Terms Of Atomic Trajectories Research Articles

Effects of extrusion ratio and direction on extruded Au nanowires investigated using molecular dynamics simulation

The effects of the extrusion ratio (diameter of the extrusion mold inlet divided by that of its outlet) and direction on extruded Au nanowires (NWs) are studied using molecular dynamics simulations based on the embedded-atom method. The effects are investigated in terms of atomic trajectories, common neighbor analysis, flow field, system pressure, and the extruded NW length-pressing ram displacement curve. The simulation results show that for the forward extrusion process, a smaller extrusion ratio leads to a larger diameter of the extruded NW and a smoother cross-sectional profile. Extruded NWs obtained at a smaller extrusion ratio have a higher face-centered cubic fraction and a less disordered structure due to lower system pressure. The adhesion effect between extruded NWs and mold outer walls increases with increasing extrusion ratio, resulting in an uneven appearance of extruded NWs and a decrease in their length. The backward extrusion process can produce amorphous NWs from crystal NWs. The forward extrusion process can produce NWs that are longer than those produced with the backward extrusion process.

Effects of tool rake angle and workpiece surface roughness on nanocutting of cu investigated using Multiscale simulation

ABSTRACT The effects of tool rake angle and workpiece surface roughness on the cutting mechanism and mechanics of Cu are studied using quasi-continuum simulations. These effects are investigated in terms of atomic trajectories, stress distribution, cutting force, resistance factor, and cutting ratio. The simulation results show that negative rake angles lead to a larger cutting force, lower resistance coefficient, and higher cutting ratio than those obtained with positive rake angles. For tools with an initial rake angle of 2° to 8°, the rake angle becomes negative during the cutting process due to the strong adhesion interaction between the tool rake face and chips. The resistance coefficient decreases with increasing tool rake angle for rake angles in the range of 2° to 8°. Tools with negative rake angles of −2° to −8° have similar cutting ratios. The cutting ratio decreases and cutting force increases with increasing surface roughness of the workpiece.

Effects of constituting material and interfacial crack on mechanical response of nanoscale metallic bilayers – a quasi-continuum study

ABSTRACT The effects of the constituting material and an interfacial crack on the deformation and mechanical response of metallic bilayer films under tension are studied using quasi-continuum simulations in terms of atomic trajectories, stress distribution and the stress–strain curve. The simulation results show that bilayer films with identical constituting monolayers have higher tensile strength than that of those with different monolayers. For bilayer films with different monolayers, the layer interface acts as a barrier that resists dislocation propagation. Dislocation emission at a crack tip blunts the tip. Necking originates at the layer interface and develops on monolayers with relatively low strength. A rectangular crack is less likely to develop than a V-shaped crack.

Mechanical response of nanoporous nickel investigated using molecular dynamics simulations.

The effect of pore size on the deformation mechanism and mechanical properties of nanoporous Ni under tension and compression tests is studied using molecular dynamic simulations in terms of atomic trajectories, dislocation extraction algorithm, and the stress-strain curve. The simulation results show that samples have a longer elastic deformation period during tension compared to that during compression. Dislocations nucleate at pore surfaces and propagate until they are terminated by neighboring pores. Samples under tension have lower ultimate stress and higher strain at ultimate stress compared to those of samples under compression. Samples with smaller pore diameter have more transformation from face-centered cubic to hexagonal close-packed structures due to more dislocation activity. The ultimate stress of samples significantly decreases with increasing pore diameter.

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.

FRACTURE AND CRACK PROPAGATION OF METALLIC BILAYERS USING QUASI-CONTINUUM SIMULATIONS

The effects of the constituting material, initial crack length, and crystal orientation on the fracture and mechanics of bilayers with an interface crack under tension are studied using quasi-continuum simulations in terms of atomic trajectories, strain distribution, the stress-strain curve, and the crack growth-strain curve. The simulation results show that at the initial tension stage, the bilayers exhibit linear elasticity regardless of initial crack length and constituting material. With an increase in initial crack length at a layer interface, the yield stress, yield strain, and ultimate stress of the bilayers decrease. Bilayers with along er initial crack under tension have faster crack growth. Bilayers with a structural orientation of [020] versus [101̅] have lower mechanical strength compared to that of those with structural orientations of [100] versus [010] and [1̅10] versus [010]. The Ni/Ni bilayer has the highest yield stress and ultimate stress under tension.

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.

Effects of mold pattern characteristics on nanoimprinted aluminum investigated using quasi-continuum simulations

The effects of mold pattern characteristics on the mechanical deformation and mechanics of nanoimprinted aluminum are studied using quasi-continuum simulations in terms of atomic trajectories, strain distribution, and the imprinting stress-mold displacement curve. The simulation results show that a mold whose pattern has a larger period (S value) produces a larger imprinting stress on the substrate for deformation. The effect of mold taper angle on imprinting stress depends on the S value. The required imprinting stress for substrate deformation decreases when mold wear occurs, resulting in the appearance of more non-local atoms in the substrate because of a nonuniform stress distribution. The nucleation and propagation of dislocations in the substrate are significantly confined when the S value is very small (e.g., 150 Å).

Effects of Temperature and Alloy Composition on Nanomechanical Properties of ZrCu Metallic Glass under Tension

Background: The nanomechanical properties of Zr50Cu50 metallic glass under tension are studied. The effects of temperature and alloy composition are investigated in terms of atomic trajectories, slip vectors, stress-strain curve, and radial distribution function. Methods: The molecular dynamics simulations based on the many-body tight-binding potential are applied to analyze the nanomechanical properties of metallic glass under tension. Results: The mechanical properties of the metallic glass are sensitive to temperature and alloy composition. Under tensile deformation, the stress increases with increasing temperature and Zr content in the alloys. At higher temperatures, the alloy atoms have high slip vectors, and plasticity becomes more homogeneous due to a better flow ability of atoms. Conclusion: The alloys with higher Zr content have larger mechanical strengths. The alloys with higher Cu content have more stable structures.

Nanowelding of nickel and copper investigated using quasi-continuum simulations

The effects of contact interference, crystal orientation, and material type on the cold nanowelding mechanism and mechanics are studied using quasi-continuum simulations. These effects are investigated in terms of atomic trajectories, strain distribution, and the stress–strain curve. The simulation results show that welding using a contact interference of 0 nm leads to the formation of the fewest defects inside the material during cold welding, and that the number of defects increases with increasing contact interference. For the Ni–Ni welding pair, welding using a contact interference of 0 nm with the structural orientation [110] versus [001] has the largest ultimate strength and longest elongation during tensile testing due to the formation of fewer defects during welding. The welding quality obtained with the Ni–Ni welding pair is higher than that obtained with Ni–Cu and Cu–Cu welding pairs. During the tensile deformation process, dislocations nucleate from the free surface, propagate along the close-packed plane, and then terminate at the other free surface.

Quasi-continuum simulations of side-to-side nanowelding of metals

The effects of contact length, crystal orientation, and material type of welded pairs on side-to-side nanowelding are studied using quasi-continuum simulations. These effects are investigated in terms of atomic trajectories, strain distributions, and stress-strain curves. The simulation results show that the welding strength of welded pairs increases with decreasing contact length, regardless of their structural orientations. Welding with structural orientations of [Formula: see text] (for one nanowire) and [111] (for the other nanowire) results in a significant yield phenomenon during the separation process due to the migration of the deformation region from the root of tips to the interior of the top tip above the welding interface. Welding with structural orientations of [111] and [Formula: see text] results in relatively poor elasticity and ductility. For welding with one type of material, during the separation process, damage may occur at the top tip instead of at the welding interface. The welding strength of the Ni-Ni welded pair is higher than those of the Ni-Cu and Cu-Cu welded pairs.

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.

Quasi-continuum Simulations of Solid-state Pressure Nanowelding of Metals

Background: Nanowelding is an attractive bottom-up fabrication technique that allows the construction, connection, and repair of nanomaterials. The effects of contact interference, crystal orientation, and material type of nanoscale welded pairs are investigated in terms of atomic trajectories, strain distribution, and the stress-strain curve. Methods: The quasi-continuum (QC) method is applied to simulate nanowelding process. The QC method is a multi-scale combined molecular dynamics approach that mixes atomistic and continuum algorithms. Results: At a small contact interference of 0-1 nm, the ultimate stress of welded pairs is independent of the magnitude of contact interference. Regarding the effect of material type, the average ultimate stress for Ni-Ni welded pairs is the highest and that for Cu-Cu welded pairs is the lowest, regardless of the contact orientation mode. The elongation of Ni-Ni welded pairs increases with increasing contact interference. Conclusion: The crystal orientation of the contact plane and material type significantly determine the welding quality. Welding on the close-packed plane of both substrates leads to the highest ultimate stress. The ultimate stress for Ni-Ni welding pairs is the highest, and that for Cu- Cu welded pairs is the lowest.

Comparison of subsurface damages on mono-crystalline silicon between traditional nanoscale machining and laser-assisted nanoscale machining via molecular dynamics simulation

Abstract Molecular dynamics is employed to compare nanoscale traditional machining (TM) with laser-assisted machining (LAM). LAM is that the workpiece is locally heated by an intense laser beam prior to material removal. We have a comprehensive comparison between LAM and TM in terms of atomic trajectories, phase transformation, radial distribution function, chips, temperature distribution, number of atoms in different temperature, grinding temperature, grinding force, friction coefficient and atomic potential energy. It can be found that there is a decrease of atoms with five and six nearest neighbors, and LAM generates more chips than that in the TM. It indicates that LAM reduces the subsurface damage of workpiece, gets a better-qualified ground surface and improves the material removal rate. Moreover, laser energy makes the materials fully softened before being removed, the number of atoms with temperature above 500 K is increased, and the average temperature of workpiece higher and faster to reach the equilibrium in LAM. It means that LAM has an absolute advantage in machining materials and greatly reduces the material resistance. Not only the tangential force (Fx) and the normal force (Fy) but also friction coefficients become smaller as laser heating reduces the strength and hardness of the material in LAM. These results show that LAM is a promising technique since it can get a better-qualified workpiece surface with larger material removal rates, less grinding force and lower friction coefficient.

Mechanical properties and crack growth behavior of polycrystalline copper using molecular dynamics simulation

This study investigated the mechanical properties and crack propagation behavior of polycrystalline copper using a molecular dynamics simulation. The effects of temperature, grain size, and crack length were evaluated in terms of atomic trajectories, slip vectors, common neighbor analysis, the material’s stress–strain diagram and Young’s modulus. The simulation results show that the grain boundary of the material is more easily damaged at high temperatures and that grain boundaries will combine at the crack tip. From the stress–strain diagram, it was observed that the maximum stress increased as the temperature decreased. In contrast, the maximum stress was reduced by increasing the temperature. With regard to the effect of the grain size, when the grain size was too small, the structure of the sample deformed due to the effect of atomic interactions, which caused the grain boundary structure to be disordered in general. However, when the grain size was larger, dislocations appeared and began to move from the tip of the crack, which led to a new dislocation phenomenon. With regards to the effect of the crack length, the tip of the crack did not affect the sample’s material when the crack length was less than 5 nm. However, when the crack length was above 7.5 nm, the grain boundary was damaged, and twinning structures and dislocations appeared on both sides of the crack tip. This is because the tip of the crack was blunt at first before sharpening due to the dislocation effect.

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.

Nanometric mechanical cutting of metallic glass investigated using atomistic simulation

The effects of cutting depth, tool nose radius, and temperature on the cutting mechanism and mechanics of amorphous NiAl workpieces are studied using molecular dynamics simulations based on the second-moment approximation of the many-body tight-binding potential. These effects are investigated in terms of atomic trajectories and flow field, shear strain, cutting force, resistance factor, cutting ratio, and pile-up characteristics. The simulation results show that a nanoscale chip with a shear plane of 135° is extruded by the tool from a workpiece surface during the cutting process. The workpiece atoms underneath the tool flow upward due to the adhesion force and elastic recovery. The required tangential force and normal force increase with increasing cutting depth and tool nose radius; both forces also increase with decreasing temperature. The resistance factor increases with increasing cutting depth and temperature, and decreases with increasing tool nose radius.

A molecular dynamics investigation into the mechanisms of material removal and subsurface damage of nanoscale high speed laser-assisted machining

Molecular dynamics is employed to study the mechanism of material removal and subsurface damage of monocrystalline silicon when it is under a nanoscale high-speed laser-assisted grinding of a diamond tip. Laser-assisted machining (LAM) is that the workpiece is locally heated by an intense laser beam before material removal. The effects of laser moving speed, laser pulse intensity and laser spot radius are thoroughly investigated in terms of atomic trajectories, phase transformation, temperature distribution, grinding temperature, grinding force and friction coefficient. The investigation shows that a higher laser moving speed reduces the subsurface damage and improves the material remove rate because of fewer atoms with five and six coordination atoms and more chips. Besides, both tangential grinding force (Fx) and normal grinding force (Fy) decrease as the laser moving speed increases. The distribution of high-temperature zone strongly depends upon the effect of laser pulse intensity and laser spot radius. Larger laser pulse intensity can make the material more fully softened before being removed. Moreover, as the laser pulse intensity becomes larger, the friction coefficients became smaller, the material remove rate improves and the depth of grinding increases. However, larger laser pulse intensity may result in a larger thermal deformation of workpiece. A larger laser spot radius reduces the grinding depth but increases the width of laser irradiation zone on machined surface. Thus, a suitable laser spot radius can improve the material removal rate. These results indicate that it is possible to control and adjust the laser parameters according to laser moving speed, laser pulse intensity and laser spot radius, and it provides a potential technology to improve a surface integrity and a smoothness of ground surface.

Molecular dynamics simulation of nanotribology properties of CuZr metallic glasses

The effects of scratch depth, scratch speed, and alloy composition on the mechanical deformation and nanotribology properties of CuZr metallic glasses are studied using molecular dynamics simulations based on the second-moment approximation of the many-body tight-binding potential. These effects are investigated in terms of atomic trajectories, slip vectors, friction force, normal force, and friction coefficient. The simulation results show that a few shear transformation zones independently develop at the contact area between the probe tip and the film. Pileup occurs in the nanoscratch process but not during nanoindentation at a depth of 2.4 nm. There are two areas on the surface where the atoms have high slip vector values during nanoscratching. These areas form due to the removal of atoms that piled up around the probe tip and those behind the probe tip, respectively. Both the friction force and the normal force increase with increasing scratch depth and scratch speed. Friction coefficients decrease with increasing scratch depth, scratch speed, and Zr content in films.

Nanoextruded NbTi superconductor nanowires investigated using molecular dynamics simulations

The effects of extrusion temperature, extrusion ratio, and workpiece size on the extrusion process of NbTi alloy nanowires (workpieces) are studied using the modified embedded atom method potential. The results are discussed in terms of atomic trajectories, potential energy, extrusion force, and stress. Simulation results show that the workpiece atoms near the ram lose their structural order when the extrusion process begins. The number of disordered atoms gradually increases with increasing ram displacement in order to relax the increasing pressure from the ram. The extrusion force and potential energy increase with increasing temperature. The effect of extrusion ratio dominates the extrusion force once the workpiece atoms start entering the mold opening. A large extrusion force is required for large workpieces with a small extrusion ratio; however, a small workpiece with a large extrusion ratio could lead to high peaks and a large oscillation in the force curve.