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.PACS numbers: 64.70.pe,65.60.+a,64.70.pv Glasses are usually defined as liquids that are trapped in a metastable state from which they slowly evolve toward the corresponding equilibrium phase [1,2]. Although aging is known since centuries, a clear picture of the dynamics in the glassy state is still missing [3,4]. Most of the experimental information available on aging concerns macroscopic quantities, such as viscosity or elastic moduli [5][6][7][8][9][10][11], or focuses on dielectric relaxation [12][13][14][15][16][17], a quantity that is often difficult to relate directly to the particle-level dynamics. From these measurements, a characteristic time for the evolution towards equilibrium can be extracted, but no direct information on the connection between aging and the underlying microscopic dynamics is available. By contrast, a full understanding of aging requires a detailed description of the particle-level dynamics, and in particular of the structural relaxation time τ , the characteristic time for a system to rearrange its structure on the length scale of its constituents. While the structural relaxation has been widely investigated in the liquid phase, only a few studies report on the behaviour of τ below the glass transition temperature, T g [14-16, 18]. Molecular dynamics simulations show that τ increases linearly or sub-linearly with the waiting time, t w , and suggest the possibility of rescaling the measured quantities on a single master curve (time-waiting time superposition principle) [18]. Studies in this direction, however, have led to debated results [5,12,13,17,19]. Consequently, several key questions remain unanswered: what is the fate of the structural relaxation process when the system falls out of equilibrium in the glassy state? How does it depend on the thermal history and the waiting time? What physical mechanism is responsible for structural relaxation? Here, we address these questions by presenting an experimental investigation of the structural relaxation on the atomic length scale in a metallic glass former, in both the supercooled liquid and the glassy state, as a function of temperature and waiting time, and for different thermal histories. We use X-Ray Photon Correlation Spectroscopy (XPCS) to study Mg 65 Cu 25 Y 10 , a well-known glass former with a relatively low T g ∼ 405 K and a st...

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.

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.Physical aging is not only of scientific interest [1,2] but also of great practical importance, as the performance stability of many technologically relevant amorphous materials such as oxide glasses, polymers and metallic glasses (MGs) depends on how aging affects their properties during the expected service life. For MGs in particular, aging can result in severe embrittlement [3] and bring about profound changes to fracture and fatigue properties [4]. Therefore, the ability to understand and characterize the micromechanisms of aging in MGs will play a central role in ensuring their success in a broad range of industrial and commercial applications.A key to understanding aging on the microscopic level is measuring the time scale on which the system rearranges its internal structure. These structural rearrangements are directly related to how fast microscopic density fluctuations in the system decay [5]. The physical quantity describing this is the density correlation function, which can be measured over a wide range of time scales using a variety of complimentary experimental techniques [6]. Additionally, it can be formally defined within the framework of statistical mechanics, allowing for detailed comparisons between experiment, theory [7] and simulation [8]. Access to the density correlation function is therefore fundamental for describing the dynamical behavior of glasses and achieving a consistent picture of structural relaxation on microscopic length scales.X-ray photon correlation spectroscopy (XPCS) has recently emerged as a novel technique for studying the microscopic dynamics of condensed matter [6,[9][10][11]. In XPCS, coherent beams of X-rays are scattered from a sample and the resulting fluctuations in intensity correlated over time. This enables measurement of the density correlation function on the atomic scale. of a unique stress-dominated dynamics characterized by subsequent aging regimes: a fast exponential growth for short waiting times, followed by a long, almost stationary regime. The latter has been observed also in network glasses [13] and is contrary to the steady slowing down of the dynamics during aging observed in macroscopic studies.Here, we investigate the atomic motion in a Pd 43 Cu 27 Ni 10 P 20 MG ribbon using XPCS and show that this system ages in a highly heterogeneous manner, consisting of quiescent periods of stationary dynamics interspersed with cooperative, avalanch...

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