analyzed by x-ray diffraction and high-resolution transmission electron microscopy and yielded unequivocal evidence that validates our MD predictions. These findings may indicate an alternative pathway for the growth of metallic nanocomposites or glassy films.

Second-moment interatomic potential for aluminum derived from total-energy calculations and molecular dynamics application

We have obtained an interatomic potential for Al within the second-moment approximation of the tight-binding theory by fitting to the volume dependence of the total energy of the metal, computed by first-principles APW calculations. This scheme was applied to calculate the bulk modulus, elastic constants, vacancy formation and surface energies of Al. The predicted values are in good agreement with the measurements. We also have used this potential to perform molecular-dynamic simulations and determine the temperature dependence of the lattice constant and atomic mean-square displacements (MSDs), as well as the phonon spectra and surface related thermodynamic properties. A satisfactory accuracy has been obtained, denoting the success of the method.

Coverage dependent self-diffusion on Cu(111) by molecular dynamics

Abstract We studied the coverage dependence of the self-diffusion on the Cu(111) surface using molecular dynamics simulations. The adatoms coalesce to a large island whose migration energy increases with coverage, attaining 0.37xa0eV above a concentration of 25%. The migration rate of the islands atoms scales with their number as N −1 , while step atoms are fast diffusing independent of the cluster size. Detailed trajectory analysis revealed that cluster diffusion occurs via ‘step running’ of the peripheral atoms that initiate concerted movements of the inner cluster atoms. In addition, adatoms and vacancies are spontaneously created and annihilated on the islands terrace, contributing to the migration of the island. These findings describe the most important processes of cluster diffusion involved in the early stages of growth on this surface.

Self-diffusion processes of copper adatom on Cu(110) surface by molecular dynamics simulations

Abstract The self-diffusion mechanisms of single adatoms on the Cu(110) surface have been studied using molecular dynamics simulation and a many-body potential within the second-moment approximation of the tight-binding model. From a detail trajectory analysis we found a variety of diffusion mechanisms, the hopping being the favoured one and we deduced the migration energies for the most important among them. At high temperatures, saturation in diffusion frequency for both hopping and exchange mechanisms is observed, indicating that the diffusion proceeds via complicated and concerted movements. In addition, we estimated the formation energy for the spontaneous creation of the vacancy-adatom pair, in good agreement with the experiment. Furthermore, from the temperature dependence of the relaxed adatom positions we found that the adatom exhibits strong contraction compared to the bulk interlayer spacing, attaining −20% at high temperatures.

Bremsstrahlung and its contribution to the gamma ray scattering spectra of solids

Abstract The bremsstrahlung radiation (BR) spectra from decelerating electrons liberated in photoelectric or Compton scattering processes are studied in the bulk of the scatterer and their influence on inelastic photon scattering experiments is considered. Using two spectrometers designed for Compton scattering studies the spectra resulting from 59.5 keV gamma rays striking W and Ti samples has been measured in the energy range between 5 and 60 keV. The spectra between 18 and 40 keV are dominated by the BR and are in good agreement with Kramers bremsstrahlung theory: BR contributes significantly at low energies to Compton spectra and its removal is crucial to the study of the infrared divergence in photon scattering experiments.