1997 TMS Annual Meeting: Tuesday Abstracts

HUME-ROTHERY AWARD SYMPOSIUM FOR BRUNO PREDEL: THERMODYNAMICS OF ALLOY FORMATION: Session II: Amorphous State

Session Chairs: R. Schmid-Fetzer Technical Universitaet Clausthal, AG Elektronische Materialien, Robert-Koch-Str. 42, D-38678 Clausthal-Zellerfeld, Germany

8:30 am

THE SHORT- AND MEDIUM-RANGE STRUCTURE CHARACTERISTIC OF AMORPHOUS ALLOYS: K. Suzuki, K. Shibata, T. Otomo* and H. Mizuseki, Institute for Materials Research, Tohoku University, Sendai, Japan, *National Laboratory for High Energy Physics, Tsukuba, Japan

Atoms in amorphous alloys are combined into a configuration of minimizing the local energy in the short-range structure instead of relaxing the total energy minimum of a system. The short-range order of amorphous alloys often shows similarity to that of their crystalline counterparts. The survival of the local coordination in amorphous alloys is also confirmed by the vibrational density-of-states. However, the medium-range structure of amorphous alloys has an quite unique nature in contrast to the crystalline alloys. The low-energy excitation in amorphous alloys appearing in the energy range of 1 to 3 meV, which is usually observed as an excess specific heat in addition to the Debye-type harmonic vibration, is contributed from the locally collective motion characterized spatially by the medium-range structure fluctuations. The experimental results mentioned above for Pd-Si, Pd-Ge, Pd-Ni-P and V-Ni amorphous alloys were observed by pulsed neutron scattering based on accelerators, which is a powerful tool for characterizing the structure of amorphous alloys, because a very wide dynamic range of energy- and momentum-transfer can be surveyed.

9:00 am INVITED

NEUTRON DIFFRACTION STUDIES OF LIQUID ZINTL-ALLOYS UP TO 2000: K.R. Winter, University of Dortmund, Department of Chemistry, Physical Chemistry I, D-44227 Dortmund, Germany

Neutron diffraction measurements have been carried out on equiatomic liquid Zntl-alloys, such as KPb, CsPb, and NaSn, over a wide temperature interval, ranging from 800 to 2000 K, and at pressures up to 150 bar. In the crystalline state, these are semiconducting compounds containing tetrahedrally coordinated polyanions. The diffraction results indicate the survival of polyanions in the expanded liquid alloys even up to high temperatures, as indicated by the persistence of the peak of S(Q) at Q1Å-1, which is indicative of intermediate-range order. The position of this peak shifts towards lower Q-values, its height decreases, and its width drastically increases with increasing temperature and correspondingly decreasing density. The experimental results and the corresponding real-space information are compared with the crystal structures in the solid state and with the results of recent ab-initio molecular dynamics calculations and computer modelling studies. In addition, we present neutron diffraction data on the effect of pressure on the structure factor of solid CsPb in the rotator phase.

9:30 am INVITED

TRANSFORMATION OF THE AMORPHOUS TO THE NANO-CRYSTALLINE STATE: R. Luck, K. Lu*, Max-Planck-Institut fur Metallforschung, SteebstraBe 75, D-70174 Stuttgart, Germany, *also with National Laboratory for RSA, Institute of Metal Research, Academia Sinica, Shenyang 110015, China

The crystallization of amorphous materials has been studied extensively during the last two decades. However, the formation of nanocrystals - that is a crystalline morphology with an average grain size of 5 to 50 nm - from t he amorphous state has been detected few years ago. Since that time it is used more and more. This procedure is performed isothermally below the crystallization temperature. We present data of the kinetics of this process. The transformation has been monitored by the measurement of several physical properties; we have applied especially the measurement of magnetic susceptibility. Magnetic measurements are able to detect the onset of the transformation of amorphous Ni-P alloys much earlier than was possible with differential scanning calorimetry. The transformation kinetics can be analyzed by means of the Avrami plot based on the Johnson-Mehl-Avrami equation. The kinetics of further solid state reactions in the nanostructured material can be investigated similarly.

10:00 am BREAK

10:15 am INVITED

THE EFFECT OF ORDER-DISORDER PHASE TRANSFORMATION ON VOLUME INTERDIFFUSION: E. Rabkin, B. Straumal, W. Gust, Institute fur Metallkunde and Max-Planck-Institut fur Metallforschung, Seestr. 75, D-70174 Stuttgart, Germany

The volume interdiffusion has been studied in Fe-Si single crystals in the vicinity of A2-B2 ordering and in Cu-Au single crystals in the vicinity of Al-L12 ordering. Decreased interdiffusivity rates were found, as expected, in the ordered Fe-Si alloys. In the Cu-Au alloys, however, the interdiffusivity in the ordered region is higher than it could be expected by extrapolation of the diffusivities in the disordered phase. Different approaches were used to calculate the thermodynamic factor for interdiffusion, and the results obtained are in qualitative agreement with the experimental data. The influence of the variation of the partial molar volumes of the components during the phase transition on the calculated interdiffusivities is discussed.

10:45 am INVITED

THE METASTABLE STATE OF AMORPHOUS ALLOYS: R. Bormann, Institute for Materials Research, GKSS Research Center, D-21494 Geesthacht, Germany

In the past, the thermodynamic state of amorphous phases has been of great concern. Due to highly non-equilibrium preparation processes involving rapid quenching and condensation techniques it has been assumed that these alloys are frozen-in liquids where a thermodynamic description cannot be applied. Recently however, the existence of a metastable state in single-phase and phase-separated amorphous alloys has been confirmed unambiguously by electromotive force (EMF) measurements and characterized by calorimetric and structural investigations. EMF measurements also allow the direct determination of the chemical potentials of the least noble component and thereby (for binary systems) the Gibbs energy of the amorphous phase. By means of thermodynamic modeling, e.g. by the CALPHAD method, phase diagrams of metastable phases can be calculated for the amorphous phase. These can predict the amorphous phase formation during rapid quenching of the melt and during solid-state reactions. The results demonstrate that the formation of amorphous alloys is strongly determined by the thermodynamics of the undercooled liquid and the amorphous phase.

11:15 am INVITED

THERMODYNAMIC CONSIDERATIONS IN THE RATIONALIZATION OF SOLID-STATE AMORPHIZATION BETWEEN METALS AND III-V SEMICONDUCTORS: Y.A. Chang, Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706; F.-Y. Shiau, Trace Storage Technology Corp., Hsinchu, Taiwan, China; S.-L. Chen, CompuTherm LLC, Middleton, WI 53562; S. Mohney, Department of Materials Science and Engineering, Penn State University, University Park, PA 16802

In addition to binary metal/metal systems, solid-state amorphization reactions have been reported to occur in numerous ternary III-V semiconductor/metal systems such as GaAs/Ni, GaAs/Co, InP/Ni, InP/Pd, InP/Co and InP/Pt. In this presentation a thermodynamic argument will be presented first to rationalize the occurrence of an amorphous phase in a model binary system. This argument will be used next to rationalize solid-state amorphization in III-V semiconductor/metal systems focusing on the GaAs/Co case. In this system, sufficient thermodynamic information is available.