Luc Billard

On the Mossbauer effect in metallic glasses

Mossbauer experiments on amorphous materials generally show an asymmetry often attributed to quadrupole splitting effects. On the other hand, some amorphous structures can be simulated with a soft-sphere relaxed model. The authors analyse this structure from the point of view of the electric field tensor, and look at the resulting distribution law for the quadrupole splittings and the contributions of dipolar terms to the internal field. By considering further a distribution of internal fields, a very simple model for analysing Mossbauer spectra is given and discussed.

Structural description of a metallic glass model

Several atom packings have been relaxed with a Johnson potential. This mere relaxation leads to states having significant differences. A procedure is given for obtaining configurations which can be considered as representative of the same amorphous structure. To give a precise geometric description of it, the authors work out a method of decomposing the whole volume into single units: tetrahedra, octahedra and more generally deltahedra.

Simulation of interstitial diffusion in an amorphous structure

The authors investigate the random walk of an interstitial atom through an amorphous structure. The interaction between the diffusing particle and the atoms in the matrix is defined through a model potential which can be used in any disordered material and has strong interrelations with the corresponding Voronoi network. The stochastic process is carefully described as consisting of two random walks: one on the net of interstitial sites, the other on the time axis. Thus, the authors can perform a Monte-Carlo experiment on a simulated amorphous sample. They show that the concept of percolation greatly reduces the complexity of the problem: one can define a percolation threshold for the saddle points, such that only those which are near to this value need be considered. Numerical results are obtained which show the gaussian character of the motion. Finally, the diffusion coefficient is evaluated and shown to be a little higher than in crystalline materials.

Simulation of a reproducible model of metallic glasses by hardsphere relaxation

The authors show that a numerical calculation to account for the structure of amorphous metallic materials needs a careful definition of the parameters used: radial distribution function, density, mean energy per atom. As far as such parameters are considered to be characteristic of the amorphous structure, they establish a reproducible model does exist, obtained by relaxation, in a Lennard-Jones potential, of different hard-sphere packings.

Binary tilings and quasi-quasicrystalline tilings

Abstract We introduce tilings which obey vertex matching rules and generalize the five-fold binary tilings to any odd symmetry. In particular, quasicrystalline, crystalline or random binary tilings do exist. We also suggest another structure, which we call a quasi-quasicrystal and which can be obtained by iteration of a substitution.

Topological and chemical ordering in a two-atoms simulated amorphous structure

Abstract We have studied simulated relaxed two-atoms amorphous structure, investigating mainly the local environment of the atoms (typically Pd and Si). No evidence for a prismatic packing has been shown. An analysis as a function of concentration has been made to try to characterize the chemical order. At low concentration, up to about 6 % Si, the Si sites have been defined. Density of several relaxed packings has been also calculated, and shown to be comparable to that determined in a simple model.

Numerical simulation of quasicrystals

Abstract Quasicrystalline structures can be built by 3D cross sections through 3D atomic motifs in a 6D space. By numerical simulation, we study the stability of a monoatomic model and of an AlMnSi model with interatomic pair potentials. Because all site environments are inequivalent, atoms move from their initial positions. The resulting structures can be related to modifications of the atomic motifs. The agreement between the computed and the experimental partial radial distribution functions of the AlMnSi quasicrystal is improved.

On the neighbours in a simulated amorphous structure

Abstract We consider a soft sphere model obtained by relaxation techniques to describe the amorphous structure. In a topologically disordered structure like that, it is not easy to define what one could call the neighbours of a given atom: first neighbours, second neighbours, etc. To do it, we first use a geometric criterion. But we show that it coincides with a metric definition in which only interatomic distances are used. In this manner, we can analyze the pair distribution function unambiguously and, in particular, we show that the first neighbour pairs thus defined represent that part of the radial distribution function which extends up to r = 2 1 2 (the unit distance corresponds to the position of the first peak). A second relaxed sample was studied. It was very much bigger, obtained with periodic conditions, from another initial state. The conclusions are very similar and confirm the reproducibility of states obtained by relaxation.

Phase Separation in Amorphous CuTi Hydrides

Neutron diffraction experiments have been performed on amorphous Cu0 5 Ti0 5 hydrides in order to study the influence of hydrogen on the structure of the amorphous CuTi matrix. The comparison of the neutron pair correlation functions before and after hydrogen loading shows that the contribution of the Cu-Ti pairs is much smaller than the CuCu and Ti-Ti ones after hydrogen loading. This indicates that phase separation takes place in these amorphous alloys upon hydrogen absorption, as confirmed by the appearance of a small-angle maximum in the interference function. These results are used to construct a model for the amorphous matrix before and after hydrogen loading, and a good agreement is obtained between experimental and simulation results for both the structure of the matrix and the location of hydrogen.

Hydrogen in Amorphous CuxTi1−x Alloys: Neutron Diffraction and Computer Simulation Studies

The technique of neutron diffraction has been used to study the structure of CuTi amorphous alloys and hydrides. Advantage has been taken of the existence of positive and negative neutron scattering lengths. The first result is that the as-quenched amorphous alloys exhibit a tendency to heterocoordination. In a second step the substitution of deuterium for hydrogen lets us identify the supplementary peaks appearing in the total pair-correlation functions of CuTiH and CuTiD alloys. These new correlations agree quite well with the corresponding ones in crystalline CuTi hydrides. From these results, a computer model of the structure of these alloys is built up. The simulated interference functions and pair-correlation functions are in good agreement with those obtained experimentally for both as—quenched and hydrogenated or deuterated alloys. The thermal evolution is followed by small-angle and large-angle neutron scattering and confirms the low thermal stability of these amorphous hydrides.