Mechanical deformation of amorphous solids can be described as consisting of an "elastic" in which the stress increases linearly with strain, up to a yield point at which the solid either fractures or starts deforming plastically. It is well established, however, that the apparent linearity of stress with strain is actually a proxy for a much more complex behavior, with a microscopic plasticity that is reflected in diverging nonlinear elastic coefficients [1,2]. Very generally, the complex structure of the energy landscape is expected to induce a singular response to small perturbations. In the athermal quasistatic regime, this response manifests itself in the form of a scale free plastic activity.The distribution of the corresponding avalanches should reflect, according to theoretical mean field calculations [3], the geometry of phase space in the vicinity of a typical local minimum. In this work, we characterize this distribution for simple models of glass forming systems, and we find that its scaling is compatible with the mean field predictions for systems above the jamming transition.These systems exhibit marginal stability, and scaling relations that hold in the stationary state are examined and confirmed in the elastic regime. By studying the respective influence of system size and age, we suggest that marginal stability is systematic in the thermodynamic limit.

Amorphous materials have a rich relaxation spectrum, which is usually described in terms of a hierarchy of relaxation mechanisms. In this work, we investigate the local dynamic modulus spectra in a model glass just above the glass transition temperature by performing a mechanical spectroscopy analysis with molecular dynamics simulations. We find that the spectra, at the local as well as on the global scale, can be well described by the Cole-Davidson formula in the frequency range explored with simulations. Surprisingly, the Cole-Davidson stretching exponent does not change with the size of the local region that is probed. The local relaxation time displays a broad distribution, as expected based on dynamic heterogeneity concepts, but the stretching is obtained independently of this distribution. We find that the size dependence of the local relaxation time and moduli can be well explained by the elastic shoving model. arXiv:1812.04527v2 [cond-mat.dis-nn] 12 Feb 2019 Nonexponential or stretched exponential relaxation is ubiquitous in amorphous materials, and is recognized as one of the key features in supercooled liquid and glassy states [1,2].It appears in many relaxation processes at equilibrium or out of equilibrium, such as aging, stress relaxation and dielectric or mechanical relaxation spectra [3,4]. However, the origin of the stretching is still controversial [5]. Two hypotheses are typically put forward to explain the stretching: one identifies the stretched relaxation as resulting from dynamic heterogeneity in different regions of space, the other assumes that the relaxation in amorphous material is uniform, with stretched relaxation being a local feature [6,7].These different views can to some extent be reconciled within the now widely accepted concept of dynamical heterogeneity, which has been confirmed both in experiment and molecular simulation [8]. The supercooled liquid, for example, can be separated into fast regions of high mobility and slow regions with lower mobility, with a "slow" or "fast" character that persists over times comparable to the total α relaxation time. Mathematically, stretched exponential relaxation can be described as a superposition of simple exponential relaxation processes [9]. It is then a natural hypothesis to assume that the slow and fast regions associated with dynamical heterogeneity each have a simple exponential relaxation, and that the global stretching results from the different relaxation times associated with different regions, which may be broadly distributed. In fact, this natural assumption was recently formalized in a series of works by Masurel et al. [10][11][12], who developed a mesoscale model to describe the viscoelastic spectrum in a polymer model near the glass transition temperature. In their model, every local region is described as a single Maxwell Voigt element, with a single relaxation time assigned randomly from a broad (log normal) distribution. Based on the idea that dynamic and elastic heterogeneity are related, Schirmacher[13] also uses a local...

Glasses have markedly different stability around their glass transition temperature ( T g ), and metallic glasses (MGs) are conventionally regarded as metastable compared to other glasses such as silicate glass or amber. Here, we show an aging experiment on a Ce-based MG around its T g (~0.85 T g ) for more than 17 years. We find that the MG with strong fragility could transform into kinetic and thermodynamic hyperstable state after the long-term room temperature aging and exhibits strong resistance against crystallization. The achieved hyperstable state is closer to the ideal glass state compared with that of other MGs and similar to that of the million-year-aged amber, which is attributed to its strong fragility and strong resistance against nucleation. It is also observed through the asymmetrical approaching experiment that the hyperaged Ce-based MG can reach equilibrium liquid state below T g without crystallization, which supports the idea that nucleation only occurs after the completion of enthalpy relaxation.

We developed a facile ultrasonic vibration route is developed to synthesize BMGs and metallic glass-glass composites.

A ternary metallic glass-forming liquid is found to be not strongly correlating thermodynamically, but its average dynamics, dynamic heterogeneities including the high order dynamic correlation length, and static structure are still well described by thermodynamic scaling with the same scaling exponent γ. This may indicate that the metallic liquid could be treated as a single-parameter liquid. As an intrinsic material constant stemming from the fundamental interatomic interactions, γ is theoretically predicted from the thermodynamic fluctuations of the potential energy and the virial. Although γ is conventionally understood merely from the repulsive part of the inter-particle potentials, the strong correlation between γ and the Grüneisen parameter up to the accuracy of the Dulong-Petit approximation demonstrates the important roles of anharmonicity and attractive force of the interatomic potential in governing glass transition of metallic glassformers. These findings may shed light on how to understand metallic glass formation from the fundamental interatomic interactions.

Atomic rearrangements induced by shear stress are fundamental for understanding deformation mechanisms in metallic glasses (MGs). Using molecular dynamic simulation, the atomic rearrangements characterized by nonaffine displacements (NADs) and their spatial distribution and evolution with tensile stress in Cu 50 Zr 50 MG were investigated. It was found that in the elastic regime the atomic rearrangements with the largest NADs are relatively homogeneous in space, but exhibit strong spatial correlation, become localized and inhomogeneous, and form large clusters as strain increases, which may facilitate the so-called shear transformation zones. Furthermore, initially they prefer to take place around Cu atoms which have more nonicosahedral configurations. As strain increases, the preference decays and disappears in the plastic regime. The atomic rearrangements with the smallest NADs are preferentially located around Cu atoms, too, but with more icosahedral or icosahedral-like atomic configurations. The preference is maintained in the whole deformation process. In contrast, the atomic rearrangements with moderate NADs distribute homogeneously, and do not show explicit preference or spatial correlation, acting as matrix during deformation. Among the atomic rearrangements with different NADs, those with largest and smallest NADs are nearest neighbors initially, but separating with increasing strain, while those with largest and moderate NADs always avoid to each other. The correlations in the fluctuations of the NADs confirm the long-range strain correlation and the scale-free characteristic of NADs in both elastic and plastic deformation, which suggests a universality of the scaling in the plastic flow in MGs.

Being a key feature of a glassy state, low temperature relaxation has important implications on the mechanical behavior of glasses; however, the mechanism of low temperature relaxation is still an open issue, which has been debated for decades. By systematically investigating the influences of cooling rate and pressure on low temperature relaxation in the ZrCu metallic glasses, it is found that even though pressure does induce pronounced local structural change, the low temperature-relaxation behavior of the metallic glass is affected mainly by cooling rate, not by pressure. According to the atomic displacement and connection mode analysis, we further demonstrate that the low temperature relaxation is dominated by the dispersion degree of fast dynamic atoms rather than the most probable atomic nonaffine displacement. Our finding provides the direct atomic-level evidence that the intrinsic heterogeneity is the key factor that determines the low temperature-relaxation behavior of the metallic glasses.

Cryogenic rejuvenation in metallic glasses reported in Ketov et al ʼs experiment (2015 Nature 524 200)has attracted much attention, both in experiments and numerical studies. The atomic mechanism of rejuvenation has been conjectured to be related to the heterogeneity of the glassy state, but the quantitative evidence is still elusive. Here we use molecular dynamics simulations of a model metallic glass to investigate the heterogeneity in the local thermal expansion. We then combine the resulting spatial distribution of thermal expansion with a continuum mechanics calculation to infer the internal stresses caused by a thermal cycle. Comparing the internal stress with the local yield stress, we prove that the heterogeneity in thermo mechanical response has the potential to trigger local shear transformations, and therefore to induce rejuvenation during a cryogenic thermal cycling.