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Session Chairperson: Prof. John H. Perepezko, Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706
8:30 am INVITED
COMPUTER SIMULATIONS OF THE STRUCTURAL, DYNAMIC, ELECTRONIC AND MAGNETIC PROPERTIES OF AMORPHOUS ALLOYS: J. Hafner, Institut für Theoretische Physik, Technische Universität Wien, Wiedner Haupstrasse 8-10/136, A-1040 Wien, Austria
There is hardly any area in materials science where the necessity to supplement the experimental information acquired in the laboratory by the results of computer experiments is felt as urgently than for liquid and amorphous materials. The classical example is the structure: whereas laboratory experiments describe a one-dimensional projection of the three-dimensional structure, the computer-simulation delivers the full set of the atomic coordinates. During the last years, a wide variety of computational tools have been developed. These techniques can essentially be classified in three groups: (a) Ab initio local-density functional techniques in the spirit of the Car-Parrinello method. While in this approach no further assumption beyond the reduction of the many-electron Hamiltonian to an effective one-electron form is required, the massive computational effort restricts the application to models with at most a few hundred inequivalent atomic sites. (b) A variety of tight-binding (TB) based techniques (ranging from TB molecular dynamics (MD) and Monte Carlo (MC) to tight-binding-bond approaches to many-atom forces) allow to treat larger systems, albeit at lower accuracy. (c) Classical MD and MC based on effective interatomic pair and volume forces derived via effective-medium approximations, pseudo-potential theory, TB-moment expansions...etc. In this talk I shall attempt to review the state-of-the-art of the computer-modeling of the structural, dynamic, electronic and magnetic properties of amorphous alloys. This work has been supported by the Bundesministerium für Wissenschaft, Forschung und Kunst through the Center for Computational Materials Science.
9:10 am INVITED
AB INITIO STUDIES OF THE ELECTRONIC STRUCTURE AND ENERGETICS OF BULK AMORPHOUS ALLOYS: G.M. Stocks, D.M.C. Nicholson, Y. Wang, X.-D. Wang, C.L. Fu, W.A. Shelton, Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008-6114, Oak Ridge, TN 37831-6114; J.C. Swihart, Physics Department, Indiana University, Bloomington, IN 47405
We present results of first-principles LDA calculations of the electronic structure and energetics of Ni40Pd40P20 and Zr60Al15Ni25 bulk amorphous alloys based on large unit cell (about 300 atoms) structural models of the amorphous state. The calculations are made tractable by the order-N locally self-consistent multiple scattering (LSMS) method implemented on massively parallel computers. For the Zr2Ni-based system, we have also studied the energetics and bonding of some competing stable and metastable ordered phases into which the amorphous state decomposes. The implications of these calculations for understanding the unusual stability of bulk amorphous alloys will be stressed. Work sponsored by ORNL Laboratory Directors Research and Development fund, BES-DMS and OCTR-MICS USDOE, under contract DE-AC05-96OR22464 with Lockheed-Martin Energy Research Corporation.
9:50 am INVITED
ELECTRONIC STRUCTURE AND RELATED PROPERTIES OF METALLIC GLASSES: LINEAR MUFFIN-TIN ORBITALS CALCULATIONS: S.K. Bose, Physics Department, Brock University, St. Catharines, Ontario L2S 3A1, Canada
We describe various aspects of electronic structure calculation for metallic glasses using the Linear Muffin-Tin Orbitals (LMTO) scheme. A method of calculating the electronic transport properties based on the LMTO-recursion method and the Kubo-Greenwood formula is discussed. The practical limitations of the method and the ways to overcome them are presented. We discuss calculations of the magnetic properties within the framework of multiple scattering formalism assuming collinear magnetic structure and using the Linear Muffin-Tin Orbitals Green's Function (LMTO-GF) method. Results for the effective exchange coupling parameters and local magnetic moments for amorphous Fe and Co are discussed with special emphasis on the dependence of these quantities on the local and global features of the structure. A simple interpolation scheme, based on the electronic properties of the pure components, is proposed for estimating the spin fluctuation effects in transition metal glasses.
10:30 am BREAK
10:50 am INVITED
REAL-SPACE ELECTRONIC STRUCTURE APPROACH TO CHEMICAL ORDER IN AMORPHOUS ALLOYS: P.E.A. Turchi, Lawrence Livermore National Laboratory (L-268), P.O. Box 808, Livermore CA 94551; D. MAYOU, LEPES-CNRS, 25 Avenue des Martyrs, BP 166, F-38042 Grenoble Cedex 9, France
A recently developed real-space approach for studying the electronic structure properties of materials which exhibit both chemical and topological disorders is presented, within the tight-binding framework. For a chemically random alloy, we show that the Coherent Potential Approximation (CPA) equations can be solved self-consistently in real-space with the same accuracy as is currently done in reciprocal space, in the case of homogeneous and inhomogeneous systems. Based on the orbital peeling technique, effective on-site and many-body interactions which build up the configurational part of the total energy are computed. With this energetics, Monte Carlo simulations based on the Kawasaki spin-exchange dynamics are performed to predict chemical short-range order in amorphous alloys. The relaxation of the lattice is performed with TB-molecular dynamics simulations, and the overall scheme can be reiterated to self-consistency. Advantages of this global scheme are discussed and application to Zr-Ni alloys are presented. Work performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48. Partial support from NATO under contract CRG 941028 is gratefully acknowledged.
11:30 am INVITED
FREE VOLUME, PERCOLATION, AND GLASS TRANSITIONS: Mo Li, W.M. Keck Laboratory of Engineering Materials 138-78, California Institute of Technology, Pasadena, CA 91125
Free volume model is examined systematically and quantitatively in a model binary liquid using molecular dynamics simulations. It is found that at the glass transition, the liquid-like cells undergo a percolation transition. However, further analysis shows that free volumes do not exhibit any cooperative behavior at the percolation threshold. The continuous change in thermodynamic and transport properties suggests that the glass transition is kinetic in nature. Implication of the percolating properties of the free volumes in transport properties of undercooled liquids will also be discussed.
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