Search

Large-scale nonequilibrium molecular dynamics (MD) simulations of shock waves in nanocrystalline iron show evidence of plasticity before the polymorphic transformation takes place. The atomistic structure in the shock direction shows an elastic precursor, plastic deformation, and shock-induced phase transformation from bcc to hcp iron. In this Rapid Communication, large-scale MD models show that the shock response of iron is highly related to the ramp time of the applied shocks. For long ramp times we observe significant plastic relaxation and formation of microstructure defects. Pressure-induced phase transformations in iron are accompanied by stress relaxation achieving almost fully relaxed three-dimensional hydrostatic final states. The evolution of the stress relaxation is in agreement with theory and experiments. Analysis of the x-ray diffraction patterns calculated from the atomistic structure using the Debye equation revealed pronounced anisotropy of the line broadening that is caused by stacking faults in hcp Fe and by dislocations in bcc Fe.

Shock induced Richtmyer-Meshkov instabilities at perturbed metal vacuum/gas interfaces result in metal material ejecta. For strong shock waves, the material ejected is initially in the form of fluid sheets when the machining grooves are two-dimensional. These sheets ultimately break up to form a distribution of droplets. Large-scale non-equilibrium molecular dynamics simulations of Richtmyer-Meshkov instabilities allow the investigation of the dynamics of the breakup process but are limited in length and time scales by rarefaction wave reflections at the boundaries, which put the material behind the perturbed interface under tension eventually leading to spall. A time-dependent boundary condition based on the self-similar character of the release wave is shown to mitigate boundary reflections and reduce unwanted tensile waves behind the perturbed interface zone and thereby can be used to increase the wavelength and time scale for breakup simulations. We discuss the details of this method and results of NEMD simulations of a shocked Cu interface with a single mode perturbation characterized by a wavelength of λ = 13 nm and a wavenumber amplitude product kh0 = 1/2.

Strain rate, temperature, and microstructure play a significant role in the mechanical response of materials. Using non-equilibrium molecular dynamics simulations, we characterize the ductile tensile failure of a model body-centered cubic metal, tantalum, over six orders of magnitude in strain rate. Molecular dynamics calculations combined with reported experimental measurements show power-law kinetic relationships that vary as a function of dominant defect mechanism and grain size. The maximum sustained tensile stress, or spall strength, increases with increasing strain rate, before ultimately saturating at ultra-high strain rates, i.e. those approaching or exceeding the Debye frequency. The upper limit of tensile strength can be well estimated by the cohesive energy, or the energy required to separate atoms from one another. At strain rates below the Debye frequency, the spall strength of nanocrystalline Ta is less than single crystalline tantalum. This occurs in part due to the decreased flow stress of the grain boundaries; stress concentrations at grain boundaries that arise due to compatibility requirements; and the growing fraction of grain-boundary atoms as grain size is decreased into the nanocrystalline regime. In the present cases, voids nucleate at defect structures present in the microstructure. The exact makeup and distribution of defects is controlled by the initial microstructure and the plastic deformation during both compression and expansion, where grain boundaries and grain orientation play critical roles.

Large-scale non-equilibrium molecular dynamics (NEMD) simulations are used to examine the role of dislocation density and strain rate on twin nucleation and deformation twinning in tantalum. The initial dislocation density was varied between 1011 − 1012 cm−2. The samples were compressed quasi-isentropically to pressures up to 100 GPa, at strain-rates in the 108 − 1012 s−1 range. The twin volume fraction, dislocation density and peak shear stress were evaluated as a function of strain and strain rate. Main results indicate that at these high strain rates, twinning in tantalum is strongly dependent on the dislocation density and strain (pressure). Deformation twinning, as measured by the twin volume fraction (TVF), increases with increase in strain rate for strain rates > 109 s−1. Below this value, a small fraction of twins nucleates but anneal out with time. Samples with lower fraction of twins equilibrate to defect states containing higher dislocation densities from those with initially higher twin volume fractions.Large-scale non-equilibrium molecular dynamics (NEMD) simulations are used to examine the role of dislocation density and strain rate on twin nucleation and deformation twinning in tantalum. The initial dislocation density was varied between 1011 − 1012 cm−2. The samples were compressed quasi-isentropically to pressures up to 100 GPa, at strain-rates in the 108 − 1012 s−1 range. The twin volume fraction, dislocation density and peak shear stress were evaluated as a function of strain and strain rate. Main results indicate that at these high strain rates, twinning in tantalum is strongly dependent on the dislocation density and strain (pressure). Deformation twinning, as measured by the twin volume fraction (TVF), increases with increase in strain rate for strain rates > 109 s−1. Below this value, a small fraction of twins nucleates but anneal out with time. Samples with lower fraction of twins equilibrate to defect states containing higher dislocation densities from those with initially higher twin volume...

We present the results of 138 million and 1.8 billion atom Non-Equilibrium Molecular Dynamics (NEMD) simulations for Al-Al sliding friction at pressures of 15 GPa. Three-dimensional samples comprised of 4 nm, 20 nm and 50 nm grains were studied to times of 117 ns for the largest systems. We discuss the evolution of the initial grain size distribution to a steady state distribution that is statistically similar for all initial grain sizes. We compare the results for the frictional force to a rate dependent model that incorporates plasticity and discuss the relationships among grain size, grain morphology, dislocations and other defect structures, and plasticity.We present the results of 138 million and 1.8 billion atom Non-Equilibrium Molecular Dynamics (NEMD) simulations for Al-Al sliding friction at pressures of 15 GPa. Three-dimensional samples comprised of 4 nm, 20 nm and 50 nm grains were studied to times of 117 ns for the largest systems. We discuss the evolution of the initial grain size distribution to a steady state distribution that is statistically similar for all initial grain sizes. We compare the results for the frictional force to a rate dependent model that incorporates plasticity and discuss the relationships among grain size, grain morphology, dislocations and other defect structures, and plasticity.

We present Non-Equilibrium Molecular Dynamics (NEMD) simulations of shock wave compression along the [001] direction in monocrystalline Tantalum, including pre-existing defects which act as dislocation sources. We use a new Embedded Atom Model (EAM) potential and study the nucleation and evolution of dislocations as a function of shock pressure and loading rise time. We find that the flow stress and dislocation density behind the shock front depend on strain rate. We find excellent agreement with recent experimental results on strength and recovered microstructure, which goes from dislocations to a mixture of dislocations and twins, to twinning dominated response, as the shock pressure increases. Fil: Tramontina Videla, Diego Ramiro. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Ministerio de Ciencia. Tecnología e Innovación Productiva. Agencia Nacional de Promoción Cientifíca y Tecnológica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentina Fil: Erhart, Paul. Lawrence Livermore National Laboratory; Estados Unidos. Chalmer University of Technology; Suecia Fil: Germann, Timothy. Los Alamos National Laboratory; Estados Unidos Fil: Hawreliak, James. Los Alamos National Laboratory; Estados Unidos Fil: Higginbotham, Andrew. University of Oxford. Department of Physics; Reino Unido Fil: Park, Nigel. Atomic Weapons Establishment. Materials Modeling Group; Reino Unido Fil: Ravelo, Ramón. Los Alamos National Laboratory; Estados Unidos. University of Texas at El Paso; Estados Unidos Fil: Stukowski, Alexander. Universitat Technische Darmstadt; Alemania Fil: Suggit, Mathew. University of Oxford. Department of Physics; Reino Unido Fil: Tang, Yizhe. University Johns Hopkins; Estados Unidos Fil: Wark, Justin. University of Oxford. Department of Physics; Reino Unido Fil: Bringa, Eduardo Marcial. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentina

When a crystal is subjected to shock compression beyond its Hugoniot Elastic Limit (HEL), the deformation it undergoes is composed of elastic and plastic strain components. In situ time-dependent X-ray diffraction, which allows direct measurement of lattice spacings, can be used to investigate such phenomena. This paper presents recent experimental results of X-ray diffraction from shocked fcc crystals. Comparison is made between experimental data and simulated X-ray diffraction using a post-processor to Molecular Dynamics (MD) simulations of shocked fcc crystals.

Generation and motion of dislocations and twinning are the main mechanisms of plastic deformation. A new mechanism of plastic deformation and stress relaxation at high strain rates (10 9 –10 12 s -1 ) is proposed, under which virtual melting occurs at temperatures much below the melting temperature. Virtual melting is predicted using a developed, advanced thermodynamic approach and confirmed by large-scale molecular dynamics simulations of shockwave propagation and quasi-isentropic compression in both single and defective crystals. The work and energy of nonhydrostatic stresses at the shock front drastically increase the driving force for melting from the uniaxially compressed solid state, reducing the melting temperature by 80% or 4,000 K. After melting, the relaxation of nonhydrostatic stresses leads to an undercooled and unstable liquid, which recrystallizes in picosecond time scales to a hydrostatically loaded crystal. Characteristic parameters for virtual melting are determined from molecular dynamics simulations of Cu shocked/compressed along the and directions and Al shocked/compressed along the direction.

We present the results of large scale non equilibrium molecular dynamics simulations of plasticity induced by a shock wave in a perfect fcc single crystal in the orientation [1 0 0]. Shockley loops are thermally nucleated behind the shock front. The algorithm used to identify and follow the time evolution of the dislocation loops is detailed. It enables the measure of the critical size that a slip fluctuation can reach before it shrinks back to a perfect configuration.

With the advent of large-scale parallel computers in the past decade, it has become possible to study the microphysics of complex nonlinear many-body systems at scales approaching the mesoscale. One of the areas where nonequilibrium deformation arises is that of dry sliding friction between ductile metals. We discuss the results of atomistic simulation studies of sliding friction in the velocity range 10 to 103 m/s in the high-pressure regime of 5 to 15 GPa for Ta/Al and Cu/Ag tribopairs. We discuss the velocity dependence and the near interface deformation seen in these simulations.

Large-scale molecular dynamics simulations are used to investigate the dislocation structure behind a shock front in perfect fcc crystals. Shock compression in both the 〈100〉 and 〈111〉 directions induces dislocation loop formation via a sequential emission of partial dislocations, but in the 〈100〉 case, this process is arrested after the first partial, resulting in stacking-fault loops. The large mobility of the bounding partial dislocations results in a plastic wave that is always overdriven in the 〈100〉 direction; the leading edges of the partials are traveling with the plastic front, as in the models of Smith and Hornbogen. In contrast, both partials are emitted in 〈111〉 shock compression, resulting in perfect dislocation loops bounded only by thin stacking fault ribbons due to the split partial dislocations. These loops grow more slowly than the plastic shock velocity, so new loops are periodically nucleated at the plastic front, as suggested by Meyers.

We use multimillion-atom molecular dynamics simulations to study shock wave propagation in fcc crystals. As shown recently, shock waves along the direction form intersecting stacking faults by slippage along {l_brace}111{r_brace} close-packed planes at sufficiently high shock strengths. We find even more interesting behavior of shocks propagating in other low-index directions: for the case, an elastic precursor separates the shock front from the slipped (plastic) region. Shock waves along the direction generate a leading solitary wave train, followed (at sufficiently high shock speeds) by an elastic precursor, and then a region of complex plastic deformation. (c) 2000 The American Physical Society.

We have performed calculations of Sn deposition on Cu(111) and Cu(100) surfaces. The atomic interactions are described by modified embedded atom method (MEAM) potentials. This is a modification of the embedded atom method (EAM) to include higher moments in the electron density. We find that at low coverages Sn deposited on Cu(111) leads to the formation of a two-dimensional (2D) alloy phase with a p((3)1/2×(3)1/2)-R 30° structure which is stable up to temperatures of 1200 K. For deposition of Sn on Cu(100), a coverage of one-quarter of a monolayer results in the formation of a stable 2D alloy phase with a p(2×2) structure. These results are in agreement with ion-scattering experiments. It is found that on both Cu(100) and Cu(111) surfaces, the resulting alloy phases are rippled with the Sn atoms displaced outward from the surfaces.

We have calculated free energy and vibrational entropy differences in Ni3Al between its equilibrium ordered structure and a disordered fcc solid solution. The free energy and entropy differences were calculated using the method of adiabatic switching in a molecular-dynamics formalism. The path chosen for the free-energy calculations directly connects the disordered with the ordered state. The atomic interactions are described by embedded-atom-method potentials. We find that the vibrational entropy difference increases with temperature from 0.14kB/atom at 300 K to 0.22kB/atom at 1200 K. We have calculated the density of states (DOS) of the disordered phase from the Fourier transform of the velocity-velocity autocorrelation function. The disordered DOS looks more like a broadened version of the ordered DOS. Analysis of the partial density of states shows that the Al atoms vibrations are most affected by the compositional disorder. The phonon partial spectral intensities along the 〈100〉 direction show that the vibrational spectrum of the disordered phase contains intensities at optical mode frequencies of the ordered alloy. We find that the volume difference between the ordered and disordered phases plays the most crucial role in the magnitude of the vibrational entropy difference. If the lattice constant of the two phases is set to the same value, the vibrational entropy difference decreases to zero.

The thermodynamic properties of various phases of tin are calculated employing the method of adiabatic switching with modified embedded atom method (MEAM) potentials. The experimental $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\beta}$ and $\ensuremath{\beta}\ensuremath{\rightarrow}\mathrm{liquid}$ transition temperatures are reproduced within an 11% accuracy. Good agreement with experiments is also obtained for other thermodynamic quantities. We demonstrate the versatility and accuracy of MEAM by how well it reproduces both metallic and covalent phases of tin over a wide temperature range and over a wide range of densities.

Utilizing a modified embedded atom method potential, we performed large-scale classical molecular dynamics simulations (with up to 50 million atoms) to investigate the response of Ga single crystals to shock compression along the three major orientations of the orthorhombic A11 ground state, i.e., $[001]$, $[010]$, and $[100]$. For weak shocks with particle velocity ${u}_{p}l30$0 m/s, these defect-free single crystals respond elastically, but for stronger shocks, they undergo a structural phase transformation and then (for even stronger shocks) melt. For intermediate shock strengths ($300\phantom{\rule{4.pt}{0ex}}\text{m/s}l{u}_{p}l1.2$ km/s) a split shock wave is formed, with an elastic precursor (uniaxial compression wave) leading the slower transformation wave. The transformed region consists of a mixed phase, with stripes of the product phase embedded into the uniaxially compressed parent phase, with the ratio of product to parent phase increasing with increasing shock strength, much like in martensitic phase transformations. Upon shock release from the free surface at the end of the sample, the transformation is reversed, leaving only some defects behind, which makes it difficult to experimentally investigate the structural transformation in shock-recovered samples. We investigated the structure produced by the shock and found it to be similar to the $\ensuremath{\beta}$ phase of Ga which is obtained by supercooling from the liquid state. However, the product phase shows only half the period along $[100]$ in the calculated diffraction pattern. Further investigation showed that this is due to a different stacking sequence in this direction, namely ABCD instead of AB for the $\ensuremath{\beta}$ phase.

We report on large-scale nonequilibrium molecular dynamics simulations of shock wave compression in tantalum single crystals. Two new embedded atom method interatomic potentials of Ta have been developed and optimized by fitting to experimental and density functional theory data. The potentials reproduce the isothermal equation of state of Ta up to 300 GPa. We examined the nature of the plastic deformation and elastic limits as functions of crystal orientation. Shock waves along (100), (110), and (111) exhibit elastic-plastic two-wave structures. Plastic deformation in shock compression along (110) is due primarily to the formation of twins that nucleate at the shock front. The strain-rate dependence of the flow stress is found to be orientation dependent, with (110) shocks exhibiting the weaker dependence. Premelting at a temperature much below that of thermodynamic melting at the shock front is observed in all three directions for shock pressures above about 180 GPa.

This study uses classical molecular dynamics to simulate infinite nuclear matter and study the effect of isospin asymmetry on bulk properties such as energy per nucleon, pressure, saturation density, compressibility, and symmetry energy. The simulations are performed on systems embedded in periodic boundary conditions with densities and temperatures in the ranges $\ensuremath{\rho}=0.02$ to $0.2\phantom{\rule{4pt}{0ex}}{\mathrm{fm}}^{\ensuremath{-}3}$ and $T=1$, 2, 3, 4, and 5 MeV, and with isospin content of $x=Z/A=0.3$, 0.4, and 0.5. The results indicate that symmetric and asymmetric matter are self-bound at some temperatures and exhibit phase transitions from a liquid phase to a liquid-gas mixture. The main effect of isospin asymmetry is found to be a reduction of the equilibrium densities, a softening of the compressibility and a disappearance of the liquid-gas phase transition. A procedure leading to the evaluation of the symmetry energy and its variation with the temperature was devised, implemented and compared to mean field theory results.

We report on recent molecular-dynamics (MD) fracture simulations of mode-I tensile loading at high strain rates. Because cracks emit sound waves, previous simulations became unreliable beyond one sound traversal time. Using massively parallel MD, we show how to eliminate unwanted boundary effects and study unimpeded crack propagation mechanisms. In order to represent tensile stress conditions near the crack tip, we employ uniaxial, homogeneously expanding periodic boundary conditions, examining the effects of strain rate, temperature, and interaction potential. Because our samples are sufficiently large, we see dislocations being emitted from the crack tip at nearly the shear-wave sound speed ${\mathit{c}}_{\mathit{s}}$. As they move many lattice spacings away from the crack, they slow down, finally moving at about 2/3${\mathit{c}}_{\mathit{s}}$. Each time dislocations are emitted, the crack tip ``fishtails,'' and at sufficiently high strain, the crack can fork; dislocations can climb and become nucleation sites for additional microcracks. We find that we can suppress ductile behavior by including viscous damping in the equations of motion, thereby demonstrating a transition to brittle crack propagation as static, zero-strain-rate conditions are approached. Finally, we show that, by altering only the attractive tail of the pair potential, we can change a ductile material into a brittle one. Under dynamic crack propagation, the distinction between ductile and brittle behavior is blurred: in brittle materials, dislocations are asymptotically bound to the crack tip, while in ductile materials, they can escape.

We report on large-scale non-equilibrium molecular dynamics (NEMD) simulations of shock wave compression in Ta single crystals. The atomic interactions are modeled via a recently developed and optimized embedded-atom method (EAM) potential for Ta, which reproduces the equation of state up to 200 GPa. We examined the elastic-plastic transition and shock wave structure for wave propagation along the low index directions: (100), (110) and (111). Shock waves along (100) and (111) exhibit an elastic precursor followed by a plastic wave for particle velocities below 1.1 km/s for (100) and 1.4 km/s for (111). The nature of the plastic deformation along (110) is dominated by twinning for pressures above 41 GPa.

Non-Equilibrium Molecular Dynamics (NEMD) simulations have exhibited characteristic velocity weakening for the tangential frictional force at smooth single crystal interfaces for velocities greater than a critical velocity, vc. This behavior has been seen in a number of material pairs including Cu-Ag, Ta-Al and Al-Al. Expressions for vc that characterize this behavior depend on system size. We discuss the size dependence for Al-Al single crystal interfaces for two cases, an Al(111)/Al(001) interface sliding along [1-10], N=1.5 106, and an Al(110)[001]/Al(110)[1-10] interface sliding along [001], N=7.5 106, where N is the number of atoms, corresponding to a three-fold increase in system size normal to the sliding direction. We find agreement with an inverse size scaling for vc.

We present molecular dynamics simulations of shock-induced plasticity and spall damage in single crystal Ta described by a recently developed embedded-atom-method (EAM) potential and a volumedependent qEAM potential. We use impact or Hugoniotstat simulations to investigate the Hugoniots, deformation and spallation. Both EAM and qEAM are accurate in predicting, e.g., the Hugoniots and γ - surfaces. Deformation and spall damage are anisotropic for Ta single crystals. Our preliminary results show that twinning is dominant for [100] and [110] shock loading, and dislocation, for [111]. Spallation initiates with void nucleation at defective sites from remnant compressional deformation or tensile plasticity. Spall strength decreases with increasing shock strength, while its rate dependence remains to be explored.

We present molecular-dynamics studies of structural transformations on metallic fcc(111) surfaces as a function of surface misfit and temperature. The particles interact through modified Lennard-Jones potentials. The studies focus on how the characteristic features of the surface-surface interatomic potential, specifically its repulsive core, determine the variation of surface misfit with temperature, and the stability of the ensuing structural phases and the nature of the observed phase transitions. The low-temperature phase consists of a striped phase of double-sine-Gordon domain walls oriented along one of the three equivalent 〈110〉 directions and stacked along the 〈112〉 direction. The temperature evolution of this phase was followed for two typical and distinct systems: in one system (system I), the surface-surface potential was chosen so as to make the effective surface misfit decrease with temperature, while in the other (system II), the misfit increased or remained constant with temperature. System I undergoes an incommensurate-commensurate phase transition which appears to be second order in nature while system II exhibits an incommensurate-incommensurate phase transition. The temperature evolution of system II closely parallels the behavior of the Au(111) surface with rise in temperature.

Using a recently proposed equation for the heat-flux vector that goes beyond Fourier’s Law of heat conduction, we model shockwave propagation in the dense Lennard-Jones fluid. Disequilibrium among the three components of temperature, namely, the difference between the kinetic temperature in the direction of a planar shock wave and those in the transverse directions, particularly in the region near the shock front, gives rise to a new transport (equilibration) mechanism not seen in usual one-dimensional heat-flow situations. The modification of the heat-flow equation was tested earlier for the case of strong shock waves in the ideal gas, which had been studied in the past and compared to Navier–Stokes–Fourier solutions. Now, the Lennard-Jones fluid, whose equation of state and transport properties have been determined from independent calculations, allows us to study the case where potential, as well as kinetic contributions are important. The new heat-flow treatment improves the agreement with nonequilibrium molecular-dynamics simulations under strong shock wave conditions, compared to Navier–Stokes.

We model strong shock-wave propagation, both in the ideal gas and in the dense Lennard-Jones fluid, using a refinement of earlier work, which accounts for the cold compression in the early stages of the shock rise by a nonlinear, Burnett-like, strain-rate dependence of the thermal conductivity, and relaxation of kinetic-temperature components on the hot, compressed side of the shock front. The relaxation of the disequilibrium among the three components of the kinetic temperature, namely, the difference between the component in the direction of a planar shock wave and those in the transverse directions, particularly in the region near the shock front, is accomplished at a much more quantitative level by a rigorous application of the Cattaneo-Maxwell relaxation equation to a reference solution, namely, the steady shock-wave solution of linear Navier-Stokes-Fourier theory, along with the nonlinear Burnett heat-flux term. Our new continuum theory is in nearly quantitative agreement with nonequilibrium molecular-dynamics simulations under strong shock-wave conditions, using relaxation parameters obtained from the reference solution. © © 2011 American Physical Society.

We have performed large‐scale molecular‐dynamics (MD) simulations of shock loading and quasi‐isentropic compression in defective copper crystals, modeling the interatomic interactions with an embedded‐atom method potential. For samples with a relatively low density of pre‐existing defects, the strain rate dependence of the flow stress follows a power law in the 109–1012 s−1 regime with an exponent of 0.40. For initially damaged, isotropic crystals the flow stress exhibits a narrow linear region in strain rate, which then bends over at high strain rates in a manner reminiscent of shear thinning in fluids. The MD results can be described by a modification of Eyring’s theory of Couette shear flow in fluids.

We propose a model embedded-atom (many-body) potential and test it against an effective, density-independent, pairwise-additive potential in a variety of nonequilibrium molecular-dynamics simulations of plastic deformation under high stress. Even though both kinds of interactions have nearly the same equilibrium equation of state, the defect energies (i.e., vacancy formation and surface energies) are quite different. As a result, we observe significant qualitative differences in flow behavior between systems characterized by purely pairwise interactions versus higher-order many-body forces.

The surface dynamics of the dislocation-mediated reconstruction of the Au(111) surface is investigated using three dimensional Molecular Dynamics. At low temperatures the surface phonon dispersions are calculated along the direction. The computational results reproduce the experimentally observed anomalies in the surface phonon curves and provide detailed interpretation of their origin, and of the polarization of the different phonon branches. The anomaly in the vertical-shear branch is found to be due to a cross-over to localization when the phonon wavelength becomes shorter than the size of the dislocations. The high temperature dynamics is followed by trajectory diagrams.

We present 3D‐nonequilibrium molecular dynamics results for the velocity dependence of the frictional force at smooth sliding interfaces for Ta and Al single crystals. For Ta/Al we consider Al(100)/Ta(100) and Al(111)/Ta(110) interfaces sliding along [001] and [110]fcc /[001]bcc respectively. These are compared with Al(111)/Al(100) interfaces at the same loads, corresponding to a pressure of 15 GPa. Both interfacial pairs show similar behavior in the velocity dependence of the frictional force: a low velocity regime with an increasing frictional force followed by a strain induced transformation regime at velocities above approximately 1/10 the transverse sound speed, followed by a fluidized interface at high velocities. For both interfacial pairs, the high velocity dependence of the frictional force exhibits power law behavior, Ft ∝ v−β with β=3/4. We discuss the structural changes that influence dissipation in each of these regimes.

Shock‐induced melting in Al, Cu and Lennard‐Jones single crystals was investigated employing large‐scale non‐equilibrium molecular dynamics (NEMD) and constant‐stress Hugoniostat simulations. The atomic interactions in Cu and Al are modeled with embedded atom method potentials. The solid and liquid Hugoniot of these materials were generated for shock propagation along the low‐index orientations: (100),(111) and (110). It is found that within sub‐nanosecond time‐scales, shock melting occurs at lower pressures (temperatures) for (110) shocks, at intermediate pressures (temperatures) for (111) and at higher pressures (temperatures) for (100) shocks.

We present an alternative equilibrium molecular dynamics method---the uniaxial constant-stress Hugoniostat---for following the dynamical evolution of condensed matter subjected to shock waves. It is a natural extension of the recently developed uniaxial constant-volume Hugoniostat [Maillet et al., Phys. Rev. E 63, 016121 (2001)]. Integral feedback is employed to reach the Hugoniot (final) state of the shock process by controlling both the normal component of the stress tensor and internal energy. The finite strain rate imposed on the system is closely related to that inherent in the front of a shock wave. The method can easily identify phase transitions along the Hugoniot shock states, even those that exhibit multiple wave structures. As an example of the method, we have simulated the Hugoniot of a Lennard-Jones crystal shocked along the $⟨110⟩$ direction. The results agree well with multi-million-atom nonequilibrium molecular-dynamics simulations.

We present the results of large-scale NonEquilibrium Molecular Dynamics (NEMD) simulations for Cu/Ag interfaces sliding in the velocity regime 0≤v≤1Km/sec. System sizes of 2.8 × 106 atoms are considered using Embedded Atom Method (EAM) potentials. Single crystals with 010 interfaces sliding along the direction are considered. We discuss the observed velocity weakening in the tangential force at high velocities, and its connection with the observed dislocation structure and nanostructure that are nucleated during dry sliding.

The physics of sliding at compressed Ta/Al interfaces is discussed based upon the results of large scale 3D NEMD simulations. A new set of Embedded Atom Method potentials has been constructed to treat the Ta‐Al interaction. Pressures of order 15 GPa are studied and the velocity dependence of the frictional force is studied for several interfacial configurations including Al(100)/Ta(100) and Al(111)/Ta(110). Generic behavior is observed, characterized by a linear increase at low velocities followed by a power law decrease at high velocities associated with near interface structural transformation in Al.

We study the nucleation mechanism of the defects responsible for plastic flow in a $〈100〉$ fcc perfect single crystal submitted to a shock wave. In the large-scale nonequilibrium molecular dynamics simulation, small dislocation loops are created from thermal fluctuations just behind the shock front, in a narrow region of a few lattice parameters width. Their critical size is measured. The activation energy for the formation of an edge dipole, under high pressure, is computed within the Peierls framework. The elastic constants and the generalized stacking fault energy are computed from the interatomic potential. This model enables a qualitative discussion of the influence of material parameters such as intrinsic and unstable stacking faults versus elastic energy release.

We have examined the effect of evolution of grain morphology on the frictional force at polycrystalline Al-Al and Al-Ta interfaces as a function of grain size and sliding velocity. We present the results of 8M, 26M and 138M particle NonEquilibrium Molecular Dynamics (NEMD) simulations for grain sizes of 13 and 20 nm. Sample sizes consisted of 3?3?3 and 5?5?5 grains on each side of a sliding interface. We have considered sliding velocities from 20 to 4000 m/s. For velocities below a size dependent critical velocity above which a fluid layer forms, we find enhanced grain coarsening leading to a highly strained, graded final steady state microstructure that exhibits a dynamic morphology for times greater than 5-10 ns. We find that the frictional force is insensitive to the initial grain size distribution due to the evolution of the initial distribution to a new nonequilibrium steady state. We discuss the relationship of these results to single crystal interfaces and the mechanisms for grain size and shape evolution.

An new equilibrium molecular-dynamics method (the uniaxial Hugoniostat) is proposed to study the energetics and deformation structures in shocked crystals. This method agrees well with nonequilibrium molecular-dynamics simulations used to study shock-wave propagation in solids and liquids.

We have performed calculations of Sn deposition on Cu(111) and Cu(100) surfaces. The atomic interactions are described by modified embedded atom method (MEAM) potentials. This is a modification of the embedded atom method (EAM) to include higher moments in the electron density. We find the at low coverages Sn deposited on Cu(111) leads to the formation of a two-dimensional (2D) alloy phase with a p (√3 × √3)-R 30° structure which is stable up to temperatures of 900K. These results are in agreement with ion-scattering experiments of thin films of Sn on Cu(111). For deposition of Sn on Cu(100), a 0.25 monolayer (ML) coverage results in the formation of a stable 2D alloy phase with a p(2 × 2) structure. This result is also in agreement with LEED measurements.

Using Molecular Dynamics, the evolution dynamics of Sn on the (111) and (100) surfaces of Cu have been investigated as a function of coverage and temperature. The interaction potentials are described by modified embedded atom method (MEAM) potentials. The calculated diffusion activation energies of Cu in Sn and Sn in Cu agree reasonably well with experimental values. We find that the structure of the overlayer depends on the morphology of the substrate and remains stable up to temperatures of the order of 70% of the melting temperature of the substrate at which diffusion of Sn into the substrate and Cu atoms onto the overlayer is observed.

The compounds in the binary system Ag2Se-Ag2S are known to undergo a phase transition to a superionic phase at a temperature which varies with concentration. The diffusion mechanism of Ag ions have been studied in a Molecular Dynamics formalism. We have calculated the diffusion coefficient as function of temperature and concentration and have calculated the activation energy for diffusion from the Arrhenius plot of DAg vs temperature. Results obtained for the end points Ag2Se and Ag2S agree with experimental measurements.

The Nos\'e-Hoover thermostat, which is often used in the hope of modifying molecular dynamics trajectories in order to achieve canonical-ensemble averages, has hidden in it a Toda ``demon,'' which can give rise to unwanted, noncanonical undulations in the instantaneous kinetic temperature. We show how these long-lived oscillations arise from insufficient coupling of the thermostat to the atoms, and give straightforward, practical procedures for avoiding this weak-coupling pathology in isothermal molecular dynamics simulations.

We have studied brittle and ductile behavior and their dependence on system size and interaction potentials, using molecular-dynamics (MD) simulations. By carefully embedding a single sharp crack in two- and three-dimensional crystals, and using a variant of the efficient sound-absorbing reservoir of Holian and Ravelo [Phys. Rev. B 51, 11275 (1995)], we have been able to probe both the static and dynamic crack regimes. Our treatment of boundary and initial conditions allows us to elucidate early crack propagation mechanisms under delicate overloading, all the way up to the more extreme dynamic crack-propagation regime, for much longer times than has been possible heretofore (before unwanted boundary effects predominate). For example, we have used graphical display of atomic velocities, forces, and potential energies to expose the presence of localized phonon-like modes near the moving crack tip, just prior to dislocation emission and crack-branching events. We find that our careful MD method is able to reproduce the ZCT brittle-ductile criterion for short-range pair potentials [static lattice Green's function calculations of Zhou, Carlsson, and Thomson, Phys. Rev. Letters 72, 852 (1994)].We report on progress we have made in large-scale 3D simulations in samples that are thick enough to display realistic behavior at the crack tip, including emission of dislocation loops. Such. calculations, using our careful treatment of boundary and initial conditions - especially important in 3D - have the promise of opening up new vistas in fracture research.

We report on Molecular Dynamics (MD) studies of free energy and vibrational entropy differences of the binary system Ag2Se-Ag2S as function of temperature. We have employed the methods of adiabatic switching and temperature integration over a reversible path to obtain the phase diagram. We discuss the nature of the entropy difference in terms of ion mobility and previous MD calculations of diffusion rates in these systems.

We discuss the nucleation from glassy to crystalline order in quenched one-component Yukawa plasmas as studied by molecular dynamics. Rapid quenches from [ital T][ge]4[ital T][sub melt] to [ital T][le]0.5[ital T][sub melt] were studied and a range of Yukawa exponents were considered for systems with up to 10 000 particles. We report on the effect of system size on nucleation and the relevance of classical nucleation theory to the observed nonequilibrium dynamics.

We study the double-sine-Gordon kink-kink collisions in a formalism which employs collective variables to describe the internal oscillations of the kinks and their translational motion in their center-of-mass frame. The equations of motion are solved in the absence of the radiation field and dynamical dressing and the results are compared with numerical molecular-dynamics simulations. We investigate the energy exchange between the translational and internal modes, and a mechanism is proposed to explain the values of the translational velocity at which maximum energy is exchanged between the two modes.

We study the stability and dynamics on the reconstructed Au(111) surface using three-dimensional molecular-dynamics simulations. The small-amplitude surface-phonon dispersions are calculated along the 〈110〉 direction and compared with inelastic-He-scattering data. The agreement with the experimental data is remarkable, given that the parameters of the surface interatomic potentials are chosen solely to reproduce the size and type of the observed surface dislocations. Low-lying localized modes (soliton modes) appear in all three polarizations and localized modes with polarization in the sagittal plane are observed around the K point with energies of about 8 meV.