Steady state dislocation motion via molecular dynamics

Nonequilibrium molecular dynamics is applied to the generation and steady propagation of edge dislocations. “Normal” propagation at about one-third the longitudinal sound speed, transonic propagation at nine-tenths that speed, climb, and multiplication are all observed.     

Reorientational motion in binary mixtures: molecular dynamics simulations

Molecular reorientational motion has been studied in a dilute solution of linear ‘tracer’ molecules in a solvent that exhibits liquid, plastic and crystalline phases. Molecular dynamics simulations have been used to extract reorientational correlation functions for both solvent and solute species as functions of temperature in all phases. The transition from the liquid to the plastic phase (upon cooling) results in less hindered tracer rotation, as evidenced by the more rapid decay of orientational correlation. These surprising dynamics are interpreted in terms of structural changes at freezing that lead to a less confining local tracer environment. The findings support a recent experimental result obtained from polarized Raman scattering on a solution of CS₂ tracers in a cyclohexane host.     

Strengthening effects of various grain boundaries with nano-spacing as barriers of dislocation motion from molecular dynamics simulations

Strengthening in metals is traditionally achieved through the controlled creation of various grain boundaries (GBs), such as low-angle GBs, high-angle GBs, and twin boundaries (TBs). In the present study, a series of large-scale molecular dynamics simulations with spherical nanoindentation and carefully designed model were conducted to investigate and compare the strengthening effects of various GBs with nano-spacing as barriers of dislocation motion. Simulation results showed that high-angle twist GBs and TBs are similar barriers and low-angle twist GBs are less effective in obstructing dislocation motion. Corresponding atomistic mechanisms were also given. At a certain indentation depth, dislocation transmission and dislocation nucleation from the other side of boundaries were observed for low-angle twist GBs, whereas dislocations were completely blocked by high-angle twist GBs and TBs at the same indentation depth. The current findings should provide insights for comprehensive understanding of the strengthening effects of various GBs at nanoscale.     

Investigation of precipitation hardening by dislocation dynamics simulations

Dislocation dynamics (DD) simulations are used to investigate precipitation-induced strengthening in a Zr–1% Nb alloy. A method is proposed to carry out simulations under dynamical conditions in connection with the microstructure of the investigated alloy. First, a sensitivity study of simulation parameters, suspected of altering simulation results, is presented. It allows setting up simulation conditions ensuring statistical representativeness. The effect of the strain rate is then investigated and analyzed in connection with the random distribution adopted to describe precipitation distribution. The strengthening induced by two different families of Nb precipitates is estimated from simulations whose simulation box is reduced to the volume of a single grain. It is shown that the obtained strengthening values are smaller than those predicted by most published models. Determination of total strengthening shows that usual superposition rules do not apply. A mixture law, fitting DD results well, is proposed in this paper.     

Determination of the activation energy by stochastic analyses of molecular dynamics simulations of dislocation processes

An investigation is reported of the probability and the probability density of thermal activation of stress-driven dislocation processes, as simulated using molecular dynamics (MD). Stochastic analyses of the survival probability are found to lead to simple relationships between the loading history and the distribution of the interaction time and strength. It is shown that the determination of the activation energy associated to a thermally activated event can be achieved by a reduction of the stochastic process to a process obeying the Poisson's distribution, preserving the activation probability at the survival time. The method is applied to the kink-pair mechanism for screw dislocations in iron. Predictions are compared with experimental results and with other methods reported in the literature, which allows the difference in the approximations and in the assumptions considered in these models to be underlined.     

Understanding acoustoplasticity through dislocation dynamics simulations

The acoustoplastic effect in metals is routinely utilised in industrial processes involving forming, machining and joining, but the underlying mechanism is still not well understood. There have been earlier suggestions that dislocation mobility is enhanced intrinsically by the applied ultrasound excitation, but in subsequent deliberations it is routinely assumed that the ultrasound merely adds extra stresses to the material without altering its dislocation density or intrinsic resistance to deformation. In this study, a dislocation dynamics simulation was carried out to investigate the interactions of dislocations under the combined influence of quasi-static and oscillatory stresses. Under such combined stress states, dislocation annihilation is found to be enhanced leading to larger strains at the same load history. The simulated strain evolution under different stress schemes also closely resembles certain previously obtained experimental observations. The discovery here goes far beyond the simple picture that the ultrasound effect is merely an added-stress one, since here, the intrinsic strain-hardening potency of the material is found to be reduced by the ultrasound, through its effect on enhancing dislocation annihilation.     

Intramolecular diffusive motion in alkane monolayers studied by high-resolution quasielastic neutron scattering and molecular dynamics simulations

Molecular dynamics simulations of a tetracosane (n-C24H50) monolayer adsorbed on a graphite basal-plane surface show that there are diffusive motions associated with the creation and annihilation of gauche defects occurring on a time scale of approximately 0.1-4 ns. We present evidence that these relatively slow motions are observable by high-energy-resolution quasielastic neutron scattering (QNS) thus demonstrating QNS as a technique, complementary to nuclear magnetic resonance, for studying conformational dynamics on a nanosecond time scale in molecular monolayers.     

Thermal diffusion in micropores by molecular dynamics computer simulations

This work focuses on the identification of the main microscopic processes that influence thermal diffusion (the Soret effect) in a fluid mixture confined in an uncorrugated slit pore. To achieve this purpose, a boundary driven nonequilibrium molecular dynamics scheme is applied on binary mixtures of super-critical Lennard–Jones (LJ) spheres representing methane and n-decane. Following previous work, we perform a systematic study of the influence of the parameters used to describe a model slit pore on an effective thermal diffusion factor. Among these parameters are: The nature of the reflection of the diffusing particles on the walls (specular or diffusive), the pore width with respect to the particle size and the fluid-wall potential strength. Simulations were run both on equimolar and non-equimolar mixtures. The results indicate that thermal diffusion is effectively lowered only for strong fluid–wall interactions. It is shown that the general trends, which are different under sub- and super-critical conditions, can be explained by a careful analysis of the relative sorption energies of the two compounds.     

Multiscale dislocation dynamics simulations of shock-induced plasticity in small volumes

Multiscale dislocation dynamics plasticity (MDDP) was used to investigate shock-induced deformation in monocrystalline copper. In order to enhance the numerical simulations, a periodic boundary condition was implemented in the continuum finite element (FE) scale so that the uniaxial compression of shocks could be attained. Additionally, lattice rotation was accounted for by modifying the dislocation dynamics (DD) code to update the dislocations’ slip systems. The dislocation microstructures were examined in detail and a mechanism of microband formation is proposed for single- and multiple-slip deformation. The simulation results show that lattice rotation enhances microband formation in single slip by locally reorienting the slip plane. It is also illustrated that both confined and periodic boundary conditions can be used to achieve uniaxial compression; however, a periodic boundary condition yields a disturbed wave profile due to edge effects. Moreover, the boundary conditions and the loading rise time show no significant effects on shock–dislocations interaction and the resulting microstructures. MDDP results of high strain rate calculations are also compared with the predictions of the Armstrong–Zerilli model of dislocation generation and movement. This work confirms that the effect of resident dislocations on the strain rate can be neglected when a homogeneous nucleation mechanism is included.     

The collapsing bubble in a liquid by molecular dynamics simulations

Molecular dynamics simulations have been made of a collapsing bubble or cavity in a simple liquid. Simulations of a Lennard-Jones liquid reveal that the collapsing process takes place in a series of stages. First, the ‘hottest’ molecules from the high kinetic energy tail in the Maxwell—Boltzmann distribution diffuse into the empty cavity. This is followed by a gradual filling in of the cavity until the density in the centre is a little lower than that of the bulk liquid. The system eventually reaches a final new equilibrium liquid state through a subsequent slower equilibration phase. The bubble fills in an oscillatory manner, by partly filling in, and then partially emptying, and so on, with ever decreasing amplitude towards the final uniform liquid state. These density oscillations are more obvious in systems with a larger bubble. Similar oscillations are observed in the kinetic energy of the molecules at selected radii from the centre of the initial bubble. The maximum temperature occurs typically at the end of the initial fillingin stage during which the density of the core undergoes a vapour-to-liquid phase transition, the released latent heat probably contributing to the temperatures achieved in this region. The average maximum temperature found in the smallest system examined is about nine times the critical temperature, which is about 6000 K for water, thus suggesting a simple mechanism for producing molecules with the sorts of kinetic energies and lifetimes required for sonoluminescence.     

Study of nanoscratching of polymers by using molecular dynamics simulations

Molecular dynamic simulations are performed to study the nanoscratching behavior of polymers.The effects of scratching depth,scratching velocity and indenter/polymer interaction strength are investigated.It is found that polymer material in the scratching zone around the indenter can be removed in a ductile manner as the local temperature in the scratching zone exceeds glass transition temperature Tg.The recovery of polymer can be more significant when the temperature approaches or exceeds Tg.The tangential force,normal force and friction coefficient increase as the scratching depth increases.A larger scratching velocity leads to more material deformation and higher pile-up.The tangential force and normal force are larger for a larger scratching velocity whereas the friction coefficient is almost independent of the scratching velocities studied.It is also found that stronger indenter/polymer interaction strength results in a larger tangential force and friction coefficient.     

Interpretation of NMR data on proteins by molecular dynamics simulations

The stability and the internal mobility of the recently determined solution structure of the activation domain of porcine procarboxypeptidase B (ADB; Vendrell J., Billeter M., Wider G., Avilés F.X., Wüthrich K.: EMBO J.10, 11–15 (1991)) was studied by unrestrained molecular dynamics. A 66 residue long polypeptide corresponding to the structurally well-defined fragment of ADB was immersed in a water bath and carefully equilibrated. A trajectory of 90 ps was then recorded at room temperature, using no constraints deduced from NMR distance measurements. The averages of the inverse third power of the proton-proton distances, for which NMR distance measurements are available, were calculated and converted to time averages of the distances. Comparison of these time averaged distances with the corresponding distance constraints used in the determination of the solution structure show that the dynamic structure described by the molecular dynamics trajectory is consistent with the NMR data for the polypeptide backbone and for most side chains. In the course of the 90 ps trajectory, the ß-sheet of ADB remains intact and well-defined. The last helix of the protein domain is also stable, but moves with respect to the rest of the protein, while the first helix shows increased instability. The structural fluctuations of the side chain do in general not exceed those observed among the 20 conformers describing the solution structure.     

The interaction of a screw dislocation with point defects in bcc iron

In this study, we calculate the interaction energy of intrinsic point defects vacancies and interstitials) with screw dislocations in body-centered cubic iron. First (we calculate the dipole tensor of a defect in the bulk crystal using molecular statics. Using a formulation based on linear elasticity theory, we calculate the interaction energy of the defect and the dislocation using both isotropic and anisotropic strain fields. Second, we perform atomistic calculations using molecular statics methods to directly calculate the interaction energy. Results from these two methods are compared. We verify that continuum methods alone are unable to correctly predict the interactions of defects and dislocations near the core. Although anisotropic theory agrees qualitatively with atomistics far from the core, it cannot predict which dumbbell orientations are stable and any continuum calculations must be used with caution. Spontaneous absorption by the core of both vacancies and dumbbells is seen. This paper demonstrates and discusses the differences between continuum and atomistic calculations of interaction energy between a dislocation core and a point defect.     

Insights into positron annihilation lifetime spectroscopy by molecular dynamics simulations

The relationship between free-volume properties measured from positron annihilation lifetime spectroscopy (PALS) and calculated from molecular dynamics simulations has been investigated for glassy and liquid glycerol in the temperature range 150–400 K. A virtual probing procedure has been developed to retrieve information on the basic free-volume properties of the simulated microstructures, i.e. mean cavity volume and free-volume cavity fractions. Our data leads us to infer on the occurrence of experimentally non-detectable small cavities with mean equivalent radius of 1.8–1.9 Å between 250 and 275 K. The size of these limiting cavities is found to be temperature dependent, being smaller at low temperatures. At high temperatures, above a characteristic PALS temperature Tb2L , the formation of very large cavities is predicted. This finding suggests that, when the dimension of the holes in the system exceeds a given value, the PALS measurements become unable to catch the complete structural information and phenomena of dynamical origin enter into play in the PALS signal decay. The calculated number of cavities is found to be almost independent on the temperature from the glassy up to the liquid phase, thus furnishing a certain support to theoretical models proposed to evaluate the free-volume cavity fractions.     

Energy flow in thermostatted non-equilibrium molecular dynamics simulations

Energy transfers between internal kinetic and potential energy reservoirs in a simple liquid are studied by setting the temperature of one energy reservoir to a different value from that of the others and computing the resultant energy flows. In the first set of simulations, the x-directional kinetic temperature was artificially raised above the other five, and in the second, the x-directional configurational temperature was artificially raised above the other five. In both cases, external energy flows balanced, but unexpected energy flows between different directional components of the potential energy were observed. Additional simulations showed that these energy flows occurred regardless of the arrangement of thermostats imposed on the six degrees of freedom and the addition of shear. Heat flow between degrees of freedom that were ostensibly at the same temperature was anomalously observed. It was concluded that a different breakdown of the contributions to the configurational energy that is consistent with the definition of the directional configurational temperatures is required.     

Diffusion behavior of hydrogen isotopes in tungsten revisited by molecular dynamics simulations

Molecular dynamics simulations were performed to study the diffusion behavior of hydrogen isotopes in single-crystal tungsten in the temperature range of 300–2000 K. The simulations show that the diffusion coefficient of H isotopes exhibits non-Arrhenius behavior, though this deviation from Arrhenius behavior is slight. Many-body and anharmonic effects of the potential surface may induce slight isotope-dependence by the activation energy; however, the dependence of the pre-factor of the diffusion coefficient on the isotope mass is diminished. The simulation results for H-atom migration near W surfaces suggest that no trap mutations occur for H atoms diffusing near either W{100} or W{111} surfaces, in contrast to the findings for He diffusion near W surfaces. Based on the H behavior obtained by our MD simulations, the time evolution of the concentration distribution of interstitial H atoms in a semi-infinite W single crystal irradiated by energetic H projectiles was calculated. The effect of H concentration on H diffusion is discussed, and the applicability of the diffusion coefficients obtained for dilute H in W is assessed.     

Computing the local pressure in molecular dynamics simulations

Computer simulations of inhomogeneous soft matter systems often require accurate methods for computing the local pressure. We present a simple derivation, based on the virial relation, of two equivalent expressions for the local (atomistic) pressure in a molecular dynamics simulation. One of these expressions, previously derived by other authors via a different route, involves summation over interactions between particles within the region of interest; the other involves summation over interactions across the boundary of the region of interest. We illustrate our derivation using simulations of a simple osmotic system; both expressions produce accurate results even when the region of interest over which the pressure is measured is very small.     

Effective interactions in molecular dynamics simulations of lysozyme solutions

On the basis of ab-initio calculations, we predict the effect of conformation and molecule-electrode distance on transport properties of asymmetric molecular junctions for different electrode materials M (M = Au, Ag, Cu, and Pt). The asymmetry in these junctions is created by connecting one end of the biphenyl molecule to conjugated double thiol (model A) and single thiol (model B) groups, while the other end to Cu atom. A variety of phenomena viz. rectification, negative differential resistance (NDR), switching has been observed that can be controlled by tailoring the interface state properties through molecular conformation and molecule-electrode distance for various M. These properties are further analyzed by calculating transmission spectra, molecular orbitals, and orbital energy. It is found that Cu electrode shows significantly enhanced rectifying performance with change in torsion angles, as well as with increase in molecule-electrode distances than Au and Ag electrodes. Moreover, Pt electrode manifests distinctive multifunctional behavior combining switch, diode, and NDR. Thus, the Pt electrode is suggested to be a good potential candidate for a novel multifunctional electronic device. Our findings are compared with available experimental and theoretical results.