Physical Aging of Glasses (original) (raw)
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Relaxation dynamics and aging in structural glasses
2013
We present a study of the atomic dynamics in a Mg 65 Cu 25 Y 10 metallic glass former both in the deep glassy state and in the supercooled liquid phase. Our results show that the glass transition is accompanied by a dynamical crossover between a faster than exponential shape of the intermediate scattering function in the glassy state and a slower than exponential shape in the supercooled liquid. While the crossover temperature is independent on the previous thermal history, both the relaxation rate and the shape of the relaxation process depend on the followed thermal path. Moreover, the temperature dependence of the the structural relaxation time displays a strong departure from the Arrhenius-like behavior of the corresponding supercooled liquid phase, and can be well described in the Narayanaswamy-Moynihan framework with a large non-linearity parameter.
Mechanics of Materials, 2015
Inorganic glasses are viscoelastic materials since they exhibit, below as well as above their glass transition temperature, a viscoelastic deformation under stress, which can be decomposed into a sum of an elastic part, an inelastic (or viscous) part and a delayed elastic part. The delayed elastic part is responsible for the non-linear primary creep stage observed during creep tests. During a stress relaxation test, the strain, imposed, is initially fully elastic, but is transformed, as the stress relaxes, into an inelastic and a delayed elastic strains. For linear viscoelastic materials, if the stress relaxation function can be fitted by a stretched exponential function, the evolution of each part of the strain can be predicted using the Boltzmann superposition principle. We develop here the equations of these evolutions, and we illustrate their accuracy by comparing them with experimental evolutions measured on GeSe 9 glass fibers. We illustrate also, by simple equations, the relationship between any kind of relaxation function based on additive contribution of different relaxation processes and the delayed elastic contribution to stress relaxation: the delayed elasticity is directly correlated to the dispersion of relaxations times of the processes involved during relaxation.
Ageing and Relaxation in Glass Forming Systems
2008
We propose that there exists a generic class of glass forming systems that have competing states (of crystalline order or not) which are locally close in energy to the ground state (which is typically unique). Upon cooling, such systems exhibit patches (or clusters) of these competing states which become locally stable in the sense of having a relatively high local shear modulus. It is in between these clusters where ageing, relaxation and plasticity under strain can take place. We demonstrate explicitly that relaxation events that lead to ageing occur where the local shear modulus is low (even negative), and result in an increase in the size of local patches of relative order. We examine the ageing events closely from two points of view. On the one hand we show that they are very localized in real space, taking place outside the patches of relative order, and from the other point of view we show that they represent transitions from one local minimum in the potential surface to another. This picture offers a direct relation between structure and dynamics, ascribing the slowing down in glass forming systems to the reduction in relative volume of the amorphous material which is liquid-like. While we agree with the well known Adam-Gibbs proposition that the slowing down is due to an entropic squeeze (a dramatic decrease in the number of available configurations), we do not agree with the Adam-Gibbs (or the Volger-Fulcher) formulae that predict an infinite relaxation time at a finite temperature. Rather, we propose that generically there should be no singular crisis at any finite temperature: the relaxation time and the associated correlation length (average cluster size) increase at most super-exponentially when the temperature is lowered.
Relaxation in glassforming liquids and amorphous solids
Journal of Applied Physics, 2000
The field of viscous liquid and glassy solid dynamics is reviewed by a process of posing the key questions that need to be answered, and then providing the best answers available to the authors and their advisors at this time. The subject is divided into four parts, three of them dealing with behavior in different domains of temperature with respect to the glass transition temperature, Tg, and a fourth dealing with “short time processes.” The first part tackles the high temperature regime T>Tg, in which the system is ergodic and the evolution of the viscous liquid toward the condition at Tg is in focus. The second part deals with the regime T∼Tg, where the system is nonergodic except for very long annealing times, hence has time-dependent properties (aging and annealing). The third part discusses behavior when the system is completely frozen with respect to the primary relaxation process but in which secondary processes, particularly those responsible for “superionic” conductivit...
Long time response of aging glassy polymers
Rheologica Acta, 2014
Aging amorphous polymeric materials undergo free volume relaxation, which causes slowing down of the relaxation dynamics as a function of time. The resulting time dependency poses difficulties in predicting their long time physical behavior. In this work, we apply effective time domain approach to the experimental data on aging amorphous polymers and demonstrate that it enables prediction of long time behavior over the extraordinary time scales. We demonstrate that, unlike the conventional methods, the proposed effective time domain approach can account for physical aging that occurs over the duration of the experiments. Furthermore, this procedure successfully describes timetemperature superposition and time -stress superposition. It can also allow incorporation of varying dependences of relaxation time on aging time as well as complicated but known deformation history in the same experiments. This work strongly suggests that the effective time domain approach can act as an important tool to analyze the long time physical behavior of aging amorphous polymeric materials. J J t t
1 8 M ar 2 00 8 Ageing and Relaxation in Glass Forming Systems
2021
We propose that there exists a generic class of glass forming systems that have competing states (of crystalline order or not) which are locally close in energy to the ground state (which is typically unique). Upon cooling, such systems exhibit patches (or clusters) of these competing states which become locally stable in the sense of having a relatively high local shear modulus. It is in between these clusters where ageing, relaxation and plasticity under strain can take place. We demonstrate explicitly that relaxation events that lead to ageing occur where the local shear modulus is low (even negative), and result in an increase in the size of local patches of relative order. We examine the ageing events closely from two points of view. On the one hand we show that they are very localized in real space, taking place outside the patches of relative order, and from the other point of view we show that they represent transitions from one local minimum in the potential surface to anot...
Theory of aging in structural glasses
The Journal of Chemical Physics, 2004
The random first order transition theory of the dynamics of supercooled liquids is extended to treat aging phenomena in nonequilibrium structural glasses. A reformulation of the idea of "entropic droplets" in terms of libraries of local energy landscapes is introduced which treats in a uniform way the supercooled liquid (reproducing earlier results) and glassy regimes. The resulting microscopic theory of aging makes contact with the Nayaranaswamy-Moynihan-Tool nonlinear relaxation formalism and the Hodge-Scherer extrapolation of the Adam-Gibbs formula, but deviations from both approaches are predicted and shown to be consistent with experiment. The nonlinearity of glassy relaxation is shown to quantitatively correlate with liquid fragility. The residual nonArrhenius temperature dependence of relaxation observed in quenched glasses is explained. The broadening of relaxation spectra in the nonequilibrium glass with decreasing temperature is quantitatively predicted. The theory leads to the prediction of spatially fluctuating fictive temperatures in the longaged glassy state, which have non-Gaussian statistics. This can give rise to "ultra-slow" relaxations in systems after deep quenches.
On the Derivation of Equilibrium Relaxation Times from Aging Experiments
The Journal of Physical Chemistry B, 2013
Physical aging below the glass transition temperature, T g , is generally understood to be governed by the same slow degrees of freedom that are responsible for structural relaxation in the equilibrium state above T g. Provided a reliable model of aging is at hand, it should thus be possible to extract very long relaxation time constants from experimental data on physical aging. Two very different models of aging are investigated in this respect using data for various molecular glass-forming liquids extending out to aging times of 3 × 10 6 s. It turns out that application of the well-known KAHR or TNM model does not provide a significant advantage over a recently proposed phenomenological approach that is much simpler.
Mathematical and Computer Modelling, 2003
Constitutive equations are developed for the linear viscoelastic behavior and volume recovery in glassy polymers. The model is based on the concept of cooperative relaxation that treats a polymer as an ensemble of cooperatively rearranged regions (CR&.). A rearrangement event is thought of as a hop of a CRR trapped in its potential well on the energy landscape to a liquid-like energy level. The energy landscape of a glassy polymer is assumed to be time-independent, whereas the position of the liquid-like state changes with time approaching the equilibrium energy level. Its evolution is described by a fictive temperature that obeys Tool's equation with a characteristic time proportional to the average time for rearrangement. Kinetic equations for volume recovery and stressstrain relations for the viscoelastic response are verified by fitting observations in dilatometric and mechanical (static and dynamic) tests. Fair agreement is demonstrated between the results of numerical simulation and the experimental data for polycarbonate, poly(arylene etherimide), poly(ether ether ketone), and poly(viny1 acetate).
Is the structural relaxation of glasses controlled by equilibrium shear viscosity?
Journal of the American Ceramic Society, 2021
The knowledge of relaxation processes is fundamental in glass science and technology because relaxation is intrinsically related to vitrification, tempering, and annealing and several applications of glasses. However, there are conflicting reports on whether the structural relaxation time of glass can be calculated using the Maxwell equation, which relates the relaxation time with the shear viscosity and shear modulus. Hence, the objective of this work was to test whether these two relaxation times are comparable. We studied the kinetics of structural relaxation of a lead metasilicate glass by measurements of the refractive index variation over time at temperatures between 5 and 25 K below the fictive temperature, which was initially set 5 K below the glass transition temperature. We also measured the equilibrium shear viscosity above and below the glass transition range, expanding the current knowledge by one order of magnitude. We found that the Kohlrausch equation describes very well the experimental structural relaxation kinetics in the whole temperature range and the Kohlrausch exponent increases with temperature, in agreement with studies for other glasses. The experimental average structural relaxation times are much longer than the values computed from isostructural viscosity, as expected. Still, they are less than one order of magnitude higher than the average relaxation time computed by the Maxwell equation. Thus, these results demonstrate that the structural relaxation process is not controlled by isostructural viscosity and that the equilibrium shear viscosity only gives a lower boundary for structural relaxation kinetics.