320 research outputs found
Thermal-activation model for freezing and the elastic robustness of bulk metallic glasses
Despite significant atomic-scale heterogeneity, bulk metallic glasses well below their glass transition temperature exhibit a surprisingly robust elastic regime and a sharp elastic-to-plastic transition. Here it is shown that, when the number of available structural transformations scales exponentially with system size, a simple thermal-activation model is able to describe these features, where yield corresponds to a change from a barrier energy dominated to a barrier entropy dominated regime of shear transformation activity, allowing the system to macroscopically exit its frozen state. A yield criterion is then developed, which describes well the existing experimental data and motivates future dedicated deformation experiments to validate the model
Shear-band arrest and stress overshoots during inhomogeneous flow in a metallic glass
At the transition from a static to a dynamic deformation regime of a shear band in bulk metallic glasses, stress transients in terms of overshoots are observed. We interpret this phenomenon with a repeated shear-melting transition and are able to access a characteristic time for a liquidlike to solidlike transition in the shear band as a function of temperature, enabling us to understand why shear bands arrest during inhomogenous serrated flow in bulk metallic glasses
The Boson peak of model glass systems and its relation to atomic structure
Abstract.: Bulk metallic glasses (BMGs) exhibit a rich variety of vibrational properties resulting from significant atomic scale disorder. The Boson peak, which reflects an enhancement of states in the low frequency regime of the vibrational density of states (VDOS), is one such experimental signature of amorphous materials that has gained much interest in recent times. However, the precise nature of these low frequency modes and how they are influenced by local atomic structure remains unclear. Past simulation work has demonstrated that such modes consist of a mixture of propagating and localized components, and have been referred to as quasi-localized modes. Using standard harmonic analysis, the present work investigates the structural origin of such modes by diagonalising the Hessian of atomistic BMG structures derived from molecular dynamics simulations using a binary Lennard Jones pair potential. It is found that the quasi-localized vibrational modes responsible for the low frequency enhancement of the VDOS exist in a structural environment characterized primarily by low elastic shear moduli, but also increased free volume, a hydrostatic pressure that is tensile, and low bulk moduli. These findings are found to arise from the long-range attractive nature of the pair-wise interaction potential, which manifests itself in the corresponding Hessian as long-range off-diagonal disorder characterized by a distribution of negative effective spring constant
How Soil Invertebrates Deal With Microplastic Contamination
Small animals living in soils, called soil invertebrates, represent a very diverse group of soil inhabitants. They include earthworms, woodlice, spiders, springtails, mites, and some insects. Soil invertebrates feed on dead plants, on fungi and bacteria, or on other soil invertebrates. The many ways soil invertebrates interact with each other, and the large number of different species, make life in soils complex and difficult to understand. Unfortunately, soil invertebrates have been dealing with soil pollution, including contamination with tiny particles of plastic called microplastics for decades now. But are microplastics harmful to these organisms? Can microplastics be passed between soil invertebrates when one feeds on another? Most questions about microplastics and soil invertebrates have been investigated using earthworms, but a few studies on others, like springtails, mites, and nematodes, also exist. In this article, we summarize the effects of microplastics on soil invertebrates
Slip statistics of dislocation avalanches under different loading modes
Slowly compressed microcrystals deform via intermittent slip events, observed as displacement jumps or stress drops. Experiments often use one of two loading modes: an increasing applied stress (stress driven, soft), or a constant strain rate (strain driven, hard). In this work we experimentally test the influence of the deformation loading conditions on the scaling behavior of slip events. It is found that these common deformation modes strongly affect time series properties, but not the scaling behavior of the slip statistics when analyzed with a mean-field model. With increasing plastic strain, the slip events are found to be smaller and more frequent when strain driven, and the slip-size distributions obtained for both drives collapse onto the same scaling function with the same exponents. The experimental results agree with the predictions of the used mean-field model, linking the slip behavior under different loading modes
BKM Lie superalgebras from counting twisted CHL dyons
Following Sen[arXiv:0911.1563], we study the counting of (`twisted') BPS
states that contribute to twisted helicity trace indices in four-dimensional
CHL models with N=4 supersymmetry. The generating functions of half-BPS states,
twisted as well as untwisted, are given in terms of multiplicative eta products
with the Mathieu group, M_{24}, playing an important role. These multiplicative
eta products enable us to construct Siegel modular forms that count twisted
quarter-BPS states. The square-roots of these Siegel modular forms turn out be
precisely a special class of Siegel modular forms, the dd-modular forms, that
have been classified by Clery and Gritsenko[arXiv:0812.3962]. We show that each
one of these dd-modular forms arise as the Weyl-Kac-Borcherds denominator
formula of a rank-three Borcherds-Kac-Moody Lie superalgebra. The walls of the
Weyl chamber are in one-to-one correspondence with the walls of marginal
stability in the corresponding CHL model for twisted dyons as well as untwisted
ones. This leads to a periodic table of BKM Lie superalgebras with properties
that are consistent with physical expectations.Comment: LaTeX, 32 pages; (v2) matches published versio
Micro-plasticity and intermittent dislocation activity in a simplified micro structural model
Here we present a model to study the micro-plastic regime of a stress-strain
curve. In this model an explicit dislocation population represents the mobile
dislocation content and an internal shear-stress field represents a mean-field
description of the immobile dislocation content. The mobile dislocations are
constrained to a simple dipolar mat geometry and modelled via a dislocation
dynamics algorithm, whilst the shear-stress field is chosen to be a sinusoidal
function of distance along the mat direction. The latter, defined by a periodic
length and a shear-stress amplitude, represents a pre-existing micro-structure.
These model parameters, along with the mobile dislocation density, are found to
admit a diversity of micro-plastic behaviour involving intermittent plasticity
in the form of a scale-free avalanche phenomenon, with an exponent for the
strain burst magnitude distribution similar to those seen in experiment and
more complex dislocation dynamics simulations.Comment: 30 pages, 12 figures, to appear in "Modelling and Simulation in
Materials Science and Engineering
Independence of Slip Velocities on Applied Stress in Small Crystals
Directly tracing the spatiotemporal dynamics of intermittent plasticity at the micro- and nanoscale reveals that the obtained slip dynamics are independent of applied stress over a range of up to ∼400 MPa, as well as being independent of plastic strain. Whilst this insensitivity to applied stress is unexpected for dislocation plasticity, the stress integrated statistical properties of both the slip size magnitude and the slip velocity follow known theoretical predictions for dislocation plasticity. Based on these findings, a link between the crystallographic slip velocities and an underlying dislocation avalanche velocity is proposed. Supporting dislocation dynamics simulations exhibit a similar regime during microplastic flow, where the mean dislocation velocity is insensitive to the applied stress. Combining both experimental and modeling observations, the results are discussed in a framework that firmly places the plasticity of nano- and micropillars in the microplastic regime of bulk crystals
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