Elementary plastic events in amorphous silica

Abstract

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 quasi-static shear. We identify two distinct types of elementary plastic events, one is a standard quasi-localized 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 non-zero eigenvalue of the Hessian matrix that vanishes at a critical strain. Remarkably, we find very high correlation between the associated eigenvectors and the non-affine displacement fields accompanying the bond breaking event, predicting the locus of structural failure. Both eigenvectors and non-affine displacement fields display an Eshelby-like quadrupolar structure for both failure modes, rearrangement or 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.Amorphous solids under applied shear deformation undergo localized plastic instabilities associated with the rearrangement of a subset of particles and an associated energy release. These particle reorganization induces structural deformation patterns, that have been identified experimentally and numerically in amorphous materials such as silica glasses [2-4], metallic glasses [5], colloidal glasses [6], foams [7], bubble rafts [5], and emulsions [8,9]. The initial destabilization can give rise to a progression of additional deformation events in some other areas of the sample, up to the global material failure. The ability to predict the plastic instabilities and characterize their spatial features is of fundamental importance to understand the mechanical response of amorphous solids and to devise mesoscale model focusing on the evolution of localized plastic events [10][11][12][13].A useful theoretical framework to analyze elementary plastic events is the limit of temperature T = 0 and of quasistatic strain where the real space structure can be easily related with a potential energy landscape description [14]. To this end, many computational studies on amorphous solids have been performed with athermal quasi static (AQS) protocol [15][16][17][18][19]: a glass sample initially quenched down to zero temperature is deformed by a quasi static shear procedure consisting in the relaxation of the system after each strain step. Within the AQS conditions the elastic and plastic features of amorphous solids can be understood by analyzing the Hessian matrixwhere U (r 1 , r 2 , · · · r N ) is the total potential energy of the system, f i is the force vector on particle i, and {r i } N i=1 are the coordinates of the particles. The explicit H element expression is reported in [1]. When the syst...

Discontinuous yielding of pristine micro-crystals

“…In the case of triangular lattices, the misalignement is much stronger and the finite size effect in the form of a gap between the predictions of discrete and continuum theories remains apparent for much smaller element sizes, see Figure 10(b). Note that the asymptotic behavior of the smallest eigenvalues near zero, shown in Figure 10, is different from the prediction of the theory of amorphous plasticity where the corresponding eigenvectors are quasilocalized [227][228][229]. The linear response of pristine crystalline solids with zero disorder is global and that is why its elastic instabilities can be largely captured by the classical continuum theory.…”

Predicting the failure of two-dimensional silica glasses

“…Each configuration is composed of N = 3456 atoms arranged in a square box of edge length L ∼ 85 Å with periodic boundary conditions along the x, y plane. In our molecular dynamics simulations, atomic interactions are described by the Watanabe interatomic potential 31,32 described in the Materials and Methods section. Starting from the initial configuration, we progressively stretch the simulated sample along the x direction by small displacement steps and subsequent relaxation using the athermal quasistatic (AQS) protocol (see Materials and Methods for details).…”

Yielding transition of a two dimensional glass former under athermal cyclic shear deformation

“…In recent years, an increasing number of studies have explored the fundamental aspects of the yielding transition in amorphous solids through experiments [12][13][14][15][16], numerical simulations [17][18][19][20][21][22][23][24][25][26][27] and theoretical models [28][29][30][31][32]. Differently from crystals [33], amorphous solid plasticity is not driven by specific defects and thus the correlation of plastic rearrangements and specific structural motifs has been a subject of several investigations, as well as the characterisation of structural changes that arise from plastic events [25,[34][35][36][37][38]. * sastry@jncasr.ac.in…”