M. Guerdane

Karlsruhe Institute of Technology, Carlsruhe, Baden-Württemberg, Germany

Are you M. Guerdane?

Claim your profile

Publications (12)26.83 Total impact

  • M. Guerdane
    [Show abstract] [Hide abstract]
    ABSTRACT: We present an atomistic approach aimed at determining the free energy gliq of binary alloy liquids, a quantity which governs the thermodynamics of phase transformations and whose evaluation has long been a challenge to modeling methods. Our approach, illustrated here for a metallic system model NiZr, combines two methods: the quasiharmonic approximation, applied for some existing (real or hypothetical) intermetallic compounds, and the liquid-solid coexistence conditions. The underlying equations for gliq are solved by means of a subregular-solution approximation. We demonstrate the high reliability of our calculated free energies in determining the phase diagram of a binary system and describing quantitatively the growth kinetics. The latter issue is illustrated by linking molecular dynamics simulations to phase-field modeling with regard to directional solidification and melting in a two-phase system [NixZr1-x]liq-Zrcryst out of chemical equilibrium.
    01/2014; 89(2).
  • Source
  • M Guerdane, H Teichler, B Nestler
    [Show abstract] [Hide abstract]
    ABSTRACT: We illustrate for a solid-liquid interface how local atomic order in a metallic melt (NiZr) transforms into a massive in-plane ordering at the surface of a crystal (bcc Zr) when commensurability is given between the solute-centered clusters of the melt and the periodic potential of the crystalline surface for a given orientation. Linking molecular dynamics simulation to phase-field modeling allows us to estimate quantitatively the influence of the surface effect on the growth kinetics. This study sheds new light on the relation between the undercooling ability (e.g., in the case of glass-forming alloys) and the pronounced local order in the melt.
    Physical Review Letters 02/2013; 110(8):086105. · 7.73 Impact Factor
  • Source
  • [Show abstract] [Hide abstract]
    ABSTRACT: We compare results from molecular dynamics simulations with those from phase-field modeling concerning the solidification and melting kinetics of a planar [NicZr1−c]liquid-Zrcrystal interface. Our study is an illustration that both approaches may predict the same quantitative physical description when the key parameters calculated within the atomistic molecular dynamics approach are used to construct the mesoscopic phase-field model. We show in this way that a thermodynamic consistent phase-field model can be applied down to the range of atomic structure. At the same time, molecular dynamics simulation seems to be capable to treat correctly relaxation dynamics, driven by thermodynamic forces, in a nonequilibrium state of solidification and melting. We discuss, in particular, how the free energy from atomistic calculations is used to design the phase dependent free-energy density in the phase-field model. Bridging the gap between both simulation approaches contributes to a better understanding of the thermodynamic and kinetic processes underlying the solidification and melting processes in alloys out of chemical equilibrium. The effective thermodynamic enhancement of the diffusivity through the strong negative enthalpy of mixing in the NiZr solution is discussed.
    Physical Review B 01/2010; 81(22):224108. · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Scherrer formula is often applied to X-ray-diffraction profiles for the size determination of small size clusters. However, for small clusters this often leads to conflicting results in comparison to other methods. A series of Pd-clusters of different size is studied by X-ray diffraction analysis and transmission electron microscopy. The influence of size and structure on the results is presented and discussed in comparison with theoretical calculations. It will be shown that the different structure of small size systems are one main origin of the conflicts. The structure problem can be overcome by using Fourier Transform of the X-ray diffraction pattern. The importance of the knowledge of the cluster structure was demonstrated by showing its Strong influence on the hydrogen solubility.
    Zeitschrift Fur Physikalische Chemie-International Journal of Research in Physical Chemistry & Chemical Physics. 01/2009; 223:169-181.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Results are presented from phase-field modelling and molecular dynamics simulations concerning the relaxation dynamics in a finite-temperature two-phase crystal–liquid sample subjected to an abrupt temperature drop. Relaxation takes place by propagation of the solidification front under formation of a spatially varying concentration profile in the melt. The molecular dynamics simulations are carried out with an interatomic model appropriate for the NiZr alloy system and provide the thermophysical data required for setting up the phase-field simulations.Regarding the concentration profile and velocity of the solidification front, best agreement between the phase-field model and molecular dynamics simulation is obtained when increasing the apparent diffusion coefficients in the phase-field treatment by a factor of four against their molecular dynamics estimates.
    Journal of Physics D Applied Physics 01/2009; 42(1):015310. · 2.53 Impact Factor
  • M Guerdane, H Teichler
    [Show abstract] [Hide abstract]
    ABSTRACT: We extend the usual static view of short range order in metallic glasses to a dynamical model of local order. We use an atomistic simulation of a NiZr glass to investigate time-dependent fluctuations of the atomic environment. We show that, even in the "frozen" glass, the solute-centered clusters change their identities between distinct polyhedron types. The frequency spectrum of these transitions exhibits a characteristic peak which we show to be related to a universal vibrational anomaly of disordered solids: the controversial boson peak.
    Physical Review Letters 09/2008; 101(6):065506. · 7.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In this work the hydrogen absorption behaviour of two types of Pd-clusters, different in structure but similar in size (3.1 nm), will be presented. First, icosahedral Pd clusters stabilised in surfactants; second, cubic Pd clusters stabilised in a Teflon-AF matrix. The phase transition in these samples was monitored by in-situ X-ray diffraction. It will be shown that the hydrogen uptake ability depends strongly on the lattice structure, which is affected by the type of stabiliser. Teflon AF stabilised clusters (cubic clusters) show the phase transition which is common for bulk Pd, whereas the surfactant stabilised clusters (icosahedral clusters) show only weak lattice dilatation upon hydrogen absorption. Pressure-lattice parameter isotherms show that the cubic clusters absorb large amounts of hydrogen in comparison to the icosahedral clusters. The measured lattice expansion is 0.130 angstrom at 10(5) Pa and 300 K, which is about 320% the amount measured for icosahedral clusters (0.04 angstrom). This suggests that surface sites are available for hydrogen in the Pd-Teflon-AF samples which are not accessible for Pd-surfactant-clusters, and that the icosahedral lattice absorbs less hydrogen for similar external pressures.
    International Journal of Materials Research. 01/2008; 99(5):528-534.
  • DPG; 01/2005
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An in situ synchrotron radiation study of quasi-free five nanometer-sized palladium clusters during hydrogen absorption is combined with molecular dynamics simulations to investigate the structural development. In the diffraction patterns, strong intensity changes are found that provide evidence for a structural phase transformation that is significantly different from the α--Pd-H bulk phase transition. The structural transition is reversible and driven by the hydrogen concentration. The intensity changes are consistent with a cubic-to-icosahedral structural phase transition obtained in molecular dynamical simulations using embedded-atom-method potentials.
    The European Physical Journal D 01/2002; 19(3):333-337. · 1.51 Impact Factor
  • Source
    M. Guerdane, H. Teichler
    [Show abstract] [Hide abstract]
    ABSTRACT: Results are reported from molecular-dynamics simulations on the structure of the amorphous, massive-metallic-glass forming Ni25Zr60Al15 alloy. The applied atomic-interaction model predicts radial distribution functions for the glass and specific-heat values for the melt that fit well to the experimental data in the literature. Analysis of bond-angle distribution and nearest-neighbor numbers reveal two types of short-range order in the system, related to the environments of Al and of Ni atoms. While Ni atoms have a trigonal prismatic neighbor shell under avoidance of direct Ni-Ni contacts, the Al atoms show an icosahedral surrounding with a tendency to form short segments of Al chains. It is the particular feature of the ternary alloy that its structure has to simultaneously accommodate both types of ordering. Even at high temperatures, the local order leads to prepeaks in the Ni and Al structure factors that experimentally should be detectable in the melt above the equlibrium melting point.
    Physical Review B 12/2001; 65(1). · 3.66 Impact Factor

Publication Stats

43 Citations
26.83 Total Impact Points

Institutions

  • 2013–2014
    • Karlsruhe Institute of Technology
      • Institute of Applied Materials
      Carlsruhe, Baden-Württemberg, Germany
  • 2001–2013
    • Georg-August-Universität Göttingen
      • Institute of Material Physics
      Göttingen, Lower Saxony, Germany
  • 2010
    • Karlsruhe University of Applied Sciences
      • Institute of Materials and Processes
      Carlsruhe, Baden-Württemberg, Germany