M. Guerdane

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

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Publications (12)28.01 Total impact

  • M. Guerdane
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    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.
    No preview · Article · Jan 2014 · Physical Review E
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    Full-text · Dataset · Sep 2013
  • M Guerdane · H Teichler · B Nestler
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    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.
    No preview · Article · Feb 2013 · Physical Review Letters
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    Full-text · Dataset · Nov 2012
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    M. Guerdane · F. Wendler · D. Danilov · H. Teichler · B. Nestler
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    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.
    Full-text · Article · Jun 2010 · Physical Review B
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    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.
    Full-text · Article · Sep 2009 · Zeitschrift für Physikalische Chemie
  • Helmar Teichler · Mohamed Guerdane
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    ABSTRACT: Results are reported from molecular dynamics (MD) simulations concerning entropy and thermodynamic potential of metallic alloy melts and, moreover, from comparing MD and phase field (PF) modeling of the dynamics of the planar [NixZr1 − x]liquid − Zrcrystal [100] interface during crystallization. Regarding the thermodynamic potentials, we consider binary Zr-rich NixZr1 − x melts and the ternary AlyNi0.4 − yZr0.6 system. Methods are sketched for estimating the entropy and free enthalpy of the liquid alloys, and results are presented from simple tests of their reliability. For the Zr-rich NixZr1−x melts, there is encouraging agreement between the MD simulated and the experimentally determined concentration-temperature phase diagram (up to overestimation of the temperature scale by about 25%). In particular, good agreement is found concerning the eutectic concentrations and concerning the ratio between the eutectic temperatures and the Ni0.5Zr0.5 and bcc Zr melting temperature. The AlyNi0.4−yZr0.6 melts show a strong composition dependence of the vibration entropy, Svib. Owing to this, below 2100 K, the structural entropy S(T) = Svib(T) + Sconfig(T) of the Ni0.4Zr0.6 melt with its local trigonal prismatic structure is larger than that of the nearly polytetrahedral Al0.4Zr0.6 melt. Above some 1050 K the configuration entropy, Sconfig(T), for Ni0.4Zr0.6 is lower than that of Al0.4Zr0.6, as expected from the local structures in the melts. — Regarding the dynamics of the planar [NixZr1 − x]liquid − Zrcrystal [100] interface, results are described demonstrating that good agreement is obtained between MD and PF modeling of the propagation velocity and the non-equilibrium concentration profile in the melt in front of the interface, provided suitable parameters are chosen in the PF treatment.
    No preview · Chapter · Mar 2009
  • M Guerdane · H Teichler
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    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.
    No preview · Article · Sep 2008 · Physical Review Letters
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    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.
    Full-text · Article · May 2008 · International Journal of Materials Research (formerly Zeitschrift fuer Metallkunde)
  • M. Suleiman · C. Borchers · M. Guerdane · A. Pundt

    No preview · Conference Paper · Jan 2005
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    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.
    Full-text · Article · Jan 2002 · The European Physical Journal D
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    M. Guerdane · H. Teichler
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    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.
    Preview · Article · Dec 2001 · Physical Review B

Publication Stats

94 Citations
28.01 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
  • 2008
    • Universitätsmedizin Göttingen
      Göttingen, Lower Saxony, Germany