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Session Chairperson: Dr. Ricardo B. Schwarz, Center for Materials Science (MS-K765), Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545
2:00 pm INVITED
STRUCTURE OF BULK AMORPHOUS ALLOYS DETERMINED BY SYNCHROTRON RADIATION: T. Egami, W. Dmowski, Department of Materials Science and Engineering (LRSM/K1), University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104-6272; Yi He, Ricardo B. Schwarz, Center for Materials Science (MS-K765), Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545
The atomic structure of Pd-Ni-P bulk amorphous alloys was studied by the anomalous (resonance) x-ray scattering technique using synchrotron radiation tuned near the Pd K-edge. Bulk samples of Pd40Ni40P20, Pd30Ni50P20, and Pd50Ni34P16 amorphous alloys were prepared by the flux method in the form of rods with the diameter of 10-14 mm. X-ray measurements were carried out at the X-7A beamline of the NSLS, Brookhaven National Laboratory. The results show that the structures are basically described by the dense random packed structure with small chemical short-range order. The implication of this result with respect to the stability of the glass will be discussed.
2:40 pm INVITED
CAN A BULK AMORPHOUS PHASE BE OBTAINED FROM LIQUID SILICON OR GERMANIUM?: Yan Shao, Frans Spaepen, Division of Engineering and Applied Sciences, Pierce Hall, Harvard University, 29 Oxford Street, Cambridge, MA 02138
A number of methods have been used to undercool liquid silicon and germanium: levitation melting, flux treatments, atomization, rapid cooling following laser melting of thin films, and quenching of droplet dispersions. An amorphous phase has been produced so far only in small volumes. The competition between the kinetics of nucleation and growth of the crystalline and the amorphous phases under these various conditions will be reviewed, and the possibility of obtaining bulk amounts of the amorphous phase will be assessed.
3:20 pm BREAK
3:40 pm INVITED
THERMODYNAMIC FRAMEWORK FOR SOLID-STATE AMORPHIZATION: D. Wolf, Materials Science Division (MSD-212), Argonne National Laboratory, Argonne, IL 60439
Atomistic computer simulations are used to expose important thermodynamic parallels between melting and solid-state amorphization and the important role of nanocrystalline microstructures. Molecular-dynamics simulations demonstrate that every crystal can, in principle, melt by two entirely different causes and underlying mechanisms; a comparison with experiments suggests that both can be operative in solid-state amorphization1. Also, simulations of a model nanocrystalline material exhibit the existence of low- and high-frequency lattice-vibrational modes not seen in the perfect crystal but also present in the amorphous phase2. The possibility of a reversible, free-energy based transition between these two metastable phases that is governed by a critical grain size is discussed together with possible mechanisms for the transition. Work supported by the US Department of Energy, BES-Materials Science under Contract No. W-31-109-Eng-38.
4:20 pm INVITED
THERMODYNAMIC AND KINETIC PROPERTIES OF AMORPHOUS AND LIQUID STATES: A.V. Granato, Physics Department, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, IL 61801-3080
The magnitude and temperature dependence of the liquid state shear modulus G, specific heat Cp, diffusivity D and viscosity are all closely related, according to the interstitialcy model. It has been proposed by Dyne, Olsen and Christensen that the viscosity is given by =h0exp(F/kT) where h0 is a reference viscosity and F is given by the work required to shove aside neighboring particles in a diffusion process, where F=GVc and Vc is a characteristic volume. In the interstitialcy model, the high frequency thermodynamic liquid state shear modulus is given by G(T)=G0exp[(-(T-T0)], where G0 is the shear modulus at a reference temperature T0 which can be taken as the glass temperature. The resulting non-Arrhenius behavior of the viscosity is compared with experimental data for the shear modulus.
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