About the plastic response of silicate glasses at the micronscale

Despite their brittleness, silicate glasses undergo plastic deformation at the micron scale. Mechanical contact and indentation are the most common situations of interest. The plasticity of glasses is characterized not only by shear flow but also by a permanent densification process. We present novel observations of the deformation and fracture of amorphous silica micropillars of various sizes using In Situ SEM Micro-Compression (Fig 1), that can help better understand the mechanisms occurring prior to its fracture [1]. Exhibiting one of the highest ratios of shear stress on shear modulus, fused silica thus further distinguishes itself from other amorphous materials. Moreover, nanocompression allows successful observations of crack initiation and growth. In parallel to this experimental investigation, atomistic simulations [2] aiming to investigate the theoretical plastic response of silicate glasses under coupled shear-pressure stress state was run. The results were interpreted in terms of volumetric and shear hardening. A buckling-like behaviour is clearly evidenced at low density (large free-volume) whereas a BMG-like is observed for samples densified until saturation. Thanks to this rich set of data, it seems now possible to define a constitutive model taking into account both nanomechanical results, i.e. nanopillars, nanoindentation, diamond anvil cell, and molecular dynamics simulation Despite their brittleness, silicate glasses undergo plastic deformation at the micron scale. Mechanical contact and indentation are the most common situations of interest. The plasticity of glasses is characterized not only by shear flow but also by a permanent densification process. We present novel observations of the deformation and fracture of amorphous silica micropillars of various sizes using In Situ SEM Micro-Compression (Fig 1), that can help better understand the mechanisms occurring prior to its fracture [1]. Exhibiting one of the highest ratios of shear stress on shear modulus, fused silica thus further distinguishes itself from other amorphous materials. Moreover, nanocompression allows successful observations of crack initiation and growth. In parallel to this experimental investigation, atomistic simulations [2] aiming to investigate the theoretical plastic response of silicate glasses under coupled shear-pressure stress state was run. The results were interpreted in terms of volumetric and shear hardening. A buckling-like behaviour is clearly evidenced at low density (large free-volume) whereas a BMG-like is observed for samples densified until saturation. Thanks to this rich set of data, it seems now possible to define a constitutive model taking into account both nanomechanical results, i.e. nanopillars, nanoindentation, diamond anvil cell, and molecular dynamics simulation

Vibrational modes as a predictor for plasticity in a model glass

The density of vibrational states in amorphous materials is known to present an unusual shape related as “boson peak”, and responsible for the very specific thermal behaviour of these systems. In this letter, we show how the vibrational modes of a model Lennard-Jones glass are affected by a mechanical load. Far from a mechanical instability, vibrational modes can be described at low frequency by weak scattering of acoustic modes. Close to a plastic instability, some of them localize. We show how the shape of the “localized” vibrational modes, juste before the plastic instability, is directly related to the spatial organization of the plastic rearrangements. A measurement of the spatial organization of the low-frequency vibrational modes could thus be used as a predictor for plastic activity

Impact of pressure on plastic yield in amorphous solids with open structure

International audiencePlasticity in amorphous silica is unusual: the yield stress decreases with hydrostatic pressure, in contrast to the Mohr-Coulomb response commonly found in more compact materials like Bulk Metallic Glasses. We show that molecular dynamics simulations of a model glass with open structure can reproduce this anomalous dependence of yield stress with pressure. We also nd that once the material is fully compacted the plastic response becomes normal. The overall shape of the yield surface is consistent with the quadratic behavior predicted assuming local buckling of the structure, a point of view which ts very well into the presents understanding of the deformation mechanisms of amorphous silica. This result points to the role of free volume as an internal variable for the continuum scale description of the plastic response of amorphous silica. In addition, we also investigated the long range correlation between rearrangement processes. We nd that strong intermittency is observed when the structure remains open while compaction results in more homogeneous rearrangements. These ndings are in agreement with recent results on the eect of compression on middle range order in silicate glasses, and also suggest that the well-known volume recovery of densied silica at relatively low temperatures is in fact a form of aging

Atomistic response of a model silica glass under shear and pressure

International audienceThe Mechanical Response of a Model Silica Glass is studied extensively at the submicrometer scale, with the help of atomistic simulations. The analysis of the response to a hydrostatic com- pression is compared to recent experimental results. The irreversible behaviour and the variation of intertetrahedral angles is recovered. It is shown that the atomistic response is homogeneous upon compression, in opposition with the localization along shear bands occuring during shear deformation with constant volume. Moreover, the Bulk Modulus anomaly is interpreted as due to a succession of such homogeneous but irreversible atomic rearrangements

Study of the effects of grain size on the mechanical properties of nanocrystalline copper using molecular dynamics simulation with initial realistic samples

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