1,025 research outputs found
Microscopic origin of nonlinear non-affine deformation and stress overshoot in bulk metallic glasses
The atomic theory of elasticity of amorphous solids, based on the nonaffine
response formalism, is extended into the nonlinear stress-strain regime by
coupling with the underlying irreversible many-body dynamics. The latter is
implemented in compact analytical form using a qualitative method for the
many-body Smoluchowski equation. The resulting nonlinear stress-strain
(constitutive) relation is very simple, with few fitting parameters, yet
contains all the microscopic physics. The theory is successfully tested against
experimental data on metallic glasses, and it is able to reproduce the
ubiquitous feature of stress-strain overshoot upon varying temperature and
shear rate. A clear atomic-level interpretation is provided for the stress
overshoot, in terms of the competition between the elastic instability caused
by nonaffine deformation of the glassy cage and the stress buildup due to
viscous dissipation.Comment: Physical Review B Rapid Comm., in pres
Rheology of hard glassy materials.
Glassy solids may undergo a fluidization (yielding) transition upon deformation whereby the material starts to flow plastically. It has been a matter of debate whether this process is controlled by a specific time scale, from among different competing relaxation/kinetic processes. Here, two constitutive models of cage relaxation are examined within the microscopic model of nonaffine elasto-plasticity. One (widely used) constitutive model implies that the overall relaxation rate is dominated by the fastest between the structural (α) relaxation rate and the shear-induced relaxation rate. A different model is formulated here which, instead, assumes that the slowest (global) relaxation process controls the overall relaxation. We show that the first model is not compatible with the existence of finite elastic shear modulus for quasistatic (low-frequency) deformation, while the second model is able to describe all key features of deformation of 'hard' glassy solids, including the yielding transition, the nonaffine-to-affine plateau crossover, and the rate-stiffening of the modulus. The proposed framework provides an operational way to distinguish between 'soft' glasses and 'hard' glasses based on the shear-rate dependence of the structural relaxation time.US Army ARO Cooperative Agreement W911NF-19-2-0055
EPSRC Theory of Condensed Matter Critical Mass Grant EP/J01763
Molecular-level relation between intra-particle glass transition temperature and stability of colloidal suspensions
In many colloidal suspensions, the dispersed colloidal particles are
amorphous solids resulting from vitrification. A crucial open problem is
understanding how colloidal stability is affected by the intra-particle glass
transition. By dealing with the latter process from a solid-state perspective,
we estabilish a proportionality relation between the intra-particle glass
transition temperature, and the Hamaker constant,
of a generic suspension of nanoparticles. It follows that
can be used as a convenient parameter (alternative to
) for controlling the stability of colloidal systems. Within DLVO
theory, we show that the novel relationship, connecting to
implies the critical coagulation ionic strength (CCIS) to be a
monotonically decreasing function of We connect our
predictions to recent experimental findings
Low-energy optical phonons induce glassy-like vibrational and thermal anomalies in ordered crystals
It is widely accepted that structural glasses and disordered crystals exhibit
anomalies in the their thermal, mechanical and acoustic properties as
manifestations of the breakdown of the long-wavelength approximation in a
disordered dissipative environment. However, the same type of glassy-like
anomalies (i.e. boson peak in the vibrational density of states (VDOS) above
the Debye level, peak in the normalized specific heat at etc)
have been recently observed also in perfectly ordered crystals, including
thermoelectric compounds. Here we present a theory that predicts these
surprising effects in perfectly ordered crystals as a result of low-lying
(soft) optical phonons. In particular, it is seen that a strong boson peak
anomaly (low-energy excess of modes) in the VDOS can be due almost entirely to
the presence of low-energy optical phonons, provided that their energy is
comparable to that of the acoustic modes at the Brillouin zone boundary. The
boson peak is predicted also to occur in the heat capacity at low . In
presence of strong damping (which might be due to anharmonicities in the
ordered crystals), these optical phonons contribute to the low- deviation
from Debye's law, producing a linear-in- behavior which is typical
of glasses, even though no assumptions of disorder whatsoever are made in the
model. These findings are relevant for understanding and tuning thermal
transport properties of thermoelectric compounds, and possibly for the
enhancement of electron-phonon superconductivity
Local inversion-symmetry breaking controls the boson peak in glasses and crystals
It is well known that amorphous solids display a phonon spectrum where the
Debye law at low frequency melds into an anomalous excess-mode
peak (the boson peak) before entering a quasi-localized regime at higher
frequencies dominated by scattering. The microscopic origin of the boson peak
has remained elusive despite various attempts to put it in a clear connection
with structural disorder at the atomic/molecular level. Using numerical
calculations on model systems, we show that the microscopic origin of the boson
peak is directly controlled by the local breaking of center-inversion symmetry.
In particular, we find that both the boson peak and the nonaffine softening of
the material display a strong positive correlation with a new order parameter
describing the local inversion symmetry of the lattice. The standard
bond-orientational order parameter, instead, is shown to be a poor correlator
and cannot explain the boson peak in randomly-cut crystals with perfect
bond-orientational order. Our results bring a unifying understanding of the
boson peak anomaly for model glasses and defective crystals in terms of a
universal local symmetry-breaking principle of the lattice.This is the author accepted manuscript. It is currently under an indefinite embargo pending publication by the American Physical Society
On mean coordination and structural heterogeneity in model amorphous solids
We propose a simple route to analytically evaluate the average coordination of model disordered solids with maximally homogeneous distribution of the particles in space. The model yields the average number of contacts (z) as a function of volume fraction (phi) of a hard-sphere connected system and recovers the critical jamming point of hard spheres (z=6 at phi=0.64). Numerical simulations of Lennard-Jones glasses with a varying attraction range are used to investigate the volume fraction dependence of the average coordination in the presence of attraction. It is observed that upon decreasing phi below 0.6, structural heterogeneity is reflected in values of the coordination number which are higher than those predicted by the model for a statistically homogeneous distribution of particles in space due to the attraction-induced local aggregation. Thus the model can be usefully employed as a quantitative reference to assess the degree of structural heterogeneity in glasses in terms of a directly accessible structural parameter such as the mean number of contacts
Nonaffine lattice dynamics with the Ewald method reveals strongly nonaffine elasticity of α-quartz
A lattice dynamical formalism based on nonaffine response theory is derived for noncentrosymmetric crystals, accounting for long-range interatomic interactions using the Ewald method. The framework takes equilibrated static configurations as input to compute the elastic constants in excellent agreement with both experimental data and calculations under strain. Besides this methodological improvement, which enables faster evaluation of elastic constants without the need of explicitly simulating the deformation process, the framework provides insights into the nonaffine contribution to the elastic constants of alpha-quartz. It turns out that, due to the noncentrosymmetric lattice structure, the nonaffine (softening) correction to the elastic constants is very large, such that the overall elastic constants are at least 3-4 times smaller than the affine Born-Huang estimate
Theory of the phonon spectrum in host-guest crystalline solids with avoided crossing
We develop an analytical model to describe the phonon dispersion relations of host-guest lattices with heavy guest atoms (rattlers). Crucially, the model also accounts for phonon damping arising from anharmonicity. The spectrum of low-energy states contains acousticlike and (soft) optical-like modes, which display the typical avoided crossing, and which can be derived analytically by considering the dynamical coupling between host lattice and guest rattlers. Inclusion of viscous anharmonic damping in the model allows us to compute the vibrational density of states (VDOS) and the specific heat, unveiling the presence of a boson peak (BP) linked to an anharmonicity-smeared van Hove singularity. Upon increasing the coupling strength between the host and the guest dynamics, and by decreasing the energy of the soft optical modes, the BP anomaly becomes stronger and it moves towards lower frequencies. Moreover, we find a robust linear correlation between the BP frequency and the energy of the soft optical-like modes. This framework provides a useful model for tuning the thermal properties of host-guest lattices by controlling the VDOS, which is crucial for optimizing thermal conductivity and hence the energy conversion efficiency in these materials
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