Elementary Plastic Events Research Articles

Origin of reversible and irreversible atomic-scale rearrangements in a model two-dimensional network glass.

In this contribution, we investigate the fundamental mechanism of plasticity in a model two-dimensional network glass. The glass is generated by using a Monte Carlo bond-switching algorithm and subjected to athermal simple shear deformation, followed by subsequent unloading at selected deformation states. This enables us to investigate the topological origin of reversible and irreversible atomic-scale rearrangements. It is shown that some events that are triggered during loading recover during unloading, while some do not. Thus, two kinds of elementary plastic events are observed, which can be linked to the network topology of the model glass.

Signature of local stress states in the deformation behavior of metallic glasses

The design of ductile heterogeneous metallic glasses (MGs) with enhanced deformability by purposely controlling the shear-band dynamics via modulation of the atomic-scale structures and local stress states remains a significant challenge. Here, we correlate the changes in the local atomic structure when cooling to cryogenic temperature with the observed improved shear stability. The enhanced atomic-level structural and elastic heterogeneities related to the nonaffine thermal contraction of the short-range order (SRO) and medium-range order (MRO) change the characteristics of the activation process of the shear transformation zones (STZs). The experimental observations corroborated by Eshelby inclusion analysis and molecular dynamics simulations disclose the correlation between the structural fluctuations and the change in the stress field around the STZ. The variations in the inclination axes of the STZs alter their percolation mechanism, affect the shear-band dynamics and kinetics, and consequently delay shear failure. These results expand the understanding of the correlation between the atomic-level structure and elementary plastic events in monolithic MGs and thereby pave the way for the design of new ductile metallic alloys.

Continuous Zachariasen carbon monolayers under tensile deformation: Insights from molecular dynamics simulations

From a series of molecular dynamics simulations, we show how the mechanical behaviour of amorphous carbon monolayers can be altered as a function of the underlying network topology. We control both the ring size as well as the ring neighbourhood distribution based on a Monte Carlo dual bond-switching algorithm. This leads to non-planar carbon monolayers whose out-of-plane displacements depend on the level of amorphousness. Subsequent tensile deformation reveals that increasing the network heterogeneity decreases both the tensile strength as well as the stiffness in the elastic range. Furthermore, we investigate the statistics of the elementary plastic events in the amorphous carbon monolayers and report on the power-law distribution of the stress-drops associated with the plastic deformation. The results of this study provide insights into the role of structural disorder of a carbon monolayer and can be used to modify its physical properties.

Elementary plastic events in amorphous silica.

Plastic instabilities in amorphous materials are often studied using idealized models of binary mixtures that do not capture accurately molecular interactions and bonding present in real glasses. Here we study atomic-scale plastic instabilities in a three-dimensional molecular dynamics model of silica glass under quasistatic shear. We identify two distinct types of elementary plastic events, one is a standard quasilocalized atomic rearrangement while the second is a bond-breaking event that is absent in simplified models of fragile glass formers. Our results show that both plastic events can be predicted by a drop of the lowest nonzero eigenvalue of the Hessian matrix that vanishes at a critical strain. Remarkably, we find very high correlation between the associated eigenvectors and the nonaffine displacement fields accompanying the bond-breaking event, predicting the locus of structural failure. Both eigenvectors and nonaffine displacement fields display an Eshelby-like quadrupolar structure for both failure modes, rearrangement, and bond breaking. Our results thus clarify the nature of atomic-scale plastic instabilities in silica glasses, providing useful information for the development of mesoscale models of amorphous plasticity.

Elementary plastic events in a Zachariasen glass under shear and pressure

In this paper, we investigate the response of a two-dimensional silica model glass under shear and pressure. The glass network is generated based on statistical data extracted from real two-dimensional silica. We present an amended dual bond-switching algorithm, which enables us to control both the level of heterogeneity as well as physically meaningful ring neighborhood statistics. The two-dimensionality of the model glass allows us to directly track all atoms during deformation and overcome the imaging problem of bulk silica glass. Firstly, we subject different sets of pressure-free samples to athermal quasistatic simple shear deformation. We observe a serrated flow for all levels of heterogeneity, where the stress drop statistics is described by a power law. Using the divergence theorem, it is shown that the outward flow of the local rearrangements associated with the stress drops is always higher than the inward flow, resulting in density fluctuations and nano-voids, the latter of which are detected using the shortest path criterion. However, the difference in the out-to-inward flow decreases with increasing heterogeneity. Secondly, we subject a set of one level of heterogeneity to various levels of compression, followed by athermal quasistatic shear deformation. After comparing the stress–strain results to those in bulk silica, we show that the statistics of the occurring stress drops of the densified material follow a power law. We identify an initial decrease of the out-to-inward flow with increasing pressure until a level of 10% densification followed by a slight increase up to a densification level of 15%.

Shearing small glass-forming systems: A potential energy landscape perspective

Understanding the yielding of glass-forming systems upon shearing is notoriously difficult since it is a strong non-equilibrium effect. Here we show that the concept of the potential energy landscape (PEL), developed for the quiescent state, can be extended to shearing. When introducing an appropriate coarse graining of the extended PEL for sheared systems, one can distinguish two fundamentally different types of plastic events, namely inherent structures (IS) vs. minimized structure (MS) transitions. We apply these concepts to non-cyclic and cyclic shearing of small systems, which allow us to characterize the properties of elementary plastic events. Whereas the general properties of the stress-strain curves are similar to larger systems, a closer analysis reveals significantly different properties. This allows one to identify the impact of the elastic coupling in larger systems. The concept of MS enables us to relate the stress overshoot of a single trajectory to the emergence of an MS transition. Furthermore, the occurrence of limit cycles can be characterized in great detail for the small systems and connections to the properties of the PEL can be formulated. Possible implications of our small-system results for the macroscopic limit are discussed.

Structural and topological nature of plasticity in sheared granular materials

Upon mechanical loading, granular materials yield and undergo plastic deformation. The nature of plastic deformation is essential for the development of the macroscopic constitutive models and the understanding of shear band formation. However, we still do not fully understand the microscopic nature of plastic deformation in disordered granular materials. Here we used synchrotron X-ray tomography technique to track the structural evolutions of three-dimensional granular materials under shear. We establish that highly distorted coplanar tetrahedra are the structural defects responsible for microscopic plasticity in disordered granular packings. The elementary plastic events occur through flip events which correspond to a neighbor switching process among these coplanar tetrahedra (or equivalently as the rotation motion of 4-ring disclinations). These events are discrete in space and possess specific orientations with the principal stress direction.

Time-dependent elastic response to a local shear transformation in amorphous solids.

The elastic response of a two-dimensional amorphous solid to induced local shear transformations, which mimic the elementary plastic events occurring in deformed glasses, is investigated via molecular-dynamics simulations. We show that for different spatial realizations of the transformation, despite relative fluctuations of order one, the long-time equilibrium response averages out to the prediction of the Eshelby inclusion problem for a continuum elastic medium. We characterize the effects of the underlying dynamics on the propagation of the elastic signal. A crossover from a propagative transmission in the case of weakly damped dynamics to a diffusive transmission for strong damping is evidenced. In the latter case, the full time-dependent elastic response is in agreement with the theoretical prediction, obtained by solving the diffusion equation for the displacement field in an elastic medium.

Yield-strain and shear-band direction in amorphous solids under two-dimensional uniaxial loading

It is well known experimentally that well-quenched amorphous solids exhibit a plastic instability in the form of a catastrophic shear localization at a well-defined value of the external strain. The instability may develop to a shear band that in some cases is followed by a fracture. It is also known that the values of the yield strain (and yield stress), as well as the direction of the shear band with respect to the principal stress axis, vary considerably with variations in the external loading conditions. In this paper we present a microscopic theory of these phenomena for two-dimensional athermal amorphous solids that are strained quasistatically. We present analytic formulas for the yield strains for different loading conditions, as well as for the angles of the shear bands. We explain that the external loading conditions determine the eigenvalues of the quadrupolar Eshelby inclusions which model the nonaffine displacement field. These inclusions model elementary plastic events and determine both the yield strain and the direction of the shear band. We show that the angles of the shear bands with respect to the principal stress axis are limited theoretically between 30° and 60°. Available experimental data conform to this prediction.

Robustness of avalanche dynamics in sheared amorphous solids as probed by transverse diffusion

Using numerical simulations, we perform an extensive finite-size analysis of the transverse diffusion coefficient in a sheared 2D amorphous solid over a broad range of strain rates at temperatures up to the supercooled liquid regime. We thus obtain direct qualitative evidence for the persistence of correlations between elementary plastic events up to the vicinity of the glass transition temperature T(g). A quantitative analysis of the data, combined with a previous study of the T and γ dependence of the macroscopic stress [Phys. Rev. Lett. 105, 266001 (2010)], leads us to conclude that the average avalanche size remains essentially unaffected by temperature up to T ~ 0.75T(g).

Intrinsic structure of acoustic emission events during jerky flow in an Al alloy

Scaling behavior is found in acoustic-emission events associated with stress drops observed in velocity-driven plastic deformation of an Al alloy, which exhibits jerky plastic flow. The occurrence of scaling proves that these acoustic-emission events, which are commonly regarded as ``elementary'' ones, have a small-scale self-organized structure comprising a group of peaks correlated in time. This structure reveals details of the temporal variation in elementary plastic events at a microsecond scale, which are hardly accessible by other measurement techniques.