Large-scale molecular dynamics simulations of hyperthermal cluster impact

Using multimillion-atom classical molecular dynamics simulations, we have studied the impact dynamics of solid and liquid spherical copper clusters (10–30 nm radius) with a solid surface, at velocities ranging from 100 m/s to 2 km/s. The resulting shock, jetting, and fragmentation processes are analyzed, demonstrating three distinct mechanisms for fragmentation. At early times, shock-induced ejection and hydrodynamic jetting produce fragments in the normal and tangential directions, respectively, while sublimation (evaporation) from the shock-heated solid (liquid) surface produces an isotropic fragment flux at both early and late times.     

A molecular dynamics approach in simulations of fluid-solid particles flow

In the paper a discrete system of particles carried by fluid is considered in a planar motion. The volumetric density of particles is assumed to be small enough such that they can be treated within the framework of a molecular dynamics model. The fluid is then considered as a carrier of particles. The Landau-Lifshitz concept of turbulence is used to describe the fluctuating part of fluid velocity. This approach is applied to simulate different regimes (laminar and turbulent) and various states of particle motion (moving bed, heterogeneous flow, and homogeneous flow) using only two parameters, which have to be determined experimentally. These two parameters, found for a particular pipe and for a particular velocity from a simple experiment, then can be used for other pipe diameters and different velocities. The computer simulations performed for the flow of particles in pipes at different flow velocities and different pipe diameters agree favorably with experimental observations of the type of flow and critical velocities identifying transitions from one type to another. Received: 8 January 1999     

Nanoscale simulations of Bauschinger effects on a nickel nanowire

In this paper, the Bauschinger effect on a nickel nanowire is studied implementing molecular dynamics simulations in nanoscale. The inter-atomic interactions are represented by employing embedded-atom potential. Initially, the stress-strain curves for tensile and compressive loading are simulated by applying suitable periodic boundary conditions on an infinitely long nanowire. The generated results demonstrate that the yield strength in compression is lower than the tensile yield strength. At the second stage, the tension-followed-by-compression process is applied to the specimen at a predetermined strain rate. It is observed that the resulted yield strength in the reloading or reverse loading is substantially lower than the compressive yield stress in the original direction, a phenomena known as the Bauschinger effect. The reverse loading process is then performed at different strain levels after yield to study the Bauschinger effect variations. To clarify the Bauschinger effects on Ni nanowire, the introduced Bauschinger stress parameter (BP) is employed in the analysis.     

Coarse-grained molecular dynamics simulations of ionic polymer networks

The stress-strain behavior of cross-linked polymeric networks was investigated using molecular dynamics simulations with a coarse-grained representation of the repeating units. The network structure was formed by dynamically cross-linking the reactants placed between two rigid layers comprised of particles of the same type. We studied two types of networks which differ only by one containing ionic pairs that amount to 7% of the total number of bonds present. The stress-strain curves were obtained after imposing deformation in tensile and shear modes to the networks and measuring their stress response. Under both forms of deformations there was improvement in the level of stress that the material could bear. Moreover, the time dependent behavior of the improvement in mechanical properties signified a self-healing mechanism.     

An instrumented structure to measure avalanche impact pressure: Error analysis from Monte Carlo simulations

Avalanche impact pressure on a flat surface has been quantified by full-scale experiments performed at the Lautaret avalanche test site (France). An inverse analysis method is used to reconstruct the pressure applied on this instrumented structure from the deformations recorded during the impact. The deformation-to-pressure transfer function of this large sensor is determined by analytical modelling and is used to quantify the pressure of an artificially released avalanche on the structure. Advanced sensitivity and error analyses are used to determine the uncertainty of the reconstructed pressure. First the uncertainties of the different parameters and processes involved are identified and estimated in the deformation-to-pressure transfer function through probability distributions. Then the sensitivity of the reconstructed pressures to these parameters is investigated to identify the most important parameters. Finally, the distribution of the uncertainty in terms of the probability density of the total error in the pressure is estimated using Monte Carlo simulations. Results are discussed on the basis of the drag coefficient to Froude number dependence and compared with previous avalanche data.     

Molecular dynamics study of cluster impact on the (0 0 1) and (1 1 0) surfaces of fcc metals

Molecular dynamics simulations of the impact deposition of metal clusters on fcc metal surfaces are presented. Two-dimensional elongated islands are formed when the incident cluster travels parallel to the surface. For perpendicular incidence the results of the impact event are very sensitive to the relative cohesive properties of the cluster and substrate atoms.     

Molecular dynamics investigation to indicate facet effects on coalescence processes of two copper clusters with different structures

Molecular dynamics simulations have been conducted to investigate facet effects on coalescence processes of Cu₅₅ and Cu₄₂₉ clusters respectively having icosahedral (Ih) and faced center cubic (FCC) geometries. It is shown that taking into account initial positions of the two clusters, structural changes of two coalescing clusters present different patterns. Simulation results establish the pathway of the structural evolution during coalescing by using shrinkage factors, average energy per atom, mean square displacements as well as atom packing configurations. The coalescence process can be separated into three stages including an approaching stage, a coalescing stage, and a coalesced stage. In the four coalescence processes, the structural transformations mainly occur in the Cu₅₅ clusters. The simulations show that the contact crystallization orientation plays an important role in the coalescing processes and resultant structures.     

Molecular dynamics simulations of carbon nanotube dispersions in water: Effects of nanotube length, diameter, chirality and surfactant structures

Molecular dynamics simulations are used to compute the potential of mean force (PMF) governing the interactions between carbon nanotubes (CNTs) in water/surfactant systems. The effects of CNT length, diameter, chirality (armchair and zigzag) and surfactant structures on CNT interaction and dispersion in water/surfactant systems are investigated for (5, 5), (5, 0), and (10, 10) single walled CNTs with two commonly used surfactants [viz., sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS)] at room conditions. An adaptive biasing force method was used to speed up the calculations. Simulations revealed that CNT length and diameter as well as optimum amount of surfactant addition and its structures can significantly affect CNT interactions (i.e., PMFs vary significantly). Surfactant molecules were found to adsorb at the CNT surface and reduced interaction strength between CNTs. SDBS surfactant contributed weaker interactions between CNTs as compared with that of SDS surfactant by a factor of about 10 indicating that SDBS is better than SDS for dispersing CNTs in an aqueous suspension. This phenomenon agrees qualitatively with the experimental results reported in the literature. The understanding of detailed atomic arrangements and atomic interactions between CNTs and surrounding molecules reported in this study is significantly helpful to computationally screening different surfactants and improving the CNT dispersion in aqueous solution. The method will also facilitate the reduction of time and cost required to produce CNT reinforced nanocomposite materials as well as homogeneous CNT dispersed solutions for many biological applications.     

Thickness and chirality effects on tensile behavior of few-layer graphene by molecular dynamics simulations

The mechanical response of few-layer graphene (FLG), consisting of 2-7 atomic planes and bulk graphite is investigated by means of molecular dynamics simulations. By performing uniaxial tension tests at room temperature, the effects of number of atomic planes and chirality angle on the stress-strain response and deformation behavior of FLG were studied using the Tersoff potential. It was observed that by increasing of the FLG number of layers, the increase of bonding strength between neighboring layers reduce the elastic modulus and ultimate strength. It was found that, while the chirality angle of FLG showed a significant effect on the elastic modulus and ultimate tensile strength of two and three graphene layers, it turns to be less significant when the numbers of layers are more than four. Finally, by plotting the deformation behavior, it was concluded that FLGs present brittle performance.     

Molecular dynamic simulations of uniaxial tension at nanoscale of semiconductor materials for micro-electro-mechanical systems (MEMS) applications

Molecular dynamic (MD) simulations of uniaxial tension at nanoscale were conducted on two semiconductor materials, namely, silicon (Si) and germanium (Ge) to determine their mechanical properties and investigate the nature of deformation under applied load at nanolevel. A general form of Tersoff-type, three-body potential was used for the interaction between the Si atoms and between the Ge atoms in the simulations. Both, Si and Ge were found to exhibit a linear elastic behavior followed by a nonlinear increase in stress in the plastic region up to the ultimate tensile stress (instead of catastrophic brittle fracture soon after the elastic limit, which is typical of most nominally brittle materials at macrolevel). Further loading beyond the ultimate tensile stress resulted in catastrophic failure of these materials by a ductile fracture mode, namely, slip at 45° to the loading direction. The strain at failure was found to be much higher than the corresponding values at macroscale possibly due to the higher loading rates used. Based on the simulation results, the Young's modulii of Si and Ge in the [100] direction were determined to be 130 and 103 GPa, respectively, and the ultimate strengths, 25 and 20 GPa, respectively, at 500 m s⁻¹. These results are in reasonable agreement with the experimental and simulation results reported in the literature. The effect of strain rate via the rate of loading (10–500 m s⁻¹, where 1 m s⁻¹ corresponds to 10⁻² Å ps⁻¹) on the nature of deformation and the measured properties were also investigated. As the rate of loading (or the strain rate) decreases, the stress–strain curves more or less overlap up to the ultimate strength with a slight decrease in the ultimate tensile stress but a significant decrease in the value of strain at failure or strain at ultimate tensile stress.     

Penetration scaling in atomistic simulations of hypervelocity impact

We present atomistic molecular dynamics simulations of the impact of copper nano particles at 5 km s⁻¹ on copper films ranging in thickness from from 0.5 to 4 times the projectile diameter. We access both penetration and cratering regimes with final cratering morphologies showing considerable similarity to experimental impacts on both micron and millimetre scales. Both craters and holes are formed from a molten region, with relatively low defect densities remaining after cooling and recrystallisation. Crater diameter and penetration limits are compared to analytical scaling models: in agreement with some models we find the onset of penetration occurs for 1.0 < f/d_p < 1.5, where f is the film thickness and d_p is the projectile diameter. However, our results for the hole size agree well with scaling laws based on macroscopic experiments providing enhanced strength of a nano-film that melts completely at the impact region is taken into account.     

Ab initio simulations of the mechanics and electrical transport of Pt nanowires

Based on first principles, theoretical studies of atomic-scale platinum contacts are presented. A short monatomic wire freely suspended between tips is seen to vibrate as its tensile load increases. The main vibration mode is transversal for lower tensions and longitudinal for higher tensions up to the breaking of the nanowire. The computed conductance exhibits oscillation in the sub-picosecond regime that can be well correlated to the mechanical oscillations of the nanowire. Both the values for the maximum tensile load and the average conductance agree well with available experimental measurements.     

Computer simulations of martensitic transformations in NiAl alloys

Ni_{1 − x}Al_x alloys in the concentration range 34% < x < 40% exhibit a martensitic transformation from an austenitic phase with bcc structure to a close-packed structured martensitic phase. Above the transformation temperature electron microscopy shows the occurrence of tweed like structures which are accompanied by a considerable softening of the phonon energies at . We have done molecular dynamics simulations employing a semi-empirical model which allows us to study the transformation on an atomistic length scale. Our results show that local distortions of the crystal lattice, which come from the atomic disorder of the alloys, are responsible for the occurrence of tweed phenomena.     

Molecular dynamics simulations of spallation in metals and alloys

We report on the results of large-scale molecular dynamics simulations of the mechanical behavior of two-dimensional metallic systems. The specific impact phenomenon studied is that in which a flyer of mass M moving with x-velocity v impacts a target of mass 2M moving with x-velocity −v/2. Simulations of such a spallation experiment have been performed for a generic metal, modelled with an embedded atom potential and also for a Cu-Ni alloy system, modelled with truncated Lennard-Jones potentials.

Our simulations indicate cold-welding upon impact, and shock wave generation, followed by rebound from the boundaries. The alloy was less ductile than the generic metal and consequently the system came apart due to the cooperative effect of the reflected shock waves.     

Molecular dynamics simulations on nanoindentation mechanisms of multilayered films

Molecular dynamics (MD) simulations were carried out to study the effects of indention deformation, contact, and adhesion on Al, Ni, and Al/Ni multilayered films. The results show that when the indention depth of the sample increased, the maximum load, plastic energy, and adhesion increased. Jump-contact behavior was observed at the beginning of the loading process. Force relaxation and adhesion took place at the holding depth and during the unloading process, respectively. The glide bands of the interface were on the {1 1 1} 1 1 0 slip systems and the maximum width of the glide bands was about 1 nm. The mechanical responses of the indented films are also discussed.     

Modelling of boron nitride: Atomic scale simulations on thin film growth

Molecular-dynamics simulations on ion-beam deposition of boron nitride are presented. A realistic Tersoff-like potential energy functional for boron nitride, which was specially fitted to ab initio-data, has been used. The impact of energetic boron and nitrogen atoms on a c-BN target is simulated with energies ranging from 10 to 600 eV. The structural analysis of the grown films shows that a loose, dominantly sp²-bonded structure arises at high ion flux. In no case the formation of a sp³-bonded phase is observed, but the obtained films partially reveal textured basal planes as found in experiment. Two different growth regimes are identified for ion energies above and below 100 eV.     

Low temperature annealing of metals with electrical wind force effects

Conventional annealing is a slow, high temperature process that involves heating atoms uniformly, i.e., in both defective and crystalline regions. This study explores an electrical alternative for energy efficiency,where moderate current density is used to generate electron wind force that produces the same outcome as the thermal annealing process. We demonstrate this on a zirconium alloy using in-situ electron back scattered diffraction(EBSD) inside a scanning electron microscope(SEM) and juxtaposing the results with that from thermal annealing. Contrary to common belief that resistive heating is the dominant factor, we show that 5 × 10~4 A/cm~2 current density can anneal the material in less than 15 min at only135?C. The resulting microstructure is essentially the same as that obtained with 600?C processing for360 min. We propose that unlike temperature, the electron wind force specifically targets the defective regions, which leads to unprecedented time and energy efficiency. This hypothesis was investigated with molecular dynamics simulation that implements mechanical equivalent of electron wind force to provide the atomistic insights on defect annihilation and grain growth.     

Atomistic simulations of spallation dynamics in multilayer thin-film interface excited by femtosecond laser

This paper presents an atomic-scale analysis of delaminating dynamics for characterizing microscopic mechanisms of interfacial spallation at multilayer thin-film interface excited by femtosecond pulse laser. For the first time via a molecular dynamics (MD) approach to investigate the interfacial spallation induced by pulse laser, the standard form of 12–6 Lennard-Jones (L-J) model and a solid-state argon interface are introduced. To allow MD modeling of interfacial spallation being conducted effectively, various laser incident energy densities and pulse durations are employed to characterize the dynamic behaviors and evolutions of interfacial spallation at multilayer thin-film interface. Based on the results of simulation, three different progressive stages, including void nucleation, coalescence leading to crack, and interfacial spallation, are classified via the transient temperature, pressure and density trajectories. The extraordinary expansive dynamics and tension stress induced by relaxation of thermal and pressure wave are major factors leading to detrimental defects growth and enlargement. The same conclusion can be further verified from the viewpoint of energy trajectories. Moreover, the ultra-high strain rate of the order 10⁹ s⁻¹ is estimated. The result is analogous to that of the experimental result of metal-film spallation excited by pulse laser. Finally, a critical strain-rate is evaluated and the dominant mechanism of the interfacial fracture is also presented.     

Molecular dynamic simulations on tensile mechanical properties of single-walled carbon nanotubes with and without hydrogen storage

Using a bond order potential, molecular dynamics (MD) simulations have been performed to study the mechanical properties of single-walled carbon nanotubes (SWNTs) under tensile loading with and without hydrogen storage. (10,10) armchair and (17,0) zigzag carbon nanotubes have been studied. Up to the necking point of the armchair carbon nanotube, two deformation stages were identified. In the first stage, the elongation of the nanotube was primarily due to the altering of angles between two neighbor carbon bonds. Young's Modulus observed in this stage was comparable with experiments. In the second stage, the lengths of carbon bonds are extended up to the point of fracture. The tensile strength in this stage was higher than that observed in the first stage. Similar results were also found for the zigzag carbon nanotube with a lower tensile strength. Hydrogen molecules stored in the nanotubes reduced the maximum tensile strength of the carbon nanotubes, especially for the armchair type. The effect may be attributed to the competitive formation between the hydrogen–carbon and the carbon–carbon bonds.