Under external mechanical loading, glassy materials, ranging from soft matter systems to metallic alloys, often respond via formation of inhomogeneous flow patterns, during yielding. These inhomogeneities can be precursors to catastrophic failure, implying that a better understanding of their underlying mechanisms could lead to the design of smarter materials. Here, extensive molecular dynamics simulations are used to reveal the emergence of heterogeneous dynamics in a binary Lennard-Jones glass, subjected to a constant strain rate. At a critical strain, this system exhibits for all considered strain rates a transition towards the formation of a percolating cluster of mobile regions. We give evidence that this transition belongs to the universality class of directed percolation. Only at low shear rates, the percolating cluster subsequently evolves into a transient (but long-lived) shear band with a diffusive growth of its width. Finally, the steady state with a homogeneous flow pattern is reached. In the steady state, percolation transitions also do occur constantly, albeit over smaller strain intervals, to maintain the stationary plastic flow in the system.

Molecular dynamics computer simulations of a binary Lennard-Jones glass under shear are presented. The mechanical response of glassy states having different thermal histories is investigated by imposing a wide range of external shear rates, at different temperatures. The stress-strain relations exhibit an overshoot at a strain of around 0.1, marking the yielding of the glass sample and the onset of plastic flow. The amplitude of the overshoot shows a logarithmic behavior with respect to a dimensionless variable, given by the age of the sample times the shear rate. Dynamical heterogeneities having finite lifetimes, in the form of shear bands, are observed as the glass deforms under shear. By quantifying the spatial fluctuations of particle mobility, we demonstrate that such shearbanding occurs only under specific combinations of imposed shear-rate, age of glass and ambient temperature. 1 arXiv:1612.01405v1 [cond-mat.soft]

In complex crystals close to melting or at finite temperatures, different types of defects are ubiquitous and their role becomes relevant in the mechanical response of these solids. Conventional elasticity theory fails to provide a microscopic basis to include and account for the motion of point defects in an otherwise ordered crystalline structure. We study the elastic properties of a point-defect rich crystal within a first principles theoretical framework derived from the microscopic equations of motion. This framework allows us to make specific predictions pertaining to the mechanical properties that we can validate through deformation experiments performed in molecular dynamics simulations.

We study the ground-state (T = 0) morphologies in the d = 3 random-field Ising model (RFIM) using a computationally efficient graph-cut method. We focus on paramagnetic states which arise for disorder strengths ∆ > ∆ c , where ∆ c is the critical disorder strength at T = 0. These paramagnetic states consist of correlated "domains" of up and down spins which are separated by rough, fractal interfaces. They show novel scattering properties with a cusp singularity in the correlation function at short distances. PACS numbers: 64.60.De -Statistical mechanics of model systems: Ising model, Monte Carlo techniques, etc.; 68.35.Rh -Phase transitions and critical phenomena; 75.60.Ch -Domain walls and domain structure

Relaxation of shear bands in a Pd 40 Ni 40 P 20 bulk metallic glass was investigated by a combination of radiotracer diffusion and molecular dynamics (MD) simulations, allowing to determine for the first time the effective activation enthalpy of diffusion along shear bands in a deformed glass. The shear bands relax during annealing below the glass transition temperature and the diffusion enhancement reveals unexpectedly a nonmonotonous behavior. The development of shear bands and the subsequent relaxation of stresses after switching off the shearing are characterized on microscopic to mesoscopic length scales by MD simulation subjecting the model glass to a constant strain rate. Mean-squared displacements as well as strain maps indicate that the heterogeneity, as manifested by shear bands in the systems under shear, persist after the shear is switched off. We observe a further relaxation of residual stresses that remain localized in regions where the shear band has been present before, although the system is -different from the macroscopic experiment -homogeneous with respect to the local density. These results indicate that even on a local scale one may expect strong dynamic heterogeneity in deformed glassy solids due to shear banding. The results thus suggest that plastically deformed metallic glasses present poly-amorphous systems that necessitate descriptions that are analogous to multiphase materials including the presence of heterophase interfaces.Although non-homogenous plastic deformation of bulk metallic glasses (BMGs) via the formation of shear bands attracted increased attention in the past [1,2], it is still far from being resolved, see e.g. the reviews in [3,4]. As a generally accepted concept, so-called "shear transformation zones" (STZ), i.e. areas in which groups of atoms collectively undergo a local shear transformation, have been introduced [5] as "unit carriers" of plastic deformation in metallic glasses. A cross-over from random 3-dimensional shear events (STZ formation) to correlated 2-dimensional dynamics has been brought forward to explain the observed shear banding in metallic glasses [6]. Although the activation of a single STZ event is not inevitably related to a change of the excess volume, shear bands are often described in terms of excess volume accumulation [3,7]. Recently, we investigated diffusion in deformed Pd 40 Ni 40 P 20 (at. %; in what follows we will use the abbreviation PdNiP) glass, in which almost a single family of shear bands was introduced, and were able to unambiguously prescribe the observed enormous enhancement of the diffusion rate to an ultra-fast atomic transport along these quasi-2-dimensional pathways [8]. The tracer concentrations in the corresponding concentration profiles, which were related to shear band diffusion, were shown to scale with the number density of the introduced shear bands (that in turn scales with the imposed strain),