The brittleness of oxide glasses
has dramatically restricted their
practical applications as structural materials despite very high theoretical
strength. Herein, using molecular dynamics simulations, we show that
silica glass prepared by consolidating glassy nanoparticles exhibit
remarkable tensile ductility. Because of dangling bonds at surfaces
and high contact stresses, the pressure applied for consolidating
glassy nanoparticles to achieve ductility is significantly lower than
that required to toughen bulk glass via permanent densification. We
have identified 5-fold silicon, with a higher propensity to carry
out local shear deformation than 4-fold silicon, as the structural
origin for the observed tensile ductility. Interestingly, the work
hardening effect has been, for the first time, observed in thus-prepared
silica glass, with its strength increasing from 4 GPa to ∼7
GPa upon cold work. This is due to stress-assisted relaxation of 5-fold
silicon to 4-fold during cold work, analogous to transformation hardening.
Mechanical properties of glassy nanowires have been intensively investigated recently by both nanomechanical experiments and atomic-level simulations. Unfortunately, there exists a huge gap in the strain rate of the nanomechanical tests between experiments and simulations, which makes it difficult to compare results even for the same material system. Using accelerated atomistic simulations based on a self-learning metabasin escape algorithm, here, we report the tensile mechanical properties of amorphous Stillinger–Weber silicon nanowires with different intrinsic ductility under strain rates ranging from 1010 to 10−1 s−1. It is found that both brittle and ductile glassy silicon nanowires display weakened strength with a decreasing strain rate, in agreement with the cooperative shear model. Moreover, as the strain rate decreases, the amount of plasticity remains unchanged for the brittle nanowires, yet it decreases for the ductile ones. Such deteriorated plasticity in ductile glassy nanowires is caused by enhanced strain localization at low strain rates. Lastly, we show that via the distance matrix of nonaffine displacement, a more hierarchical potential energy landscape is responsible for the higher strain localization propensity in ductile silicon glassy nanowires.
Temperature-dependent viscosity is critical to decipher two profound questions in condensed matter physics, namely the glass transition and the relaxation of amorphous solids. However, direct measurement of viscosity over a large temperature range is extremely difficult. Here, using classical molecular dynamics (MD) simulations, we report a novel method to calculate the equilibrium viscosity of supercooled liquid both above and below the glass transition temperature (T g ) and to estimate the nonequilibrium viscosity of glass down to room temperature. Based on the shoving model, we derived an analytical formula showing that the shear viscosity in logarithmic scale changes linearly with the shear-induced variation in shear modulus or potential energy of the glass-forming system. The shear viscosity as a function of steady-state potential energy of liquid under different shear strain rates can be directly calculated in MD simulations; together with its equilibrium potential energy, one can extrapolate the zero-strain-rate equilibrium viscosity. We verified the proposed model by reliably calculating equilibrium viscosity near T g of four glass-forming systems (Kob-Andersen system, silica, Cu 45.5 Zr 45.5 Al 9 , and silicon) with different fragilities. Furthermore, our model can estimate the nonequilibrium viscosity of glass below T g ; the upperbound nonequilibrium viscosity of amorphous silica and silicon at room temperature are calculated to be ~10 32 and 10 25 Pa•s, respectively.
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