Aging, Fragility and Reversibility Window in Bulk Alloy Glasses (original) (raw)
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Ageing, fragility and the reversibility window in bulk alloy glasses
Journal of Physics: Condensed Matter, 2005
Non-reversing relaxation enthalpies ( H nr ) at glass transitions T g (x) in the P x Ge x Se 1−2x ternary display wide, sharp and deep global minima ( 0) in the 0.09 < x < 0.145 range, within which T g s become thermally reversing. In this reversibility window, glasses are found not to age, in contrast to ageing observed for fragile glass compositions outside the window. Thermal reversibility and lack of ageing seem to be paradigms of self-organization which molecular glasses share with protein structures which repetitively and reversibly change conformation near T g and the folding temperature respectively.
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.
The change of physical properties during aging and the associated microscopic dynamics of the Au49Cu26.9Si16.3Ag5.5Pd2.3 bulk metallic glass are investigated using a broad collection of laboratory and synchrotron-based techniques, such as differential- and fast-scanning calorimetry, thermal-mechanical testing, and x-ray photon correlation spectroscopy. At low annealing temperatures, we observe multiple decays in the enthalpy change, which is reflected by a microscopic ordering pathway consisting of distinct stationary dynamical regimes interconnected by abrupt aging regimes, representative of states of local and transient equilibrium with increasingly higher activation energies. Furthermore, the aging study is conducted with the kinetically fragile frozen-in structure and the underlying fragile-to-strong transition is accessed by the ultra-viscous liquid state during annealing on a long-time scale and corresponds to the last observed enthalpy equilibration decay. The experimental work verifies, for the first time, that in a metallic glass forming system, the fragile-to-strong transition can also occur below the conventional glass transition temperature. Upon reheating, the reverse transformation, i.e. the strong-to-fragile transition, is observed with an entropy change of 0.19 J/(g-atom K), which is 2.4% of the entropy of fusion.
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.
Reversibility Window, Aging, and Nanoscale Phase Separation in GexAsxS1-2x Bulk Alloy Glasses
The non-reversing enthalpy (∆H nr) near T g , in bulk Ge x As x S 1-2x glasses is found to display a global minimum (~0) in the 0.11 < x < 0.15 range, the reversibility window. Furthermore, the ∆H nr term is found to age for glass compositions below (x < 0.11) and above (x > 0.15) the window but not in the window. In analogy to corresponding selenides, glass compositions in the window represent the intermediate phase, those at x < 0.11 are floppy, and those at x > 0.15 stressed-rigid. Raman scattering shows floppy and stressed rigid networks to be partially nanoscale phase separated, an aspect of structure that contributes to a narrowing of the reversibility window width and to suppression of the ∆H nr term in S-rich glasses qualitatively in relation to corresponding Se-rich glasses. Ideas based on Lagrangian bonding constraints have proved to be central to understand the elastic behavior of network glasses 1,2. In the 1980s, these considerations led to the
Decelerated aging in metallic glasses by low temperature thermal cycling
Physical Review Research
Differential scanning calorimetry measurements on different bulk metallic glasses show no measurable rejuvenation upon deeply cooled (cryogenic) thermal cycling. This applies both to as-quenched and wellannealed samples. Extensive molecular dynamics simulations of a generic model glass former corroborate these observations. We disentangle the effects of aging from those of thermal treatment and show that aging is slowed down but not stopped-neither reversed-during thermal cycling. These observations are corroborated further by a survey of energy distribution, which continues narrowing, albeit with a smaller rate.
Theory of the structural glass transition: a pedagogical review
Advances in Physics, 2015
The random first-order transition (RFOT) theory of the structural glass transition is reviewed in a pedagogical fashion. The rigidity that emerges in crystals and glassy liquids is of the same fundamental origin. In both cases, it corresponds with a breaking of the translational symmetry; analogies with freezing transitions in spin systems can also be made. The common aspect of these seemingly distinct phenomena is a spontaneous emergence of the molecular field, a venerable and well-understood concept. In crucial distinction from periodic crystallisation, the free energy landscape of a glassy liquid is vastly degenerate, which gives rise to new length and time scales while rendering the emergence of rigidity gradual. We obviate the standard notion that to be mechanically stable a structure must be essentially unique; instead, we show that bulk degeneracy is perfectly allowed but should not exceed a certain value. The present microscopic description thus explains both crystallisation and the emergence of the landscape regime followed by vitrification in a unified, thermodynamics-rooted fashion. The article contains a self-contained exposition of the basics of the classical density functional theory and liquid theory, which are subsequently used to quantitatively estimate, without using adjustable parameters, the key attributes of glassy liquids, viz., the relaxation barriers, glass transition temperature, and cooperativity size. These results are then used to quantitatively discuss many diverse glassy phenomena, including: the intrinsic connection between the excess liquid entropy and relaxation rates, the non-Arrhenius temperature dependence of α-relaxation, the dynamic heterogeneity, violations of the fluctuation-dissipation theorem, glass ageing and rejuvenation, rheological and mechanical anomalies, super-stable glasses, enhanced crystallisation near the glass transition, the excess heat capacity and phonon scattering at cryogenic temperatures, the Boson peak and plateau in thermal conductivity, and the puzzling midgap electronic states in amorphous chalcogenides.
Physical Review B, 2008
Physical aging effects caused by prolonged natural storage ͑ϳ22 years͒ in binary As x Se 100−x glasses are probed by temperature-modulated differential scanning calorimetry. It is shown that all aged samples with x Ͻ 40 reveal nonzero out-of-phase component of complex heat flow testifying physical aging effect. The first composition, which can be attributed to a so-called self-organized phase characterized by the absence of natural physical aging effect, is the stoichiometric As 2 Se 3 glass; lack of aging within the so-called reversibility window, 28Ͻ x Ͻ 38, is not found. High-resolution x-ray photoelectron spectroscopy fails to show any evidence of fourfold-coordinated As atoms, which is consistent with the aging ability of glasses with x Ͻ 40.