Kinetics of structural relaxation and regularities of plastic flow of metallic glasses

On the basis of a calculation of the structural relaxation rate and an experimental acoustical-emission determination of the temperature of the transition from localized to uniform flow it is argued that the type of plastic deformation of metallic glasses is uniquely determined by the kinetic structure of the relaxation. In the case of a kinetically hindered structural relaxation, which is characteristic for tests of initial samples at temperatures T<380–420 K, a localized dislocational deformation is realized. At higher temperatures, “memory” of the thermal prehistory of the samples is lost (aging at room temperature), the structural relaxation rate grows abruptly and plastic flow becomes uniform viscoplastic flow. Fiz. Tverd. Tela (St. Petersburg) 41, 2167–2173 (December 1999)     

Structural relaxation and viscous flow in amorphous ZrAlCu

The ternary metallic glass Zr₆₅Al_7.5Cu_27.5 offers a wide temperature range between glass transition temperature and crystallization temperature and is therefore well suited for investigation of the glass transition and the state of the super cooled liquid. The non-linear viscosity change caused by structural relaxation has been measured caused by structural relaxation has been measured using tensile creep experiments on as quenched samples. The increase of viscosity can be described by bimolecular annihilation kinetics of flow defects. The Arrhenius plot of equilibrium viscosity shows a kink at a temperature which seems to be the glass transition temperature. The activation energies of viscous flow below and above that glass transition temperature differ by nearly a factor two. Different microscopic processes responsible for viscous flow in the two regimes of temperature are therefore conceivable. This view is also encouraged by Dynamic-Mechanical-Analysis on relaxed samples, a method to examine the viscoelastic behaviour of glassy materials on different time scales and by recent diffusion measurements on a different system.     

Structural relaxation and viscous flow in amorphous ZrAlCu

The ternary metallic glass Zr₆₅Al_7.5Cu_27.5 offers a wide temperature range between glass transition temperature and crystallization temperature and is therefore well suited for investigation of the glass transition and the state of the super cooled liquid. The non-linear viscosity change caused by structural relaxation has been measured caused by structural relaxation has been measured using tensile creep experiments on as quenched samples. The increase of viscosity can be described by bimolecular annihilation kinetics of flow defects. The Arrhenius plot of equilibrium viscosity shows a kink at a temperature which seems to be the glass transition temperature. The activation energies of viscous flow below and above that glass transition temperature differ by nearly a factor two. Different microscopic processes responsible for viscous flow in the two regimes of temperature are therefore conceivable. This view is also encouraged by Dynamic-Mechanical-Analysis on relaxed samples, a method to examine the viscoelastic behaviour of glassy materials on different time scales and by recent diffusion measurements on a different system.     

Low frequency relaxation conductance theory of superionic glasses

Based on the characteristic of superionic glasses and a physical picture of unified theory of the low frequency fluctuation, dissipation and relaxation phenomenon, the transport features of a non-Markovian process of conducting ions are explored the theory predicts that the frequency independent conductivity follows a simple Arrhenius dependence on temperature at high temperatures. The theory can also explain the frequency dispersion which depend on the infrared divergence exponent n.     

Evidence of distributed interstitialcy-like relaxation of the shear modulus due to structural relaxation of metallic glasses

The interstitialcy theory is used to calculate the kinetics of shear modulus relaxation induced by structural relaxation of metallic glasses. A continuous distribution of activation energies is shown to be a salient feature of the relaxation. High precision in situ contactless electromagnetic acoustic-transformation shear modulus (600- kHz) measurements performed on a Zr-based bulk metallic glass are found to strongly support the approach under consideration. It is revealed that the activation energy spectra derived from isothermal and isochronal shear modulus measurements are in good agreement with each other. It is concluded that the increase of the shear modulus during structural relaxation can be understood as a decrease of the concentration of structural defects similar to dumbbell interstitials in simple crystalline metals.     

Kinetic theory of nonlinear viscous flow in two and three dimensions

On the basis of a nonlinear kinetic equation for a moderately dense system of hard spheres and disks it is shown that shear and normal stresses in a steady-state, uniform shear flow contain singular contributions of the form ¦X¦^3/2 for hard spheres, or ¦X¦ log ¦X¦ for hard disks. HereX is proportional to the velocity gradient in the shear flow. The origin of these terms is closely related to the hydrodynamic tails t^–d/2 in the current-current correlation functions. These results also imply that a nonlinear shear viscosity exists in two-dimensional systems. An extensive discussion is given on the range ofX values where the present theory can be applied, and numerical estimates of the effects are given for typical circumstances in laboratory and computer experiments.Supported by National Science Foundation grant No. CHE-73-08856 (to HvB, JRD, and JS) and the Center for Theoretical Physics of the Univ. of Md. (to HvB).On leave from Institute of Theoretical Physics, Warsaw University, Warsaw, Poland.     

Phenomenological theory of structural relaxation based on a thermorheologically complex relaxation time distribution

The aim of this work is to explore the consequences on the kinetics of structural relaxation of considering a glass-forming system to consist of a series of small but macroscopic relaxing regions that evolve independently from each other towards equilibrium in the glassy state. The result of this assumption is a thermorheologically complex model. In this approach each relaxing zone has been assumed to follow the Scherer-Hodge model for structural relaxation (with the small modification of taking a linear dependence of configurational heat capacity with temperature). The model thus developed contains four fitting parameters. A least-squares search routine has been used to find the set of model parameters that fit simultaneously four DSC thermograms in PVAc after different thermal histories. The computersimulated curves are compared with those obtained with Scherer-Hodge model and the model proposed by Gómez and Monleón. The evolution of the relaxation times during cooling or heating scans and also during isothermal annealing below the glass transition has been analysed. It has been shown that the relaxation times distribution narrows in the glassy state with respect to equilibrium. Isothermal annealing causes this distribution to broaden during the process to finally attain in equilibrium the shape defined at temperatures above T _g .     

Toward a theory of nuclear relaxation in dielectric glasses at ultralow temperatures

The temperature and frequency dependence of the nuclear relaxation rate in dielectric glasses is investigated. It is shown that at low and ultralow temperatures nuclear relaxation is due to an interaction between the nuclear quadrupole moment and fluctuations of the electric field created by dipole moments of two-level systems. Fluctuations of this field can be associated with the background relaxation or are due only to the dipole-dipole interaction between two-level systems. It is shown that at lower temperatures the second relaxation mechanism begins to dominate. Expressions are obtained for the temperature and frequency of crossover between different nuclear relaxation regimes. The possibility of experimental confirmation of our results is discussed. Zh. éksp. Teor. Fiz. 115, 2254–2262 (June 1999) Russian Scientific Center “Kurchatov Institute”     

An advanced NMR protocol for the structural characterization of aluminophosphate glasses

In this work a combination of complementary advanced solid-state nuclear magnetic resonance (NMR) strategies is employed to analyse the network organization in aluminophosphate glasses to an unprecedented level of detailed insight. The combined results from MAS, MQMAS and (31)P-{(27)Al}-CP-heteronuclear correlation spectroscopy (HETCOR) NMR experiments allow for a detailed speciation of the different phosphate and aluminate species present in the glass. The interconnection of these local building units to an extended three-dimensional network is explored employing heteronuclear dipolar and scalar NMR approaches to quantify P-O-Al connectivity by (31)P{(27)Al}-heteronuclear multiple quantum coherence (HMQC), -rotational echo adiabatic passage double resonance (REAPDOR) and -HETCOR NMR as well as (27)Al{(31)P}-rotational echo double resonance (REDOR) NMR experiments, complemented by (31)P-2D-J-RESolved MAS NMR experiments to probe P-O-P connectivity utilizing the through bond scalar J-coupling. The combination of the results from the various NMR approaches enables us to not only quantify the phosphate units present in the glass but also to identify their respective structural environments within the three-dimensional network on a medium length scale employing a modified Q notation, Q(n)(m),(AlO)(x), where n denotes the number of connected tetrahedral phosphate, m gives the number of aluminate species connected to a central phosphate unit and x specifies the nature of the bonded aluminate species (i.e. 4, 5 or 6 coordinate aluminium).     

Effective temperature in relaxation of Coulomb glasses

We study relaxation in two-dimensional Coulomb glasses up to macroscopic times. We use a kinetic Monte Carlo algorithm especially designed to escape efficiently from deep valleys around metastable states. We find that, during the relaxation process, the site occupancy follows a Fermi-Dirac distribution with an effective temperature much higher than the real temperature T. Long electron-hole excitations are characterized by T(eff), while short ones are thermalized at T. We argue that the density of states at the Fermi level is proportional to T(eff) and is a good thermometer to measure it. T(eff) decreases extremely slowly, roughly as the inverse of the logarithm of time, and it should affect hopping conductance in many experimental circumstances.     

Interaction-driven relaxation of two-level systems in glasses

We study how the elastic interaction of two-level systems contributes to their relaxational motion. Evaluating the Mori-Zwanzig memory function in terms of a perturbation series in powers of the couplings J(ij), we find a null result at second order, which means that interacting pairs of two-level systems do not give rise to relaxation, yet a finite relaxation rate does occur in fourth order; i.e., a diffusive band is formed by resonant triples. Our results provide a simple explanation for several puzzling experimental observations. Regarding the temperature dependence of the sound velocity deltanu approximately lnT in the kHz range, we find that its slope below and above the maximum takes opposite signs but the same absolute value, in agreement with the measured ratio 1 : - 1. Below the relaxation plateau, the internal friction is shown to vary linearly with T, in agreement with experiment.     

Correlating dynamic relaxation and viscoelasticity in metallic glasses

Relaxation dynamics,essential for the structural evolution of non-equilibrium systems like glassy materials,remain enigmatic.Here,we explore relaxation dynamics and viscoelastic properties in three types of metallic glasses with distinct β relaxation behavior.In systems with significant β relaxation,stress relaxation and creep experiments reveal a transition from two-step to one-step relaxation with rising temperature.However,such a phenomenon is absent in systems with weaker β relaxation.We mod...     

Dislocation glasses: aging during relaxation and coarsening

The dynamics of dislocations is reported to exhibit a range of glassy properties. We study numerically various versions of 2D edge dislocation systems, in the absence of externally applied stress. Two types of glassy behavior are identified (i) dislocations gliding along randomly placed, but fixed, axes exhibit relaxation to their spatially disordered stable state; (ii) if both climb and annihilation are allowed, irregular cellular structures can form on a growing length scale before all dislocations annihilate. In all cases both the correlation function and the diffusion coefficient are found to exhibit aging. Relaxation in case (i) is a slow power law, furthermore, in the transient process (ii) the dynamical exponent z approximately 6, i.e., the cellular structure coarsens relatively slowly.     

Microscopic theory of network glasses

A theory of the glass transition of network liquids is developed using self-consistent phonon and liquid state approaches. The dynamical transition and entropy crisis characteristic of random first-order transitions are mapped as a function of the degree of bonding and density. Using a scaling relation for a soft-core model to crudely translate the densities into temperatures, theory predicts that the ratio of the dynamical transition temperature to the laboratory transition temperature rises as the degree of bonding increases, while the Kauzmann temperature falls explaining why highly coordinated liquids are "strong" while van der Waals liquids without coordination are "fragile."