Since their discovery in the 1960s, metallic glasses have continuously attracted much interest across the physics and materials science communities. In the forefront are their unique properties, ...which hold the alluring promise of broad application in fields as diverse as medicine, environmental science and engineering. However, a major obstacle to their wide-spread commercial use is their inherent temporal instability arising from underlying relaxation processes that can dramatically alter their physical properties. The result is a physical aging process which can bring about degradation of mechanical properties, namely through embrittlement and catastrophic mechanical failure. Understanding and controlling the effects of aging will play a decisive role in our on-going endeavor to advance the use of metallic glasses as structural materials, as well as in the more general comprehension of out-of-equilibrium dynamics in complex systems. This review presents an overview of the current state of the art in the experimental advances probing physical aging and relaxation processes in metallic glasses. Similarities and differences between other hard and soft matter glasses are highlighted. The topic is discussed in a multiscale approach, first presenting the key features obtained in macroscopic studies, then connecting them to recent novel microscopic investigations. Particular emphasis is put on the occurrence of distinct relaxation processes beyond the main structural process in viscous metallic melts and their fate upon entering the glassy state, trying to disentangle results and formalisms employed by the different groups of the glass-science community. A microscopic viewpoint is presented, in which physical aging manifests itself in irreversible atomic-scale processes such as avalanches and intermittent dynamics, ascribed to the existence of a plethora of metastable glassy states across a complex energy landscape. Future experimental challenges and the comparison with recent theoretical and numerical simulations are discussed as well.
Understanding and controlling physical aging, that is, the spontaneous temporal evolution of out-of-equilibrium systems, represents one of the greatest tasks in material science. Recent studies have ...revealed the existence of a complex atomic motion in metallic glasses, with different aging regimes in contrast with the typical continuous aging observed in macroscopic quantities. By combining dynamical and structural synchrotron techniques, here for the first time we directly connect previously identified microscopic structural mechanisms with the peculiar atomic motion, providing a broader unique view of their complexity. We show that the atomic scale is dominated by the interplay between two processes: rearrangements releasing residual stresses related to a cascade mechanism of relaxation, and medium range ordering processes, which do not affect the local density, likely due to localized relaxations of liquid-like regions. As temperature increases, a surprising additional secondary relaxation process sets in, together with a faster medium range ordering, likely precursors of crystallization.
Upon cooling, glass-forming liquids experience a dynamic decoupling in the fast β and slow α process, which has greatly influenced glass physics. By exploring an extremely wide temporal and ...temperature range, we find a surprising gradual change of the relaxation profile from a single-step to a two-step decay upon cooling in various metallic glasses. This behavior implies a decoupling of the relaxation in two different processes in a glass state: a faster one likely related to the anomalous stress-dominated microscopic dynamics, and a slower one associated with subdiffusive motion at larger scales with a broader distribution of relaxation times.
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We use x-ray photon correlation spectroscopy to investigate the structural relaxation process in a metallic glass on the atomic length scale. We report evidence for a dynamical crossover between the ...supercooled liquid phase and the metastable glassy state, suggesting different origins of the relaxation process across the transition. Furthermore, using different cooling rates, we observe a complex hierarchy of dynamic processes characterized by distinct aging regimes. Strong analogies with the aging dynamics of soft glassy materials, such as gels and concentrated colloidal suspensions, point at stress relaxation as a universal mechanism driving the relaxation dynamics of out-of-equilibrium systems.
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Vitrification from physical vapor deposition is known to be an efficient way for tuning the kinetic and thermodynamic stability of glasses and significantly improve their properties. There is a ...general consensus that preparing stable glasses requires the use of high substrate temperatures close to the glass transition one, T
. Here, we challenge this empirical rule by showing the formation of Zr-based ultrastable metallic glasses (MGs) at room temperature, i.e., with a substrate temperature of only 0.43T
. By carefully controlling the deposition rate, we can improve the stability of the obtained glasses to higher values. In contrast to conventional quenched glasses, the ultrastable MGs exhibit a large increase of T
of ∼60 K, stronger resistance against crystallization, and more homogeneous structure with less order at longer distances. Our study circumvents the limitation of substrate temperature for developing ultrastable glasses, and provides deeper insight into glasses stability and their surface dynamics.
X-ray photon correlation is used to probe the slow dynamics of the glass-former B2O3 across the glass transition. In the undercooled liquid phase, the decay times of the measured correlation ...functions are consistent with visible light-scattering results and independent of the incoming flux; in the glass they are instead temperature independent and show a definite dependence on the x-ray flux since here the dynamics is artificially induced by the interaction with the x-ray beam. The decay times in this regime provide a measure of the number of atoms that rearrange on a length scale on the order of the inverse of the exchanged wave vector following an absorption event. Quite surprisingly, this number for the B2O3 glass is on the order of thousands of atoms. The induced dynamics persists also at higher temperatures until, close to the glass transition, it gets much slower than the intrinsic relaxation of the material. We suggest a possible scenario in which the single absorbed photon induces a collective motion of a volume of the glass with a size of the order of the nanometer.
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Still very little is known on the relaxation dynamics of glasses at the microscopic level due to the lack of experiments and theories. It is commonly believed that glasses are in a dynamical arrested ...state, with relaxation times too large to be observed on human time scales. Here we provide the experimental evidence that glasses display fast atomic rearrangements within a few minutes, even in the deep glassy state. Following the evolution of the structural relaxation in a sodium silicate glass, we find that this fast dynamics is accompanied by the absence of any detectable aging, suggesting a decoupling of the relaxation time and the viscosity in the glass. The relaxation time is strongly affected by the network structure with a marked increase at the mesoscopic scale associated with the ion-conducting pathways. Our results modify the conception of the glassy state and asks for a new microscopic theory.
The propagation and damping of the acoustic excitations in vitreous silica is measured at terahertz frequencies using inelastic x-ray scattering. The apparent sound velocity shows a marked dispersion ...with frequency while the sound attenuation undergoes a crossover from a fourth to a second power law frequency dependence. This finding solves a recent controversy concerning the location of this crossover in vitreous silica, clarifying that it occurs at the position of the glass-characteristic excess of vibrational modes known as boson peak, and thus establishing a direct connection between boson peak and acoustic dispersion curves.
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Nowadays powerful X-ray sources like synchrotrons and free-electron lasers are considered as ultimate tools for probing microscopic properties in materials. However, the correct interpretation of ...such experiments requires a good understanding on how the beam affects the properties of the sample, knowledge that is currently lacking for intense X-rays. Here we use X-ray photon correlation spectroscopy to probe static and dynamic properties of oxide and metallic glasses. We find that although the structure does not depend on the flux, strong fluxes do induce a non-trivial microscopic motion in oxide glasses, whereas no such dependence is found for metallic glasses. These results show that high fluxes can alter dynamical properties in hard materials, an effect that needs to be considered in the analysis of X-ray data but which also gives novel possibilities to study materials properties since the beam can not only be used to probe the dynamics but also to pump it.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
We report the observation, by means of high-resolution inelastic x-ray scattering, of an unusually large temperature dependence of the sound attenuation of a network glass at terahertz frequency, an ...unprecedentedly observed phenomenon. The anharmonicity can be ascribed to the interaction between the propagating acoustic wave and the bath of thermal vibrations. At low temperatures the sound attenuation follows a Rayleigh-Gans scattering law. As the temperature is increased the anharmonic process sets in, resulting in an almost quadratic frequency dependence of the damping in the entire frequency range. We show that the temperature variation of the sound damping accounts quantitatively for the temperature dependence of the density of vibrational states.
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