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Ageing is a non-linear, irreversible process that defines many properties of glassy materials. Now, it is shown that the so-called material-time formalism can describe ageing in terms of equilibrium-like properties.

The particle motion associated to the a-relaxation in supercooled liquids is still challenging scientists due to its difficulty to be probed experimentally. By combining synchrotron tech- niques, we report the existence of microscopic structure-dynamics relationships in Pt42.5 Cu27 Ni 9.5P 21 and Pd 42.5Cu 27 Ni9.5 P21 liquids which allows us to disentangle structural and kinetic contributions to the a-process. While the two alloys show similar kinetic fragilities, their structural fragilities differ and correlate with the temperature dependence of the stretching parameter describing the decay of the density fluctuations. This implies that the evolution of dynamical heterogeneities in supercooled alloys is determined by the rigidity of the melt structure. We find also that the atomic motion not only reflects the topological order but also the chemical short-range order, which can lead to a surprising slowdown of the a- process at the mesoscopic length scale. These results will contribute to the comprehension of the glass transition, which is still missing., We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA III beamline P02.1 and we would like to thank Alexander Schoekel and Michael Wharmby for assistance. Further we want to thank our colleagues B. Adam, L. Ruschel, S.S. Riegler, H. Jiang, V. Lemkova, H. Voigt and J. Schmauch for collaboration and fruitful discussions concerning the topic. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and inno- vation programme (Grant Agreement No 948780). The research activity has been sup- ported by the project CALIPSOplus under the Grant 307 Agreement 730872 from the EU Framework Program for Research 308 and Innovation HORIZON 2020. The authors declare that they have no competing interests. All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials., Peer Reviewed, Postprint (published version)

One of the hallmarks of disordered matter is the large amplitude of the anelastic deformation, i.e., the fraction of reversible deformation that is not instantaneously recovered after the release of load but is delayed in time. In this paper, this delayed elasticity is studied for the glass-forming Zr46.25Ti8.25Cu7.5Ni10Be27.5 alloy by means of stress step and recovery experiments. Even at high temperatures, not far from the glass transition, the delayed elasticity can recover an important fraction of the deformation and endure for a long time. Analyzing the effects of loading time and waiting time on the strain evolution, we reveal the presence of an anelastic response with a timescale dependent on loading time and an invariant shape, which indicates the presence of a distribution of reversible relaxation modes following a t-n law with exponent n between 0.5 and 1. The underlying distribution of energy barriers activated at different temperatures is accordingly shape invariant. Moreover, we found that a distribution of reversible modes corresponding to the high-frequency side of the a-relaxation peak can reproduce the experimental results. The results establish a direct link between the dynamical spectrum and the distribution of activation energies, revealing the origin of the transient creep and anelastic recovery behaviors of metallic glasses., Peer Reviewed, Postprint (published version)

The stress relaxation dynamics of metallic glass Pd 40Ni40P20 was studied in both supercooled liquid and glassy states. Time-temperature superposition was found in the metastable liquid, implying an invariant shape of the distribution of times involved in the relaxation. Once in the glass state, the distribution of relaxation times broadens as temperature and fictive temperature decrease, eventually leading to a decoupling of the relaxation in two processes., We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities and we would like to thank Y. Chushkin and F. Zontone for assistance in using beamline ID10. E.P. acknowledges financial support from MINECO (grant FIS2017-82625-P) and Generalitat de Catalunya (grant 2017SGR0042)., Postprint (author's final draft)

We studied structural relaxation in the bulk metallic glass forming alloy Zr 46.8 Ti 8.2 Cu 7.5 Ni 10 Be 27.5 on different timescales and length scales, with emphasis on the supercooled liquid state. Using x-ray photon correlation spectroscopy, we determined the microscopic structural relaxation time covering timescales of more than two decades in the supercooled liquid region, down to the subsecond regime. Upon heating across the glass transition, the intermediate scattering function changes from a compressed to a stretched decay, Postprint (published version)

Liquid-liquid transitions are interesting to many researchers since they occur in systems as diverse as monoatomic liquids, multicomponent oxides, and metallic glass formers. In some cases, the crossover is accompanied by changes in the dynamical properties. By combining state-of-the-art synchrotron techniques, we followed the structure and atomic motion during quasistatic cooling of the Au49Cu26.9Si16.3Ag5.5Pd2.3 metallic glass former from the low-temperature supercooled liquid. With this thermal protocol, we were able to lower the glass transition temperature far enough to reveal a liquid-liquid crossover between two amorphous structures corresponding to two ultraviscous liquids with different kinetic behavior. This transition is in competition with vitrification, which occurs at conventional cooling rates, and is accompanied by structural changes not affecting the average density. Our results provide a direct connection between polyamorphism and dynamical crossover, and an alternative case to add to the highly debated topic on the low-temperature divergence of the dynamics in supercooled liquids., Peer Reviewed, Postprint (published version)

This is the author final draft version of article https://doi.org/10.1103/PhysRevLett.120.135504, As ultrastable metallic glasses (UMGs) are promising candidates to solve the stability issues of conventional metallic glasses, their study is of exceptional interest. By means of x -ray photon correlation spectroscopy, we have investigated the stability of UMGs at the atomic level. We find a clear signature of ultrastability at the atomic level that results in slower relaxation dynamics of UMGs with respect to conventional (rapidly quenched) metallic glasses, and in a peculiar acceleration of the dynamics by near T g annealing. This surprising phenomenon, called here anti-aging, can be understood in the framework of the potential energy landscape. For all samples, the structural relaxation process can be described with a highly compressed shape of the density fluctuations, unaffected by thermal treatments and regardless of the ultrastability of the glass., Peer Reviewed, Postprint (updated version)

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, thermomechanical testing, and x-ray photon correlation spectroscopy. We observe multiple decays in the enthalpy change during aging. This is reflected by a microscopic ordering consisting of distinct stationary dynamical regimes interconnected by abrupt aging processes. The stationary regimes are 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.

This article has been published in a revised form in Journal of Materials Research, https://doi.org/10.1557/jmr.2017.187. This version is free to view and download for private research and study only. Not for re-distribution, re-sale or use in derivative works. © Materials Research Society., Several recent X-ray photon correlation spectroscopy works have reported an anomalous atomic dynamics in hyperquenched metallic glasses. Here, we compare and contrast these microscopic dynamics with that found in a Zr44Ti11Ni10Cu10Be25 bulk metallic glass, prepared with a cooling rate some 6 orders of magnitude lower. In both cases, structural relaxation in the glass is governed by internal stresses, giving rise to highly compressed density correlation functions. Differently from the fast aging reported in previous studies, here the atomic dynamics displays a slow linear atomic-level aging, while not affecting the shape parameter. Traditional macroscopic phenomenological models fail to capture the temperature dependence of the microscopic structural relaxation time, suggesting a length scale dependence of the aging. Interestingly, the dynamics does not seem to be affected by the presence of a low percentage of frozen nanocrystals and displays a temperature dependence similar to that observed in macroscopic viscosity measurements., Peer Reviewed, Postprint (author's final draft)

The current view of structural relaxation in metallic glasses assumes the presence of primary and secondary processes with different activation energies. While the faster, secondary process can be well characterized in the out-of-equilibrium state below the glass transition temperature Tg, the experimental direct determination of the primary process in this temperature region is more difficult due to the long relaxation times. In this work, we merge new and literature data to analyze the temperature behavior of the primary relaxation time below Tg as observed by mechanical spectroscopy and stress relaxation of metallic glasses of different fragility. We compare these results with the microscopic structural relaxation times previously measured with X-ray photon correlation spectroscopy. The coincidence between the macroscopic and microscopic relaxation times allows us to discuss the underlying mechanisms responsible of primary relaxation over different length scales, as well as to propose an overall picture of the primary relaxation behavior in the glassy regime near Tg., Postprint (author's final draft)

Peer Reviewed, Postprint (published version)

We use coherent x rays to probe the aging dynamics of a metallic glass directly on the atomic level. Contrary to the common assumption of a steady slowing down of the dynamics usually observed in macroscopic studies, we show that the structural relaxation processes underlying aging in this metallic glass are intermittent and highly heterogeneous at the atomic scale. Moreover, physical aging is triggered by cooperative atomic rearrangements, driven by the relaxation of internal stresses. The rich diversity of this behavior reflects a complex energy landscape, giving rise to a unique type of glassy-state dynamics., Peer Reviewed, Postprint (published version)

The structural relaxation, glass transition and crystallization processes of Mg65Cu25Y10 metallic glass are studied by Differential Scanning Calorimetry (DSC) and Mechanical spectroscopy. The relaxation model derived from the mechanical measurements is compared with the kinetics of these transformations obtained from the DSC curves. The structural relaxation kinetics is found to be controlled by the glassy dynamics following an Adams-Gibbs-Vogel function. The glass transition and crystallization kinetics are controlled by the dynamics of the supercooled melt following a Vogel-Fulcher-Tammann behaviour. The results suggest that the microscopic processes responsible of structural relaxation and aging below the glass transition correspond to the same processes generating the a-relaxation peak. (C) 2013 Elsevier B.V. All rights reserved., Peer Reviewed, Postprint (author’s final draft)

We present a temperature investigation of the structural and dynamical evolution of rapidly quenched metallic glasses of Mg65Cu25Y10 at the atomic length scale by means of in situ high energy x-ray diffraction and time resolved x-ray photon correlation spectroscopy. We find a flattening of the temperature evolution of the position of the first sharp diffraction peak on approaching the glass transition temperature from the glassy state, which reflects into a surprising slowing down of the relaxation ynamics of even one order of magnitude with increasing temperature. The comparison between structural and dynamical properties strengthens the idea of a stress-induced, rather than pure diffusive, atomic motion in metallic glasses, Postprint (published version)