182 research outputs found
Plastic deformation of a model glass induced by a local shear transformation
The effect of a local shear transformation on plastic deformation of a
three-dimensional amorphous solid is studied using molecular dynamics
simulations. We consider a spherical inclusion, which is gradually transformed
into an ellipsoid of the same volume and then converted back into the sphere.
It is shown that at sufficiently large strain amplitudes, the deformation of
the material involves localized plastic events that were identified based on
the relative displacement of atoms before and after the shear transformation.
We found that the density profiles of cage jumps decay away from the inclusion,
which correlates well with the radial dependence of the local deformation of
the material. At the same strain amplitude, the plastic deformation becomes
more pronounced in the cases of weakly damped dynamics or large time scales of
the shear transformation.Comment: 19 pages, 7 figure
The effect of a reversible shear transformation on plastic deformation of an amorphous solid
Molecular dynamics simulations are performed to investigate the plastic
response of a model glass to a local shear transformation in a quiescent
system. The deformation of the material is induced by a spherical inclusion
that is gradually strained into an ellipsoid of the same volume and then
reverted back into the sphere. We show that the number of cage-breaking events
increases with increasing strain amplitude of the shear transformation. The
results of numerical simulations indicate that the density of cage jumps is
larger in the cases of weak damping or slow shear transformation. Remarkably,
we also found that, for a given strain amplitude, the peak value of the density
profiles is a function of the ratio of the damping coefficient and the time
scale of the shear transformation.Comment: 19 pages, 7 figure
Dynamical heterogeneity in periodically deformed polymer glasses
The dynamics of structural relaxation in a model polymer glass subject to
spatially-homogeneous, time-periodic shear deformation is investigated using
molecular dynamics simulations. We study a coarse-grained bead-spring model of
short polymer chains below the glass transition temperature. It is found that
at small strain amplitudes, the segmental dynamics is nearly reversible over
about cycles, while at strain amplitudes above a few percent, polymer
chains become fully relaxed after a hundred cycles. At the critical strain
amplitude, the transition from slow to fast relaxation dynamics is associated
with the largest number of dynamically correlated monomers as indicated by the
peak value of the dynamical susceptibility. The analysis of individual monomer
trajectories showed that mobile monomers tend to assist their neighbors to
become mobile and aggregate into relatively compact transient clusters.Comment: 22 pages, 9 figure
Mechanical annealing of model glasses: Effects of strain amplitude and temperature
Molecular dynamics simulations are performed to examine the dynamic response
of amorphous solids to oscillatory shear at finite temperatures. The data were
collected from a poorly annealed binary glass, which was deformed periodically
in the elastic regime during several hundred shear cycles. We found that the
characteristic time required to reach a steady state with a minimum potential
energy is longer at higher temperatures and larger strain amplitudes. With
decreasing strain amplitude, the asymptotic value of the potential energy
increases but it remains lower than in quiescent samples. The transient decay
of the potential energy correlates well with a gradual decrease in the volume
occupied by atoms with large nonaffine displacements. By contrast, the maximum
amplitude of shear stress oscillations is attained relatively quickly when a
large part of the system starts to deform reversibly.Comment: 22 pages, 10 figure
Reversible plastic events during oscillatory deformation of amorphous solids
The effect of oscillatory shear strain on nonaffine rearrangements of
individual particles in a three-dimensional binary glass is investigated using
molecular dynamics simulations. The amorphous material is represented by the
Kob-Andersen mixture at the temperature well below the glass transition. We
find that during periodic shear deformation of the material, some particles
undergo reversible nonaffine displacements with amplitudes that are
approximately power-law distributed. Our simulations show that particles with
large amplitudes of nonaffine displacement exhibit a collective behavior;
namely, they tend to aggregate into relatively compact clusters that become
comparable with the system size near the yield strain. Along with reversible
displacements there exist a number of irreversible ones. With increasing strain
amplitude, the probability of irreversible displacements during one cycle
increases, which leads to permanent structural relaxation of the material.Comment: 16 pages, 6 figure
Atomistic modeling of heat treatment processes for tuning the mechanical properties of disordered solids
We investigate the effect of a single heat treatment cycle on the potential
energy states and mechanical properties of metallic glasses using molecular
dynamics simulations. We consider the three-dimensional binary mixture, which
was initially cooled with a computationally slow rate from the liquid state to
the solid phase at a temperature well below the glass transition. It was found
that a cycle of heating and cooling can relocate the glass to either
rejuvenated or relaxed states, depending on the maximum temperature and the
loading period. Thus, the lowest potential energy is attained after a cycle
with the maximum temperature slightly below the glass transition temperature
and the effective cooling rate slower than the initial annealing rate. In
contrast, the degree of rejuvenation increases when the maximum temperature
becomes greater than the glass transition temperature and the loading period is
sufficiently small. It was further shown that the variation of the potential
energy is inversely related to the dependence of the elastic modulus and the
yield stress as functions of the maximum loading temperature. In addition, the
heat treatment process causes subtle changes in the shape of the radial
distribution function of small atoms. These results are important for
optimization of thermal and mechanical processing of metallic glasses with
predetermined properties.Comment: 22 pages, 9 figure
The potential energy states and mechanical properties of thermally cycled binary glasses
The influence of repeated thermal cycling on mechanical properties,
structural relaxation, and evolution of the potential energy in binary glasses
is investigated using molecular dynamics simulations. We consider a binary
mixture with strongly non-additive cross interactions, which is annealed across
the glass transition with different cooling rates and then exposed to one
thousand thermal cycles at constant pressure. We found that during the first
few hundred transient cycles, the potential energy minima after eachcycle
gradually decrease and the structural relaxation proceeds via collective,
irreversible displacements of atoms. With increasing cycle number, the
amplitudes of the volume and potential energy oscillations are significantly
reduced, and the potential energy minima saturate to a constant value that
depends on the thermal amplitude and cooling rate. In the steady state, the
glasses thermally expand and contract but most of the atoms return to
theircages after each cycle, similar to limit cycles found in periodically
driven amorphous materials. The results of tensile tests demonstrate that the
elastic modulus and the yielding peak, evaluated after the thermal treatment,
acquire maximum values at a particular thermal amplitude, which coincides with
the minimum of the potential energy.Comment: 23 pages, 10 figure
Heterogeneous relaxation dynamics in amorphous materials under cyclic loading
Molecular dynamics simulations are performed to investigate heterogeneous
dynamics in amorphous glassy materials under oscillatory shear strain. We
consider three-dimensional binary Lennard-Jones mixture well below the glass
transition temperature. The structural relaxation and dynamical heterogeneity
are quantified by means of the self-overlap order parameter and the dynamic
susceptibility. We found that at sufficiently small strain amplitudes, the mean
square displacement exhibits a broad sub-diffusive plateau and the system
undergoes nearly reversible deformation over about cycles. Upon
increasing strain amplitude, the transition to the diffusive regime occurs at
shorter time intervals and the relaxation process involves intermittent bursts
of large particle displacements. The detailed analysis of particle hopping
dynamics and the dynamic susceptibility indicates that mobile particles
aggregate into clusters whose sizes increase at larger strain amplitudes.
Finally, the correlation between particle mobilities in consecutive time
intervals demonstrates that dynamic facilitation becomes increasingly
pronounced at larger strain amplitudes.Comment: 20 pages, 7 figure
Molecular dynamics simulations of the rotational and translational diffusion of a Janus rod-shaped nanoparticle
The diffusion of a Janus rod-shaped nanoparticle in a dense Lennard-Jones
fluid is studied using molecular dynamics (MD) simulations. The Janus particle
is modeled as a rigid cylinder whose atoms on each half-side have different
interaction energies with fluid molecules, thus comprising wetting and
nonwetting surfaces. We found that both rotational and translational diffusion
coefficients are larger for Janus particles with higher wettability contrast,
and these values are bound between the two limiting cases of uniformly wetting
and nonwetting particles. It was also shown that values of the diffusion
coefficients for displacements parallel and perpendicular to the major axis of
a uniformly wetting particle agree well with analytical predictions despite a
finite slip at the particle surface present in MD simulations. It was further
demonstrated that diffusion of Janus particles is markedly different from that
of uniform particles; namely, Janus particles preferentially rotate and orient
their nonwetting sides along the displacement vector to reduce drag. This
correlation between translation and rotation is consistent with the previous
results on diffusive dynamics of a spherical Janus particle with two
hemispheres of different wettability.Comment: 27 pages, 9 figure
Distributions of pore sizes and atomic densities in binary glasses revealed by molecular dynamics simulations
We report on the results of a molecular dynamics simulation study of binodal
glassy systems, formed in the process of isochoric rapid quenching from a
high-temperature fluid phase. The transition to vitreous state occurs due to
concurrent spinodal decomposition and solidification of the matter. The study
is focused on topographies of the porous solid structures and their dependence
on temperature and average density. To quantify the pore-size distributions, we
put forth a scaling relation that provides a robust data collapse in systems
with high porosity. We also find that the local density of glassy phases is
broadly distributed, and, with increasing average glass density, a distinct
peak in the local density distribution is displaced toward higher values.Comment: 22 pages, 6 figure
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