Program Organizer: Professor Carl C. Koch, Materials Science and Engineering Department, North Carolina State University, Box 7907, Raleigh, NC 27695; Dr. Robert D. Shull, NIST, Bldg. 223 B152, Gaithersburg, MD 20899
Monday, AM Room: Orange County 1
February 5, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: Carl C. Koch, Materials Science and Engineering Department, North Carolina State University, Box 7907, Raleigh, NC 27695
OPENING REMARKS: Carl C. Koch
8:40 am Keynote
THERMODYNAMIC AND KINETIC ASPECTS OF GLASSY PHASE FORMATION IN NON-EQUILIBRIUM METALLIC SYSTEMS: William L. Johnson, California Institute of Technology, 138-78, Pasadena, CA 91125
A glass or amorphous solid is a condensed phase, which like the related liquid phase, lacks long range periodicity in the arrangement of atoms. Amorphous solids can be formed when the liquid phase is deeply undercooled and configurationally freezes without crystallizing, when a vapor condenses under restricted kineteics, when a crystalline solid is driven from equilibrium by a disordering processes so that long range order collapses, or during chemical reactions in the solid state where kinetic contstraints prevent the attainment of full equilibrium. The latter two cases can be usefully described as "solid state melting" or "solid state amorphization". The fundamental thermodynamic and kinetic factors which govern these various types of glass formation will be discussed and compared. Experimental results relating to glass formation, melting, and amorphization in metallic systems will be surveyed, reviewed, and analyzed to illustrate how a fundamental understanding of these phenomena has developed.
9:20 am Invited
UNIVERSAL ATOMIC SIZE CRITERION FOR GLASS FORMATION: T. Egami, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104
The importance of the atomic size difference in the stability of metallic alloy phases has long been known ever since the work of Hume-Rothery. However, the rules remained largely semi-empirical. We show that the size factor can be quantitatively understood from the view point of the local topological instability, and a single principle explains melting, glass transition, solid state amorphization by irradiation as well as range of composition for glass formation by liquid quenching. The local topological instability occurs when the relation between the local volume and the local coordination number deviates by a certain amount from the ideal relation which can be deduced from purely geometrical considerations. Glasses become relatively stable when crystals of the same composition are destabilized by the size effects. These effects are demonstrated by computer simulation.
TIGHT-BINDING APPROACH TO ORDER AND METASTABILITY IN AMORPHOUS ALLOYS: P.E.A. Turchi, LLNL (L-268) P.O. Box 808, Livermore, CA 94551. Dicker Mayou, LEPES-CNRS, 25 Avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France
We present a real-space approach based on the tight-binding approximation to study order and metastability in amorphous alloys. The extended recursion coupled with the coherent potential approximation and the generalized perturbation method allows a real-space treatment of ordering processes in this class of materials. The interactions obtained from these electronic structure calculations which build up the ordering part of the total energy can then be used with a statistical model to study ordering trends. Finally, in combination with molecular dynamics simulations and the full tight-binding expression for the energetics, the most probable atomic configuration of an alloy can be predicted and analyzed on the basis of fundamental electronic parameters. The advantages and the future of this new approach together with some preliminary results will be discussed. 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 No. CRG-941028 is gratefully acknowledged.
10:10 am BREAK
10:30 am Invited
THERMODYNAMICS AND METASTABILITY IN GLASS FORMING SYSTEMS: H.J. Fecht, TU Berlin, Hardenbergstr. 36 PN 2-3, 10623 Berlin, Germany
Glasses are generally produced from the highly undercooled liquid state by rapid quenching methods or quasi-statically at slow cooling by the effective control of potent heterogeneous nucleation sites. For metallic systems the latter method recently has led to the development of bulk metallic glass with a complex multicomponent chemistry and advanced engineering properties. As long as crystallization can be avoided the relevant thermodynamic properties of the metastable glassy and undercooled liquid phases can be measured below and above the glass transition temperature, respectively. The obtained data give new insight into the nature of the glass transition suggesting that it is not a phase transition in the classical sense but kinetic freezing triggered by an underlying entropic instability. Furthermore, glass formation can be achieved by solid-state-processing without passing through the liquid state. Conceptually, the formation of the glassy state from the liquid and the solid can be described within a thermodynamic framework under appropriate kinetic constraints resulting in a universal phase diagram with a pseudo-critical point.
11:00 am Invited
DEFECTS, DEFECT MEDIATED TOPOLOGIAL ORDER-TO-DISORDER TRANSITIONS AND SOLID STATE AMORPHIZATION: Mo Li, W.M. Keck Laboratory, 138-78, California Institute of Tecnology, Pasadena, CA 91125
Crystal-to-glass transition can be considered as one of the defect induced topological order-to-disorder transitions. Stability of a crystalline phase is closely related to the presence of defects that are either introduced during experiments or pre-exist in the host crystalline phases. It was shown, through computer simulations, that it is the topological defects that are ultimately responsible for the crystalline instability, or amorphization. The strain fluctuations caused by the defects give rise to softening of elastic constants, leading to the so-called "mechanical instability" at amorphization. The amorphization can occur in different modes depending on the state of the defects. We show that the normally observed nucleation and growth in amorphization can change if a mechanical spinodal precipitates preemptively. We will show that the (static) displacement fields of atoms from their stable lattice positions can be used as a general order parameter to describe the topological order-to-disorder transition. Connections of this order parameter to experimental measurements will be discussed.
CALCULATION OF PHASE SELECTION IN NON-EQUILIBRIUM PROCESSING: G. Shaw and P. Tsakiropoulos, Department of Materials Science and Engineering, University of Surrey, Guildford, Surrey GU 25XH, United Kingdom
Predominant nucleation maps in rapidly solidified alloys were calculated by
combining phase equilibria data, derived using the CALPHAD method and extended
to undercooled melts, with time dependent nucleation and steady state
nucleation theories. The study includes AlxTMy compounds in undercooled gas
atomised droplets of Al-TM alloys, stoichiometric compounds and a solid
solution phase in melt spun Al-Ti-V alloy ribbons and the fcc, hcp and ordered
L1o([[gamma]]) phases in TiAl-V aluminides. Phase selection under rapid
solidification conditions will be discussed in terms of transient nucleation
and steady state nucleation at different melt cooling rates or melt
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