Model Binary Glass Research Articles

Optimally rejuvenated model binary glasses

Using the creation relaxation algorithm developed for the atomistic modeling of the high-dose irradiation limit of crystalline systems, we explore the limits of the structural rejuvenation of a highly excited model binary glass. This high-energy athermal amorphous structure exhibits a direct transition to homogeneous plastic flow and a microstructure that is largely insensitive to this flow, being characterized by a porous system-spanning network of minimally frustrated structural motifs. The observed homogeneous plasticity is mediated by the same string-like structural excitations, which mediate structural relaxation and microplasticity at finite temperature in more relaxed structures. This highly rejuvenated structural asymptote is not far from the structural state of regions, which have experienced athermal shear localization in more relaxed samples, suggesting an optimally rejuvenated glassy structure will always be limited by that produced by shear localization.

Viscosity and transport in a model fragile metallic glass

How thermally activated structural excitations quantitatively mediate transport and microplasticity in a model binary glass at the microsecond timescale is revealed using atomistic simulation. These local excitations, involving a stringlike sequence of atomic displacements, admit a far-field shear-stress signature and underlie the transport of free-volume and bond geometry. Such transport is found to correspond to the evolution of a disclination network describing the spatial connectivity of topologically distinct bonding environments, demonstrating the important role of geometrical frustration in both glass structure and its underlying dynamics.

Micro-plasticity in a fragile model binary glass

Atomistic deformation simulations in the nominally elastic regime are performed for a model binary glass with strain rates as low as 104/s (corresponding to 0.01 shear strain per 1 µs). A strain rate dependent elastic softening due to a micro-plasticity is observed, which is mediated by thermally-activated localized structural transformations (LSEs). A closer inspection of the atomic-scale structure indicates the material response is distinctly different for two types of local atomic environments. A system spanning iscosahedrally coordinated substructure responds purely elastically, whereas the remaining substructure admits both elastic and microplastic evolution. This leads to a heterogeneous internal stress distribution which, upon unloading, results in negative creep and complete residual-strain recovery. A detailed structural analysis in terms of local stress, atomic displacement, and SU(2) local bonding topology shows such microscopic processes can result in large changes in local stress and are more likely to occur in geometrically frustrated regions characterized by higher free volume and softer elastic stiffness. The thermally-activated LSE activity also mediates structural relaxation, and in this way should be distinguished from stress-driven shear transformation activity which only rejuvenates glass structure. The frequency of LSE activity, and therefore the amount of micro-plasticity, is found to be related to the degree to which the glassy state is relaxed. These insights shed atomistic light onto the structural origins that may govern recent experimental observations of significant structural evolution in response to elastic loading protocols.

Correlated disorder in a model binary glass through a local SU(2) bonding topology

A quantitative understanding of the microscopic constraints which underlie a well relaxed glassy structure is the key to developing a microscopic theory of structural evolution and plasticity for the amorphous solid. Here we demonstrate the applicability of one such theory of local bonding constraints developed by D. R. Nelson [Phys. Rev. B 28, 5515 (1983)], for a model binary Lennard-Jones glass structure that has undergone an isothermal annealing simulation spanning over 10 micro-seconds of physical simulation time. By introducing a modified radical Voronoi tessellation which removes some ambiguity in how nearest neighbour bonds are enumerated, it is found, that a large proportion ($>95\%$) of local atomic environments follow the connectivity rules of the SU(2) topology of Nelson's work resulting in a dense network of disclination lines characterizing the defect bonds. Furthermore, it is numerically shown that a low energy glass structure corresponds to a reduced level of bond-length frustration and thus a minimally defected bond-defect network. It is then demonstrated that such a defect network provides a framework in which to analyse thermally-activated structural excitations, revealing those high-energy/low-density regions not following the connectivity constraints are more likely to undergo structural rearrangement that often results in a local relaxation that ends with the creation of new SU(2) local topology content.

A new approach for dynamic mechanical thermal analysis: The energy dissipation rate method

We report for the first time a new approach for analyzing the DMTA spectra of the internal friction of binary systems (polymer – additive) - the energy dissipation rate method. The method involves representation of the loss spectrum in the free damp oscillation (FDO) mode in the form: absorption coefficient α(ν) = Λ(ν)*f(ν), where Λ(ν) is logarithmic decrement of oscillation and f(ν) is frequency of free damp oscillation (ν ≈ 0,7Т°К). The experimental data presented in these coordinates allows for: (1) correct application of all mathematical operations with the spectra α(ν), as with ordinary optical absorption spectra; (2) quantification of the individual components' contributions to the spectra of internal friction in multicomponent systems (composites, polymers with residual solvent, porous sorbents, glass forming systems and many others). The study investigated a model binary glass forming system made up of poly (vinyl-formal) - glycerol. The results of analysis of the loss spectra in FDO mode, associated with the additive (glycerol), correspond well to data received from far IR and dielectric spectroscopy of the dopant.

Emergent structural length scales in a model binary glass - The micro-second molecular dynamics time-scale regime

It is now possible to routinely perform atomistic simulations at the microsecond timescale. In the present work, we exploit this for a model binary Lennard-Jones glass to study structural relaxation at a timescale spanning up to 80 μs. It is found that at these longer time-scales, significant mobility and relaxation occurs, demonstrating a reproducible relaxation trajectory towards a class of asymptotic structural states which are strongly heterogeneous. These structures are discussed both in terms of the ideal glass state and a production route towards novel amorphous crystalline nano-composite micro-structures with strong interface stability.

Local volume as a robust structural measure and its connection to icosahedral content in a model binary amorphous system

Molecular dynamics simulations are performed for a model binary glass to investigate the temperature dependence of the thermally driven localized structural excitations mediating glass relaxation. This is done under fixed zero pressure by increasing the temperature from zero to values close to and beyond the glass transition regime. It is found that for a given ramp rate, fixed pressure simulations admit a similar amount of structural relaxation as fixed volume simulations (Acta Materialia 143 (2018) 205–213), but at a reduced temperature scale corresponding to a reduced glass transition temperature. Furthermore, analysis of the volume evolution under fixed pressure, and stress evolution under fixed volume, demonstrates an equivalence between NPT and NVT ensemble molecular dynamics simulations in achieving a particular structural state. The observed relaxation induces a local densification which may be quantitatively connected to the creation of icosahedral structure that minimizes both bond energy frustration and maximizes local atomic packing. Together these results establish a robust link between the structural state of an amorphous solid and its free volume content, both of which now directly relate to the degree of relaxation.

Evolution of glassy gratings with variable aspect ratios under surface diffusion

The structural evolution of surface gratings on a glassy material is investigated by means of molecular simulations. The gratings provide a means to probe surface diffusion in the vicinity of the glass transition temperature. A theory by Mullins [J. Appl. Phys. 30, 77 (1959)] is used to extract qu-antitative measures of surface diffusivity that rely on calculation of grating amplitude as a function of time. The simulations are implemented in the context of a model binary glass mixture [S. S. Ashwin and S. Sastry, J. Phys.: Condens. Matter 15, S1253 (2003)]. We find that surface diffusion is faster than bulk diffusion by several orders of magnitude, consistent with recent experimental data for an organic glass former. The diffusivities extracted by the grating-decay approach are consistent with those estimated on the basis of mean-squared particle displacements. The grating-decay approach, however, is more efficient than traditional techniques based on Einstein's diffusion equation. Grating decay is also more versatile and is shown to be applicable in a variety of sample geometries.

Simulations of glasses: multiscale modeling and density of states Monte-Carlo simulations

We show the application of two recent developments in molecular simulations, density of states (DOS) (Wang–Landau) Monte-Carlo and multiscale modeling, to the understanding of the glass transition. First, we review DOS Monte-Carlo using the two-dimensional Ising model without external field on lattices of varying size. We point out that we can analyze the resulting densities of state in a canonical and a microcanonical way starting from the same simulations. The heat capacity is discussed in both ensembles. Results from both ensembles of off-lattice simulations of a model binary glass former are compared as well. Subsequently, a self-consistent systematic mapping procedure for molecular models from the atomistic to the mesoscale is presented. It allows to efficiently derive mesoscale models with fewer interaction centers from atomistic models preserving the molecular identity. We use the optimization of a corresponding mesoscale model for atactic polystyrene (a good glass former) in the melt as an example. Simulations of different temperatures using this model allow some insight into the glass formation of this system. We point out the strengths and weaknesses of these approaches and give an outlook towards their combination.

A model for calculation of isothermal kinetics of the nucleation-and-growth type phase separation in the course of one-step heat treatment

A model is described for calculating the kinetics of a diffusion-limited nucleation-and-growth type phase separation. A homogeneous melt of glass is cooled to a temperature, at which the nucleation, growth and Ostwald ripening of particles of a new phase are possible. In the model, a spatial inhomogeneity of nucleation rate, caused by the formation of diffusion zones around particles, and an effect of interaction of these zones on growth of particles are taken into account. Criteria for application of the model are formulated and parameters of the model are calculated for a model binary glass. A conclusion is drawn that the proposed model is applicable to systems where supersaturation is not too high. In particular, nucleation must not be so plentiful as to suppress a growth from supersaturated solution and to initiate Ostwald ripening at the nucleation stage. It is noted that such situations are observed in experiments on phase separation in glasses. The model seems to be applicable to some other types of phase transformations as well.

Molecular-dynamics simulation of structural anisotropy in glassy metals and its relationship to magnetic anisotropy

Creep-induced magnetic anisotropy in metallic glass was investigated using a molecular-dynamics simulation. A model binary glass structure of 2048 particles interacting via Lennard–Jones potentials was creep deformed below its glass transition temperature. Creep deformation resulted in the bond-orientational anisotropy (BOA) within the first nearest-neighbor shell with a large sixth-order spherical harmonic component. The magnetic anisotropy estimated on the basis of the point-charge model shows that a small two-fold component of the BOA can induce a sizable magnetic anisotropy in amorphous rare-earth alloys.

Molecular dynamics simulation of structural anisotropy in glassy metals

Molecular dynamics simulations have been carried out to study the creep-induced structural anisotropy in amorphous alloys. A model binary glass structure 2048 particles interacting via Lennard-Jones potentials was creep-deformed below its glass transition temperature. Creep-deformation resulted in the bond-orientational anisotropy (BOA) within the first nearest-neighbor shell with a large sixth-order spherical harmonic component. The BOA caused an anisotropy characterized by uniform elongation along the tensile principal axis of the creep-deformation beyond the third nearest-neighbor shell.