M.P. Saksena

Structure and dynamics of a two-component metallic glass

Computations of the frequencies of longitudinal and transverse phonon modes in a two-component metallic glass (Mg70Zn30) based on the pseudopotential approach used for the calculation of interatomic pair potential have been presented. The theory employed for these calculations is a self-consistent phonon theory as developed by Takeno and Goda (1971) for amorphous solids. The computed results are compared with molecular dynamics and neutron inelastic scattering experiments. It appears that the short-wavelength collective excitations detected in this glass arise from longitudinal phonon excitations and probably not from a diffuse Umklapp scattering from transverse acoustic waves.

Structure and vibrational dynamics of Ca70Mg30 glass

Abstract A new effective pair potential is proposed for Ca 70 Mg 30 glass in Ashcroft form using the concept of a Wigner-Seitz sphere treating it as a one-component system. A theory of phonon in amorphous solids in a generalized random phase approximation is employed to compute the eigenfrequencies of the longitudinal and transverse phonons making use of the so obtained potential. The eigenfrequencies obtained agree qualitatively with the neutron inelastic scattering results of Suck et al . and also with the theoretical results of Hafner and with the results of Bhatia and Singh, both qualitatively as well as quantitatively. In addition, the elastic and thermodynamic properties of the glass have also been studied using the longitudinal and transverse sound velocities.

Crystallization kinetics and specific heat of Cu60Zr40 glassy alloy

Abstract CuZr alloy with Cu-40at.%Zr was splat quenched to the non-crystalline state using a rotating-drum technique. The glass transition temperature and crystallization kinetics of the splat-quenched material have been studied using differential scanning calorimetry (DSC). The DSC traces were taken at seven different heating rates. In all the thermograms, a double glass transition phenomenon as well as double-stage crystallization have been detected at all the heating rates. The enthalpy release during the crystallization process has been determined by measuring the areas below various crystallization peaks. The activation energy has been evaluated to be 435 kJ mol−1 bycontinuous heating. In addition to the crystallization kinetics, the specific heat of the sample has been determined at four heating rates. The difference in specific heat before and after the glass transition has also been studied as a function of the heating rate. It has been found that this difference decreases with increasing heating rate.

Temperature-dependent free energy of crystallization and viscosity of glassy materials

Abstract Elementary thermodynamic procedures have been used to calculate the Gibbs free energy difference ΔG from a knowledge of the specific heat difference ΔC p between the undercooled melt and solid phase. However, the metastability of the liquid makes the experimental determination of ΔC p very difficult over a wide range of temperatures. In the case of glass-forming materials, ΔG can be best measured between the melting temperature T f and the glass transition temperature T g . Several investigators have attempted to propose an appropriate expression for ΔG in terms of more easily measured parameters. The present work incorporates a new expression for the difference in Gibbs free energy proposed in the power law form ΔG = ΔS m ( ΔT ) x . This relation contains a relatively more physical parameter x and is able to predict non-linearity between ΔG and ΔT and hence the deviation from the Turnbull formula. The calculated values using this approach agree fairly well with experimental results in liquids as well as binary and ternary glasses over the temperature range of interest. The temperature dependence of the viscosity can also be arrived at from the C p data up to the glass transition temperature. The variation in viscosity above T g has been very well accounted for by two different proposed expressions in the case of O -terphenyl and Na 0.32 K 0.68 glassy systems.