Ali Kerrache

CEA, DEN, International Scientific Associate

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My research focuses on applying high-performance computing, physics, computational physics, condensed matter physics and experiments to understand at atomic level the behaviour and the properties of different materials ranging from simple metallic components to more complex materials such as multi-components metallic systems and glasses (Silica, Silicon, Oxide Glasses, Bulk Metallic Glasses, Amorphous Materials, Borosilicate Glasses, Nuclear Glasses, ...). The topics of interest include, but not limited to, the following: simulation of static and dynamical properties of amorphous and disordered materials, liquids, glasses and multi-component metallic systems, bulk metallic glasses, melting, crystallization and solid-liquid interfaces on metallic materials; phase transitions in condensed materials (liquid-glass transition); solid-solid interfaces, inter-diffusion, diffusion mechanism in super-cooled and glassy materials, effect of mechanical shear deformations on the properties of amorphous and complex materials; structural defects; structure of the glasses used for nuclear waste storage; He and water diffusion in nuclear glasses, irradiation effects ... The computer simulation methods used to develop my scientific projects are: Monte Carlo (MC) and classical Molecular Dynamics (MD) simulations. In the near future I would like to apply continuum modelling and first principle simulations to study the properties of complex materials.
Address: Montreal (Qc) Canada.

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Papers by Ali Kerrache

Silicon under mechanical shear: molecular dynamics study

Bulletin of the American Physical Society, Mar 20, 2009

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Ordering Kinetics in L12 and L10 Structures via a General Monte-Carlo Approach

Defect and Diffusion Forum, Apr 1, 2001

ABSTRACT Ordering kinetics in L12 and L10 structures are investigated through Monte-Carlo simulat... more ABSTRACT Ordering kinetics in L12 and L10 structures are investigated through Monte-Carlo simulations using the model previously developed for B2 and DO3 structures : a vacancy jump mechanism between nearest neighbour sites with a Glauber algorithm using an Ising Hamiltonian. General tendencies have been explored with pair interaction energies between first (V2) and second (V2) nearest neighbours. Ordering kinetics are well described by exponential like behaviours with two relaxation times. Without saddle point energy, the temperature dependence of the long relaxation time follows an Arrhenius law, yielding an effective migration energy (EM) that represents the influence of long range order on the atom migration energy. For a given value of V1, EM increases linearly with TC, the order-disorder transition temperature. For the same set of parameters, EM and its slope with TC are much higher in the L12 structure than in the L10 one, in agreement with experimental trends observed in the Co-Pt and Ni-Pt compounds. The contribution of order to the migration energy has been calculated in some L12 phases where the pair interactions have been measured.

Aging effects in supercooled silica, a molecular dynamics investigation

HAL (Le Centre pour la Communication Scientifique Directe), 2003

Using molecular dynamics simulations we investigate the presence of dynamical heterogeneities in ... more Using molecular dynamics simulations we investigate the presence of dynamical heterogeneities in supercooled silica, a &amp;amp;amp;amp;amp;#x27;strong&amp;amp;amp;amp;amp;#x27;glass former, and the changes as a function of time of these heterogeneities during aging. We compare the radial distribution functions for the 10% most mobile silicon or oxygen atoms with the corresponding mean radial distribution functions. We measure changes of these radial distribution functions and the changes of the non-gaussian parameters with time after a quench. We find that dynamical ...

Phénomène de viellissement dans les verres de silice

HAL (Le Centre pour la Communication Scientifique Directe), 2005

Crystallization of amorphous silicon induced by mechanical shear deformations

Physical Review B, Jul 26, 2011

We have investigated the response of amorphous silicon (a-Si), in particular crystallization, to ... more We have investigated the response of amorphous silicon (a-Si), in particular crystallization, to external mechanical shear deformations using classical molecular dynamics (MD) simulations and the empirical Environment Dependent Inter-atomic Potential (EDIP) [Phys. Rev. B 56, 8542 (1997)]. In agreement with previous results we find that, at low shear velocity and low temperature, shear deformations increase disorder and defect density. At high temperatures, however, the deformations are found to induce crystallization, demonstrating a dynamical transition associated with both shear rate and temperature. The properties of a-Si under shear deformations and the extent at which the system crystallizes are analyzed in terms of the potential energy difference (PED) between the sheared and non-sheared material, as well as the fraction of defects and the number of particles that possess a crystalline environment.

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Aging process in amorphous materials out of equilibrium

Ce travail de thèse a été réalisé au laboratoire des Propriétés Optiques des Matériaux et Applica... more Ce travail de thèse a été réalisé au laboratoire des Propriétés Optiques des Matériaux et Applications (POMA) de l'université d'Angers. Avant tout, je remercie le professeur André Monteil de m'avoir accueilli au sein de son laboratoire. Je souhaiterai remercier mon directeur de thèse Victor Teboul pour sa grande disponibilité, sa persévérance et son enthousiasme. Ce fut une chance pour moi de travailler sous sa direction. A ses qualités, tant scientifiques qu'humaines, je dois beaucoup, aussi bien sur le plan professionnel que personnel. Ce que j'ai appris avec lui m'accompagnera encore longtemps, même dans des domaines a priori éloignés des domaines de la physique et de la simulation. Je remercie chaleureusement monsieur Gilles TARJUS (Directeur de recherche à l'université de Paris VI) d'avoir accepté de présider le jury de ma thèse. Je tiens à remercier madame Christiane ALBA-SIMIONESCO (Directeur de recherche à l'université de Paris Sud) et monsieur Denis MORINEAU (Chargé de recherche à l'université de Rennes I) d'avoir accepté de rapporter cette thèse. Leurs remarques m'ont été très précieuses pour la mise en forme finale de ce manuscrit. Je tiens aussi à remercier monsieur André Monteil (Professeur à l'université d'Angers), monsieur Jean Michel Nunzi (Professeur à l'université d'Angers) et monsieur Michel CHRYSOS (Professeur à l'université d'Angers), d'avoir accepté de faire partie des membres de jury de ma thèse. Je tiens à remercier toutes les personnes qui ont contribuées, de près ou de loin, au bon déroulement de mon travail de thèse. Je remercie tous les membres du laboratoire POMA, à commencer par le directeur et tout le personnel administratif, ainsi que tous les enseignants. Je remercie aussi tout le personnel de l'école doctorale de l'université d'Angers pour tous les efforts fournis pour le bon déroulement de la formation des doctorants, et notamment pour les formations pluridisciplinaires et les forums organisés. Je ne vais pas oublier bien sûr mes collègues thésards avec qui j'ai passé de bons moments. J'aimerai aussi remercier monsieur Hamid Bouzar (maître de conférences à l'université Mouloud Mammeri de Tizi-Ouzou), avec qui j'ai commencé l'apprentissage des méthodes de simulation, pour son aide précieuse au cours de cette thèse et pour ses encouragements. Je tiens à remercier vivement l'agence universitaire de la francophonie (AUF) qui m'a attribué un financement pour préparer cette thèse dans de bonnes conditions. Cette thèse i 1. Un pocessus markovien est un processus sans mémoire. 1. Il existe d'autres méthodes pour obtenir un matériau vitreux. 2. La cristallisation commence par l'apparition des germes ayant une structure cristalline bien définie.

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Phase Behavior and Microscopic Transport Processes in Binary Metallic Alloys: Computer Simulation Studies

Wiley-VCH Verlag GmbH & Co. KGaA eBooks, Mar 26, 2009

In a binary liquid mixture, different kinds of phase transitions can occur that are associated wi... more In a binary liquid mixture, different kinds of phase transitions can occur that are associated with various mass transport phenomena in the liquid. First, there is the possibility that the liquid undergoes a liquidliquid demixing transition [1]. Near the critical point of this transition, a slowing ...

Molecular Dynamics Computer Simulation of Multicomponent Liquids: From Effective Potentials to Crystallisation from the Melt

The molecular dynamics (MD) simulation technique is a powerful tool for the investigation of mult... more The molecular dynamics (MD) simulation technique is a powerful tool for the investigation of multicomponent liquids and solids. A realistic description of such systems relies on the quality of the effective potential with which the interactions between the atoms are modelled in a MD simulation. We propose a fitting scheme to derive effective potentials from ab initio simulations. This scheme is used to parametrize a new potential for silica. In a second case study, MD simulations are used to investigate crystallization in an AlNi alloy, elucidating the crystal growth mechanism on an atomistic scale.

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Etude du vieillissement des matériaux amorphes hors d'équilibre : Hétérogénéités dynamiques et vieillissement de la silice surfondue

L'étude de la dynamique hétérogène et le vieillissement des matériaux amorphes hors d'équ... more L'étude de la dynamique hétérogène et le vieillissement des matériaux amorphes hors d'équilibre, est à la fois difficile et d'un grand intérêt pratique et théorique. Ce travail est consacré à l'étude par simulation de dynamique moléculaire des propriétés structurales et dynamiques de la silice surfondue à l'approche de la transition vitreuse. Deux aspects importants ont été étudiés : hétérogénéités dynamiques et processus de vieillissement. Ces aspects ont suscité un grand intérêt dans la communauté scientifique durant les deux dernières décades. La silice est considérée comme l'archétype des matériaux à réseaux (dits systèmes "forts"). L'étude de la silice vitreuse a permis de nombreuses avancées, mais a soulevé aussi de nombreuses polémiques. De nombreuses considérations théoriques et expérimentales incitaient à associer l'existence des hétérogénéités dynamiques à la dynamique vitreuse des liquides dits "fragiles". Dans ce travai...

Order and Migration Energies in a CoPt3 Single Crystal: Experimental Determination from Neutron Scattering and Monte-Carlo Simulations

Defect and Diffusion Forum, 2001

ABSTRACT The phonon dispersion curves have been measured in a CoPt3 single crystal using inelasti... more ABSTRACT The phonon dispersion curves have been measured in a CoPt3 single crystal using inelastic neutron scattering. Measurements were performed at room temperature and at 930 K in the L12 ordered structure and at 1060 K in the disordered state. The average migration energies EM of the ordered (EMO = 1.36 eV) and disordered states (EMD = 1.13 eV) are deduced from the phonon density of states. Moreover, the contribution to the migration energy of the effective pair interaction energies that determine the stability of the ordered phase is obtained through a Monte Carlo simulation based on an atom-vacancy jump model, using the five interactions deduced from the analysis of the elastic neutron diffuse scattering cross-sections. The obtained value of 0.25 eV is in very satisfying agreement with the difference (EMO - EMD) got experimentally.

SiO2–Na2O–B2O3 density: A comparison of experiments, simulations, and theory

Journal of Non-Crystalline Solids, 2013

This paper is devoted to a comparison of experimental, simulation, and theoretical results on the... more This paper is devoted to a comparison of experimental, simulation, and theoretical results on the density of SiO 2-B 2 O 3-Na 2 O glasses. It is found that theoretical and simulation densities do compare favorably with experimental values yet simulations give a better estimate of the density of the samples. Furthermore, the structural make-up (i.e. types of borate and silicate units) of the ternary glasses and the volume of the elementary units have also been investigated with simulations and compared to theory. These results are found to compare favorably when R b R d1 R ¼ Na2O ½ B2O3 ½ ; K ¼ SiO2 ½ B2O3 ½ and R d1 ¼ 0:5 þ 0:25K yet variations do exists when R N R d1. These variations include more Na + ions attaching to the borate network in simulations than theorized.

$This process continues until all borate units are classified as pyroborate units (f,) and all silica units are free (i.e. no bridging ox- ygen atoms). Thus, the fractions of each are estimated as follows [10,13,14,28,29]: where Rg3 = 2 + K. Based on Eq. (6) after Rg3, the fractions can be estimated as follow:$

$borons contain at least 1 NBO (ie. f, = 0) or Rag = 1.5 + .75 K. Thus the fractions of each are estimated as follows [10,13,14,28,29]:$

$The table is the fraction of borate and silicate units in MD simulations (gray rows) compared to the fraction of borate (Eqs. (3)-(6)) and silicate (Eq. (2)) units from theory for systems o interest (Table 3). Uncertainties in all fractions for MD simulations presented in the table are less than 0.001. Appendix C (Table 10) provides the fraction of borate and silicate units base on ICP-AES measurements.$

$Fig. 4. Pictorial representation of the borate unit fractions presented in Table 5: (top left) f,; (top right) f,; (bottom left) f3; and (bottom right) f,. Colors represent the K grouping: red diamonds K ~ 3.75; green triangles K ~ 2.12; and blue squares K ~ 1.83. Solid face markers depict simulation results; and open face markers depict theoretical results. The visualization of f3 and f4 units is enhanced by a zoom of the y-axis. Normally the axis ranges from 0 to 1. Lines connecting data point are for the eye.$

$Elementary volume elements of the borate and silicate units extracted from MD simula- tions using Voronoi volumes for each glass. The units of which are cm?/mol. The weighted average takes into consideration the fraction of each unit in the glasses. Inoue's values are shown for comparison.$

$Fig. 5. Pictorial representation of the silicate unit fractions presented in Table 5: (top left) Qu; (top right) Q3; (bottom left) Qo; and (bottom right) Q;. Colors represent the K grouping: red diamonds K ~ 3.75; green triangles K ~ 2.12; and blue squares K ~ 1.83. Solid face markers depict simulation results; and open face markers depict theoretical results. The visualization of Q and Q, units is enhanced by a zoom of the y-axis. Normally the axis ranges from 0 to 1. Lines connecting data point are for the eye. As stated above, MD simulations give access to the structure of various glasses atom-by-atom. Thus once the borate and silicate units have been identified elementary volume units are estimated via Voronoi volumes. Average results for each SBN glass are presented in Table 7. The volume of planar BO3, groups (V,) is somewhat smaller than what is expected for pure B03 and what is theorized by Inoue et al. [14], Feil and Feller [13], and Budhwani and Feller [12] fits. The origin of these differences could originate from the fact that B2O3's short range order is a planar BO3/2 group. Thus extracting a volume from a plane is somewhat prob- lematic. Yet Voronoi cells do give us an insight on what it could be. Anoth- er interesting observation is that V; has a tendency to decrease as [Na20] increases (Fig. 6a). Furthermore this decrease appears to be dependent on K. It is known that adding Na atoms to a silicate network leads to a de- crease of the free volume of the glass structure because the alkali ions$

$Acomparison of densities obtained from experiments (Pexp), MD simulations (Psim) and theory. Inoue's equations for target densities (pratge') and as measured ICP-AES densities (piCP:AFS) invoke the fraction of borate (Eqs. (3)-(6)) and silicate (Eq. (2)) units from theory along with Inoue's theoretical volumes (Table 7). Voronoi volumes for extracting densities from each chemical composition (py), and the weighted average Voronoi volume (p-y-.) invoke the fraction of borate and silicate units and Voronoi volumes estimated from simulations.$

$The fraction of f; and fz borate units for the different chemical com- positions is globally coherent between the model and the MD simula- tions for R < Ra. On the other hand the f3 proportion is significantly larger and the f, proportion is smaller in the MD simulations than in the theoretical models. Another major difference between the simula- tions and theory is the percentage of Na2O associating with the silica network after Rq; is lower than theorized. At this point in time we are unsure if this is an artifact of the potential or factual. More precise ex- periments are required to respond to this question.$

$Appendix B. Details of the binary Na20-SiO, glass fractions Commonly it is assumed that low quantities of Na2O will convert bridging oxygen atoms to NBO (i.e. Q, — Q3) ina sliding rule fashion:$

$There exists a difference in the target molar composition of glasses and in the molar compositions of fabricated glass samples as measured from ICP-AES. The fraction of borate (Eqs. (3)-(6)) and silicate (Eq. (2)) units are calculated from theory. Table 10$

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Molecular-dynamics computer simulation of crystal growth and melting in Al 50 Ni 50

EPL (Europhysics Letters), 2008

PACS 81.10.Aj-Theory and models of crystal growth; physics of crystal growth, crystal morphology ... more PACS 81.10.Aj-Theory and models of crystal growth; physics of crystal growth, crystal morphology and orientation PACS 64.70.Dv-Solid-liquid transitions PACS 02.70.Ns-Molecular dynamics and particle methods Abstract.-The melting and crystallization of Al50Ni50 are studied by means of molecular dynamics computer simulations, using a potential of the embedded atom type to model the interactions between the particles. Systems in a slab geometry are simulated where the B2 phase of AlNi in the middle of an elongated simulation box is separated by two planar interfaces from the liquid phase, thereby considering the (100) crystal orientation. By determining the temperature dependence of the interface velocity, an accurate estimate of the melting temperature is provided. The value k = 0.0025 m/s/K for the kinetic growth coefficient is found. This value is about two orders of magnitude smaller than that found in recent simulation studies of one-component metals. The classical Wilson-Frenkel model is not able to describe the crystal growth kinetics on a quantitative level. We argue that this is due to the neglect of diffusion processes in the liquid-crystal interface.

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An investigation of string-like cooperative motion in a strong network glass-former

The European Physical Journal B, 2004

The presence of string-like cooperative motion in silica, a strong network glass former, is inves... more The presence of string-like cooperative motion in silica, a strong network glass former, is investigated using large scale molecular dynamics simulations based on a realistic potential. We observe that dynamical heterogeneities and string-like motion in this strong glass-former show similarities with the results obtained by Donati et al. on a fragile model system. We observe that silicon and oxygen atoms are heterogeneously moving in a string-like fashion on different time scales corresponding to the maximum of their respective non-Gaussian parameter. We observe that the average string length increases when the temperature decreases. This result suggests that the length of the strings is not the determinant parameter for discriminating strong from fragile glass formers. We also observe that in silica the excess of cooperativity of mobile atoms is entirely due to the string-like motion of some of them.

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Monte-Carlo simulation of order–disorder kinetics in 2D and fcc binary alloys

Computational Materials Science, 2000

The thermodynamic parameters that drive the atomic migration in 2D and 3D binary alloys are studi... more The thermodynamic parameters that drive the atomic migration in 2D and 3D binary alloys are studied using Monte-Carlo simulations. The model is based on a vacancy±atom jump mechanism between nearest neighbour sites, with a constant vacancy concentration, so that only the migration counterpart E M of the total activation energy E A E M E F , where E F is the vacancy formation energy) is determined. The ordering kinetics are well described by exponential like behaviours with one (respectively two) relaxation time in the 2D (respectively 3D) cases. The temperature dependence of the long relaxation time is an Arrhenius law yielding eective migration energies. Slowing down of the relaxation is observed at the Order±Disorder transitions. In both cases, we observe a linear variation of the eective migration energy with the Order±Disorder critical temperature.

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Topography of borosilicate glass reacting interface under aqueous corrosion

Chemical Physics Letters, 2013

Molecular dynamics simulations were performed to calculate the structure of a Na-borosilicate gla... more Molecular dynamics simulations were performed to calculate the structure of a Na-borosilicate glass. The topography of an initial flat surface of this glass caused by the instantaneous release of atoms weakly bonded to the silicate network was studied, giving the minimum roughness of the surface. The resulting profiles were compared to experimental data obtained by atom probe tomography on a complex nuclear glass, after having verified that simulations are valid for the complex glass. As experimental profiles are much thicker, these comparisons support the concept that interdiffusion is a key mechanism controlling the long-term corrosion rate of nuclear glasses.

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Screening dependence of the dynamical and structural properties of BKS silica

Chemical Physics, 2006

Molecular dynamics simulations of amorphous silica are carried out on a large temperature range u... more Molecular dynamics simulations of amorphous silica are carried out on a large temperature range using a modified version of the BKS inter-atomic potential. We investigate the dependence on the screening procedure of the structural and dynamical properties of amorphous silica. We show that an increased screening of the electrostatic interaction leads to a decrease of the diffusion constants and then to better agreement with experimental data, while structural properties are unchanged. We show that the Arrhenius dependence of the diffusion constants may be reproduced in this case up to a temperature of 4000 K with activation energies very similar to the experimental data.

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Etude des processus de vieillissement des matériaux amorphes hors d'équilibre

Dans ce chapitre, nous donnerons un aperçu sur un certain nombre de concepts et des généralités s... more Dans ce chapitre, nous donnerons un aperçu sur un certain nombre de concepts et des généralités sur les liquides surfondus et les matériaux vitreux. Dans un premier temps, nous parlerons des liquides surfondus en général et des verres de silice en particulier. Nous verrons comment varie le volume spécifique ou l'enthalpie libre au cours du refroidissement, qui conduit à l'état vitreux. Nous aborderons aussi la classification des liquides surfondus selon le comportement de leur viscosité en fonction de l'inverse de la température. Nous ...

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Phys. Rev. B 84, 014110 (2011)

Multicomponent Liquids: From Effective Potentials to Crystallization from the Melt

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Amorphous silicon under mechanical shear deformations: Shear velocity and temperature effects

Physical Review B, 2011

Mechanical shear deformations lead, in some cases, to effects similar to those resulting from ion... more Mechanical shear deformations lead, in some cases, to effects similar to those resulting from ion irradiation. Here we characterize the effects of shear velocity and temperature on amorphous silicon (a-Si) modelled using classical molecular dynamics simulations based on the empirical Environment Dependent Inter-atomic Potential (EDIP). With increasing shear velocity at low temperature, we find a systematic increase in the internal strain leading to the rapid appearance of structural defects (5-fold coordinated atoms). The impacts of externally applied strain can be almost fully compensated by increasing the temperature, allowing the system to respond more rapidly to the deformation. In particular, we find opposite power-law relations between the temperature and the shear velocity and the deformation energy. The spatial distribution of defects is also found to strongly depend on temperature and strain velocity. For low temperature or high shear velocity, defects are concentrated in a few atomic layers near the center of the cell while, with increasing temperature or decreasing shear velocity, they spread slowly throughout the full simulation cell. This complex behavior can be related to the structure of the energy landscape and the existence of a continuous energy-barrier distribution.

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Ali Kerrache

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