Constant-stress Molecular Dynamics Research Articles

Role of Local Structure in the Enhanced Dynamics of Deformed Glasses.

External stress can accelerate molecular mobility of amorphous solids by several orders of magnitude. The changes in mobility are commonly interpreted through the Eyring model, which invokes an empirical activation volume. Here, we analyze constant-stress molecular dynamics simulations and propose a structure-dependent Eyring model, connecting activation volume to a machine-learned field, softness. We show that stress has a heterogeneous effect on the mobility that depends on local structure through softness. The barrier impeding relaxation reduces more for well-packed particles, which explains the narrower distribution of relaxation time observed under stress.

Unraveling Mg 〈c + a〉 slip using neural network potential

ABSTRACT Magnesium (Mg) activates 〈c + a〉 dislocation slip on the second order pyramidal slip plane. This slip mode is very complex compared to other modes including several metastable structures. Due to the complexity and very similar energies of the different structures, reliably modelling this slip mode is challenging. The problem is exacerbated when considering alloying, in which a combination of 1st order and 2nd order pyramidal slip is usually observed. Motivated by the need for a high fidelity potential for Mg alloys, we have developed first a highly accuracy potential for pure Mg. The new potential shows better agreement with density functional theory and experimental calculations than previous interatomic potentials for Mg. With the help of this new potential, we demonstrate that the basal dissociated 〈c + a〉 core is not sessile, as previously thought, and that constant stress molecular dynamics demonstrate clear preference for the 2nd order pyramidal system over the 1st order system.

Atomistic calculations of lattice constants of mullite with its compositions

We calculate lattice constants a and b of mullite with its compositions (Al 2[Al 2+2 x Si 2−2 x ]O 10− x ) using the constant stress molecular dynamics (MD) method. The Matsui's and Winkler's interatomic models for aluminosilicate are adapted. Two sets of mullite models are considered, one with a random distribution of oxygen vacancies and Al/Si substitutions and the other with a partially ordered Al occupancy at the T * sites. All the simulation results predict the trend observed in experiments in which the lattice constant a increases while b decreases when the x-value increases. The expansion mechanism induced by Al/Si substitutions is supported by bond length analysis. The reduction of lattice constant b is supported by the locally anisotropic atom displacement in the a and b directions sensed by the oxygen vacancies. We conclude that the causes of lattice expansion and reduction are tightly interwoven with the coexistence of vacancies and Al/Si substitutions and their inherent influence on atom displacements.

Cross-slip and annihilation of screw dislocations in Cu: a molecular dynamics study

Cross-slip of screw dislocations and the annihilation of screw dislocation dipoles in Cu were simulated using constant-temperature constant-stress molecular dynamics. The cross-slip mechanism and annihilation process of flexible dislocations in a large dipole configuration was identified as a dynamic variant of the Friedel–Escaig mechanism. The cross-slip rate was found to exhibit exponential dependence on the temperature, from which the activation enthalpy for the cross-slip process was calculated by the Arrhenius relation.

Simulation of the plastic behavior of amorphous glassy bis-phenol-A-polycarbonate.

A protocol for studying the plastic deformation of amorphous glassy polymers is presented. The protocol is based on a viable computational procedure which combines constant-stress molecular dynamics simulations and fixed-cell energy minimizations, followed by kinetic, configurational, and energy analyses. It is shown that the computational results can be accounted for within a "potential energy landscape" theoretical framework, in which the plastic transition is interpreted as a crossing between and a collapse onto each other of "ideal (thermodynamic) structures." The procedure is applied to bis-phenol-A-polycarbonate (BPA-PC), but is equally valid for a wide variety of polymeric species. Allowing for the limited size of the simulation cell, the high strain rate, and the fact that the simulation are conducted at low temperature, the values of the density, Young's modulus, yield strain, yield stress, activation energy, and activation volume are in fair agreement with the experimental data on BPA-PC. The analysis of the results shows that the plastic relaxation for this polymer has both a collective and cooperative character (as in classical percolation theories), involves a significant fraction of the simulation cell, and can be viewed as a "nanoscopic shear band."

Simulation of pressure-driven phase transitions from tetrahedral crystal structures

Pressure-driven transitions of ionic materials from the zinc-blende to rocksalt and $\ensuremath{\delta}\ensuremath{-}{\mathrm{ZnCl}}_{2}$ to ${\mathrm{CdCl}}_{2}$ crystal structures are studied using constant-stress molecular dynamics with a polarizable-ion potential model. Both transformations are characterized by a change in cation coordination environment from tetrahedral to octahedral and are nonmartensitic. Transformation mechanisms are identified and characterized and similarities discussed. The blende to rocksalt transformation is observed to proceed via a diatomic $\ensuremath{\beta}$-tin-like structure, though this is shown to be a transition state and not a true intermediate phase in this system. The relationship of the observed mechanisms to those deduced from experiments on halide systems is discussed. The development of displacive motion across the simulation cell is discussed. The ${\mathrm{ZnCl}}_{2}$ system is a layered structure, and while the coordination changes are highly cooperative within each layer, the overall transformation takes place on a layer-by-layer basis. In the blende, the interlayer correlations required to produce a grain-boundary-free final structure are associated with a shearing motion which propagates across the cell. These differences have characteristic effects on the kinetics of the transformations.

NpH-MD-Simulations of the Elastic Moduli of Cellulose II at Room Temperatue

We have used molecular dynamics modeling to investigate the stucture and mechanical properties of regenerated cellulose fibres. This work is motivated by continued interest in replacing the environmentally hazardous viscose process by alternative spinning methods. An important input parameter for any realistic model of the elastic properties is the stiffness tensor of the crystalline constituent, cellulose II. Conventional molecular mechanics techniques can be used to estimate the elastic reaction of a material with respect to small external stresses or strains, i.e. the compliance and stiffness tensors, and the elastic moduli derived therefrom, at zero temperature. In order to access non-zero temperatures, it is necessary to use either the quasi-harmonic approximation for the vibrational free energy or molecular dynamics (MD) simulations. In the present work, Parrinello-Rahman constant-stress MD was performed to generate trajectories in constant particle number (N), constant external stress tensor (p or t) and constant enthalpy H (NpH or HtN) ensemble. This was found to be less time consuming than working with isothermal conditions, as done by other authors. The fluctuations in kinetic energy and MD cell vectors were then used to calculate adiabatic elastic constants, thermal expansion coefficients and heat capacity. The isothermal elastic constants were found by applying a standard thermodynamic relation. The Young′s modulus along the chain direction, El, was determined to be 155 GPa, whereas the values in the perpendicular directions vary between 51 and 24 GPa. These results are of the same order of magnitude as those obtained by Tashiro and Kobayashi [1] with the static (T = 0K) method, but our value of El is 5% lower and, unexpectedly, the lateral values are up to six times higher. A strong anisotropy is found for shear along the chains in planes containing the chain axis, the shear modulus ranging from 5 to 20 GPa. Convergence was achieved in the simulations, to the extend that the elastic constants become stationary, but significant internal stresses remain, pointing to shortcomings in the software used. Further work is necessary to resolve these problems, although the major conclusions should be unaffected.

Constant-pressure and constant-stress molecular dynamics studies of the elastic properties of FeB glasses

Abstract The equations-of-motion for isothermal constant-pressure and constant-stress molecular dynamics in the Andersen-Nose and Parrinello-Ray-Rahman-Nose schemes are generalized for systems with density-dependent pair- and volume-forces. Applications to the calculation of the elastic constants of FeB glasses are presented.

Stress/strain analysis of polymeric materials by molecular dynamics simulations

AbstractMechanical properties number themselves among the most important properties of any polymeric material used in engineering applications. The Molecular Mechanics approach has been used to calculate the mechanical moduli of various polymers, but this approach has the limitation of requiring experimental densities and neglecting both entropy contributions and local volume fluctuations which can be significant in condensed phase polymers such as glasses and melts. Although constant stress molecular mechanics can provide an estimate of the bulk density, constant stress molecular dynamics (CSMD) can overcome all these limitations. However, CSMD is limited by an inherently short simulation time scale. CSMD was used to calculate the bulk moduli, Young's moduli and density of various amorphous polymers, including: polyethylene, poly(propylene), polystyrene, poly(vinyl chloride), poly(ethylene terephthalate), poly(butylene terephthalate), nylon 6, poly(methyl methacrylate), polydimethylsiloxane, bisphenol‐A polycarbonate, poly(phenylene sulfide). Results indicate that CSMD is a useful tool in characterizing mechanical properties of amorphous polymers despite this time scale limitation.

Structure as a function of relaxation in glassy polymers

AbstractPreviously, amorphous glasses were simulated by carrying out energy minimization on initial conformations generated by growing polymer chains in a periodic cube. It was not known what degree of relaxation these simulated glasses possessed. The degree relaxation is determined by the thermal history of the bulk polymer and in turn determines numerous important properties. Constant stress molecular dynamics (CSMD) followed by energy minimization was used to simulate different thermal histories of an isotactic poly(propylene) glass. This simulation approach produced glasses in which the degree of energetic relaxation was a function of the thermal history. Based on the simulated cohesive energy density, the simulation using minimization alone produced a glass that was energetically annealed. However, the local geometry and x‐ray structure factor indicate that it has a different structure than those obtained using CSMD followed by energy minimization and may not be structurally relaxed.

Constant-stress molecular dynamics: The phase diagram of silver iodide.

The method of constant-stress molecular dynamics using the Parrinello-Rahman Lagrangian is described, particularly as applied to Coulomb systems. The method is used to simulate the phase behavior of silver iodide. The \ensuremath{\beta}, \ensuremath{\alpha}, rocksalt, and liquid phases are reproduced and the boundaries are similar to those observed for real AgI. Evidence is found for the order-disorder transition in the \ensuremath{\alpha} phase proposed by Perrott and Fletcher. This is characterized by a heat-capacity anomaly and a rise in the cation diffusion coefficient. The supercooled \ensuremath{\alpha} phase and the equilibrium rocksalt phase both have diffuse fast-ion transitions and the latter cuts the rocksalt--\ensuremath{\alpha}-phase boundary, causing it to cusp in to an apparent triple point with the \ensuremath{\alpha}-phase order-disorder transition.

The phase behaviour of silver iodide. Constant stress molecular dynamics simulation

The constant stress molecular dynamics (MD) technique using the Parrinello-Rahman Lagrangian is used to investigate the phase behaviour of AgI using a simple rigid-ion model potential. The system of 216 or 256 ions spontaneously transforms from one pure phase to another upon superheating or supercooling and the known phase diagram of AgI is remarkably well mimicked. The high-pressure rocksalt phase has a diffuse fast-ion transition as does the α-phase supercooled below the α-β transition at 300 K, 0 GPa. Some authors have argued that a divergence in defect densities or a diffuse fast-ion transition in the β-phase drives the β-α transition. No evidence is found for this. A heat capacity, compressibility and diffusivity anomaly in the α-phase appears to confirm the order-disorder transition there proposed by Perrott and Fletcher. This perturbs the shape of the rocksalt-α-phase boundary as predicted earlier. Heats of transition are determined and the volume change across the β-α boundary is indeed negative as for real AgI. These results are a remarkable vindication of the MD technique, and should encourage application to further important model systems.

Molecular-dynamics simulation of the phase behavior of AgI.

The phase diagram of silver iodide is simulated by the method of constant-stress molecular dynamics with use of the Parrinello-Rahman Langrangean. The $\ensuremath{\beta}$, $\ensuremath{\alpha}$, rocksalt, and liquid phases are reproduced and the boundaries are similar to those observed for real AgI. The supercooled $\ensuremath{\alpha}$ phase and the equilibrium rocksalt phase both have diffuse fast-ion transitions and the latter cuts the rocksalt-$\ensuremath{\alpha}$ boundary causing it to cusp into an apparent triple point with an order-disorder transition in the $\ensuremath{\alpha}$ phase. This is characterized by a heat-capacity anomaly and a rise in the cation diffusion coefficient.