Many-body Embedded-atom Potential Research Articles

Effects of grain size and zirconium concentration on mechanical properties of nanocrystalline copper grain boundary doping

ABSTRACT Alloying nanocrystalline Cu (nc–Cu) with immiscible elements (i.e. Zr) is a promising approach for reducing microstructural instability and inhibiting grain growth. Understanding the relationship between the grain size, Zr concentration, deformation, and mechanical properties of the nc–Cu–Zr system is essential for practical applications. Molecular dynamics simulations based on the many-body embedded-atom potential were used for the related analysis from an atomistic point of view. The grain size of nc–Cu was varied from 3 to 9 nm and the Zr concentration was varied from 0% to 7%. The simulation results show that doping Zr atoms into an nc–Cu system enhances tensile strength and ductility, but weakens compressive and shear strengths. Doping 1% Zr into nc–Cu greatly increases both ultimate tensile strength and ultimate tensile strain; the increase is insignificant for higher Zr concentrations (2% to 7%). For a given Zr concentration, the relationship between ultimate tensile strength and grain size is unclear. Grain boundary sliding dominates the elastic deformation mechanism of the nc–Cu–Zr system under tensile, compressive, and shear tests. Tensile fracture occurs faster for an nc–Cu–Zr system with a larger grain size.

Molecular dynamics simulation of strengthening of nanocrystalline Cu alloyed with Zr

Nanocrystalline Cu-Zr alloys are an emerging class of immiscible high-strength materials with significant potential for structural and high-temperature applications. The effects of Zr concentration and tensile velocity on the strengthening and mechanics of Cu alloyed with Zr is studied using molecular dynamics simulations based on the many-body embedded-atom potential. The simulation results show that the potential energy of Cu-Zr systems significantly decreases with increasing Zr concentration, resulting in an increase in structural stabilization. Doping Zr atoms at the grain boundaries can strengthen the boundaries and reduce their mobility during the tensile test. The Young’s modulus of Cu-Zr systems decreases with increasing Zr concentration. The plasticity of Cu-Zr systems increases with increasing Zr concentration. The mechanical strength of Cu-Zr systems reaches its maximum value, which is 1.13 times that of the pure Cu counterpart, when the Zr concentration is 5%.

Molecular dynamics simulation of effects of microstructure and loading mode on mechanical properties of Au nanowires

ABSTRACT The effects of microstructure and loading mode on the mechanical properties of Au nanowires (NWs) are studied using molecular dynamics simulations based on the many-body embedded-atom potential. The effects are investigated in terms of common neighbour analysis, shear strain distribution, and stress-strain curves. The microstructure effect is examined using single-crystalline (SC), nanocrystalline (NC), and amorphous (AC) structures. The simulation results show that during tension, SC, NC, and AC NWs have visually similar deformations and similar ultimate strains, but completely different deformation mechanisms. For all loading modes, the SC NWs have the highest tensile and shear strength values and the NC NWs have the highest compressive strength. The AC NWs have poor strength under tensile, compressive, and shear loadings. The presence of grain boundaries increases the geometric instability of the NC NWs under compression and shear loadings.

Effects of interfacial defect on deformation and mechanical properties of Cu/Ni bilayer—A molecular dynamics study

The deformation and mechanical properties of Cu/Ni bilayer films with and without an interfacial defect, void, or asperity under tensile and shear tests are studied using molecular dynamics simulations based on the many-body embedded-atom potential. These properties are investigated in terms of atomic trajectories, the dislocation extraction algorithm, common neighbor analysis, shear strain distribution, and the stress-strain curve. The simulation results show that during the tensile test, defect-free bilayer films have higher ultimate strength and strain than those of bilayer films with a void or asperity. The bilayer films with an interfacial void cause dislocation and crack nucleation at the void. For a given defect size, a void has more influence than that of an asperity on the ultimate stress and strain of bilayer films under tension. The presence of an asperity increases the shear deformation resistance of bilayer films.

Effects of alloy composition, cavity aspect ratio, and temperature of imprinted ZrCu metallic glass films: a molecular dynamics study

The effects of alloy composition, cavity aspect ratio, and temperature on the pattern transfer mechanism and mechanics of ZrCu metallic glass (MG) films under a nanoimprinting process are studied using molecular dynamics simulations based on the many-body embedded-atom potential. The simulation results show that the glass transition temperature of ZrCu MGs increases with increasing Zr content. Imprinting at higher temperatures or with a higher cavity aspect ratio shortens filling time. Shear origin zones are more concentrated for imprinting at lower imprint temperatures or a higher cavity aspect ratio. The loading force increases with increasing cavity aspect ratio and decreasing Zr content. The springback ratio of a pattern increases with increasing cavity aspect ratio and decreasing Zr contare the film heights including and ent. For imprinting with a high cavity aspect ratio, the pattern width has much more springback than that of the pattern height. The sharpness of patterns significantly decreases with increasing unloading temperature.

Atomistic analysis of nanoextrusion process for fabrication of gold nanowires

The effects of extrusion temperature and velocity on extruded gold nanowires (NWs) are studied using molecular dynamics simulations based on the many-body embedded-atom potential. The effects are investigated in terms of atomic trajectories, common neighbor analysis, flow field, and the extruded NW length-pressing ram displacement curve. The simulation results show that NWs extruded at a temperature of 300 K are longer and have a more uniform cross-sectional area compared to those extruded at higher temperatures. Higher temperature increases the cross-sectional area of extruded NWs, whereas higher extrusion velocity decreases it. In the extrusion process, dislocations first nucleate around the mold outlet and propagate along the close-packed plane {111} toward the interior. The number of disordered structures significantly increases with increasing extrusion temperature and velocity.

Molecular dynamics study on deformation and mechanics of nanoscale Au/Cu multilayers under indentation.

The effects of individual layer thickness, indentation velocity, and temperature on the mechanical properties and mechanics of nanoscale Au/Cu multilayers under indentation were studied using molecular dynamics simulations based on the many-body embedded-atom potential. The simulation results show that layer interfaces act as strong barriers that resist the propagation of dislocations, even at an extremely small individual layer thickness of 3nm. The number of dislocations increases significantly and the growth of dislocations decreases with decreasing individual layer thickness. There is no clear relationship between the magnitude of the required indentation force and the number of film layers; however, the average required indentation force increases with increasing indentation velocity and decreasing temperature. During indentation at a relatively low velocity, dislocation propagation is more significant; the number of disordered atoms significantly increases at a relatively high indentation velocity.

Atomistic simulation of effects of surface notches and loading mode on deformation and mechanics of ZrNi metallic glass

The effects of surface notches and loading mode on the mechanical deformation and mechanics of ZrNi metallic glass (MG) are studied using molecular dynamics simulations based on the many-body embedded-atom potential. The effects are investigated in terms of atomic trajectories, shear strain distributions, and stress-strain curves. The simulation results show that for ZrNi MG, resistance to shear deformation (shear strain > 200%) before breaking is much greater than that to tensile and compressive deformation. For ZrNi MG under tension, a pre-existing notch leads to earlier necking and breaking. Significant stress concentration occurs around the notch root when the notch length (L) is 3 nm or above, and dominates plastic deformation. For ZrNi MG under compression, a pre-existing notch is completely filled by neighboring atoms at the initial stage of compression. A pre-existing notch leads to single-edge barreling and weakens a sample’s ultimate strength when the L value is 3 nm or above. For ZrNi MG under shear loading, a pre-existing notch does not influence the shear modulus of samples; however, their ultimate strength decreases with increasing L value.

Atomic study of effects of crystal structure and temperature on structural evolution of Au nanowires under torsion

The effect of temperature on the structural evolution of nanocrystalline (NC) and single-crystalline (SC) Au nanowires (NWs) under torsional deformation is studied using molecular dynamics simulations based on the many-body embedded-atom potential. The effect is investigated using common neighbor analysis and discussed in terms of shear strain distribution and atomic flow field. The simulation results show that deformation for NC NWs is mainly driven by the nucleation and propagation of dislocations and the gliding of grain boundaries (GBs) and that for SC NWs is mainly driven by dislocations and the formation of disordered structures. Dislocations for NC and SC NWs easily nucleate at GBs and free surfaces, respectively. For NC NWs, torsional buckling occurs easily at GBs with large gliding. SC NWs have a more uniform and larger elastic deformation under torsion compared to that for NC NWs due to the former's lack of grains. SC NWs have a long period of elastic deformation transforming into plastic deformation. Increasing temperature facilitates stress transmission throughout NWs.

Molecular dynamics analysis of plastic deformation and mechanics of imprinted metallic glass films

The effects of mold cavity geometry and imprinting temperature on imprinted amorphous ZrCu films are studied using molecular dynamics simulations based on the many-body embedded-atom potential. The effects are investigated in terms of shear strain, flow field, loading force, relationship between pattern filling height and mold displacement, and pattern elastic recovery. The simulation results show that imprinting should be conducted at high temperatures (slightly above the glass transition temperature), followed by unloading at 300 K. Imprinting at higher temperatures shortens the pattern filling time, decreases the required loading force, and lowers the shear strain concentration inside the films. Unloading at 300 K leads to a low pattern elastic recovery, which increases with increasing temperature. Imprinting using a groove pattern on the mold requires a larger loading force and less filling time compared to those required when using a tip-like pattern on the mold. For imprinting using a tip-like pattern on the mold at temperatures of 300–475 K, shear transformation zones form inside the films in the form of a ring surrounding the mold.

Atomistic Simulation of ZrNi Metallic Glasses Under Torsion Test

ZrNi metallic glass alloy nanowires (NWs) under torsion are studied using molecular dynamics simulations based on the many-body embedded-atom potential. The effect of cooling rate on the deformation mechanism and mechanical properties of ZrNi NWs is evaluated in terms of shear strain, torque, potential energy and radial distribution function. Simulation results show that for slower cooling rates, the NWs have larger packing density, whereas for faster cooling rates, the packing density of atoms decreases. The amount of deformation increases with increasing torsional angle before it reaches a critical torsional angle ([Formula: see text]. The torque required for deformation and the [Formula: see text] value increase with decreasing cooling rate, indicating a larger mechanical strength. Localized shear bands concentrate at regions with high shear strains, leading to the formation of torsional buckling.

Atomistic simulation of effects of temperature and velocity on tensile-deformed Au/Cu/Au/Cu films

The effects of temperature and tensile velocity on the mechanical properties and mechanics of Au/Cu/Au/Cu nanofilms under tensile testing are studied using molecular dynamics simulations based on the many-body embedded-atom potential. The simulation results show that at the initial tensile deformation, dislocations nucleate at layer interfaces, propagate along close-packed plane {111} toward the interior of the film, and are then terminated by layer interfaces, which act as a barrier. The proportions of atoms in face-centered cubic and hexagonal close-packed structures during tensile deformation decrease and that in other structures (disordered atoms) increases when the temperature increases from 273 to 700K. Breaking occurs at layer interfaces at higher temperatures or tensile velocities, indicating that the interface is the weakest structure in the films. Breaking occurs earlier when a higher tensile velocity is used. The mechanical strength of the films increases with decreasing temperature and increasing tensile velocity.

Atomistic simulation study of tensile deformation in nanocrystalline and single-crystal Au.

The effect of notch size (r) on nanocrystalline (NC) and single-crystal (SC) Au at a temperature of 300 K under tension testing is studied using molecular dynamics simulations based on the many-body embedded-atom potential. The effect is investigated in terms of atomic trajectories, common neighbor analysis, and the stress-strain curve. The simulation results show that for the NC samples, stacking faults nucleate at grain boundaries (GBs), propagate inside grains, and eventually are terminated by GBs or transect them. For tensile-deformed NC and SC samples with larger r values, necking and breaking occur faster, which indicates that ductility decreases. SC samples exhibit larger yield strength and ultimate strength than those of NC samples. SC samples have a longer elastic deformation period due to their lack of GBs. For NC samples, increasing the r value increases Young's modulus but decreases the yield point; in addition, it decreases mechanical strength and speeds up the formation of necking.

Structure and Stability of an Amorphous Metal

Using molecular dynamics simulations, with a realistic many-body embedded-atom potential, and a novel method to characterize local order, we study the structure of pure nickel during the rapid quench of the liquid and in the resulting glass. In contrast with previous simulations with pair potentials, we find more crystalline order and fewer icosahedra for slower quenching rates, resulting in a glass less stable against crystallization. It is shown that there is not a specific amorphous structure, only the arrest of the transition from liquid to crystal, resulting in small crystalline clusters immersed in an amorphous matrix with the same structure of the liquid.

On the structure and mobility of dislocations in NiAl

Dislocation core structures of 〈100〉, 〈110〉 and 〈111〉 dislocations in NiAl have been studied by molecular statics calculations using a new many-body embedded atom potential. The response of dislocation cores to applied homogeneous shear stresses is investigated and the Peierls stresses of straight dislocations having edge, screw and mixed character determined. The results are generally in good agreement with experimental observations. Finally, the mobility of dislocations in single crystals oriented for hard slip is discussed in a simple line-tension model.

Unimolecular decomposition in the sputtering of metal clusters.

The stability of metal clusters ${\mathit{M}}_{\mathit{n}}$ sputtered from single-crystal surfaces by keV and sub-keV ${\mathrm{Ar}}^{+}$ ions was investigated by a molecular-dynamics (MD) simulation using a many-body embedded-atom potential. It is shown that the clusters identified within the sputtered flux immediately after the ion impact contain an average internal energy of roughly 1 eV per constituent atom. Consequently, most of the sputtered trimers and tetramers and virtually all ejected clusters with n\ensuremath{\ge}5 are found to be unstable and therefore decompose into stable fragments on a time scale of several tens of picoseconds after the ejection. The contribution of fragmentation to the yields, translational and internal energy distributions of stable and, hence, experimentally detectable dimers, trimers, and tetramers were calculated. It is found that this contribution increases strongly with increasing primary-ion energy ${\mathit{E}}_{\mathit{B}}$ and becomes comparable to the contribution of ``intrinsic'' stable molecules (i.e., such molecules that are identified immediately after the sputtering process) at ${\mathit{E}}_{\mathit{B}}$=5 keV. It is shown that the experimental data available on ${\mathrm{Ag}}_{\mathit{n}}$ multimers sputtered from polycrystalline silver can be reproduced very well by the MD simulation if fragmentation processes are included in predicting the yields and distributions.

Rh atom ejection from keV ion-bombarded p(2×2)O/Rh{111}: Adsorption site and coverage determination from angle-resolved desorption measurements

Angular distributions of Rh atoms desorbed by energetic ion bombardment of an oxygen covered Rh{111} surface are measured accurately using a multiphoton resonance ionization (MPRI) detection technique. The results, in conjunction with molecular dynamics calculations of the ion impact event show that these distributions reflect the near-surface crystal structure. The molecular dynamics calculations were performed using a many-body embedded-atom potential to describe the dynamics of the Rh atoms and a pair-wise additive potential to describe the oxygen–Rh interactions. Several oxygen overlayer structures were considered for molecular dynamics modeling of the desorption process, including p(2×2) overlayers with a coverage of 0.25 monolayer (ML), and p(2×1) overlayers with a coverage of 0.50 ML, both of which are consistent with low energy electron diffraction (LEED) data. Three different adsorption sites were tested: threefold symmetric sites over second layer Rh atoms, threefold symmetric sites over third layer Rh atoms, and atop sites. The calculated azimuthal angular distributions of desorbed Rh atoms for each of these cases are unique, matching the experimental data best in the case of a p(2×1) overlayer with oxygen atoms adsorbed in threefold symmetric sites over third layer Rh atoms. The calculated Rh atom desorption yield (ejected atoms per incident ion) is sensitive to the oxygen coverage in the range 0.25–0.50 ML. These calculations are important in developing a surface bonding site and coverage consistent with LEED and our experiments. The peak in energy distribution of ejected Rh atoms from the oxygen covered surface is at a lower energy value than that of the clean metal. This indicates that collisional energy loss processes contribute to determining the peak position as well as the well known binding energy effect.

Many-body embedded-atom potential for describing the energy and angular distributions of Rh atoms desorbed from ion-bombarded Rh{111}

In this paper, we show that many-body interactions are important for describing the energy- and angle-resolved distributions of neutral Rh atoms ejected from keV-ion-bombarded Rh{111}. We compare separate classical-dynamics simulations of the sputtering process assuming either a many-body potential or a pairwise additive potential. The many-body potential is constructed using the embedded-atom method to describe equilibrium properties of the crystal, parameters from the Moli\`ere potential to describe close encounters between energized atoms, and parameters from a ${\mathrm{Rh}}_{2}$ potential to aid the description of the desorption event. The most dramatic difference between the many-body potential and the pair potential is in the predicted kinetic energy distributions. The pair-potential kinetic energy distribution peaks at \ensuremath{\sim}2 eV, whereas the many-body potential predicts a broader peak at \ensuremath{\sim}4 eV, giving much better agreement with experiment. This difference between the model potentials is due to the predicted nature of the attractive interaction in the surface region through which all ejecting particles pass. Variations of the many-body-potential parameters are examined in order to ascertain their effect on the predicted energy and angular distributions. A specific set of parameters has been found which leads to excellent agreement with recent experimental trajectory measurements of desorbed Rh atoms.