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Stacked graphene (GE) membranes with cascading nanoslits can be synthesized economically compared to monolayer nanoporous GE membranes, and have potential for molecular separation. This study focuses on investigating the seawater desalination performance of these stacked GE layers as forward osmosis (FO) membranes by using molecular dynamics simulations. The FO performance is evaluated in terms of water flux and salt rejection and is explained by analysing the water density distribution and radial distribution function. The water flow displays an Arrhenius type relation with temperature and the activation energy for the stacked GE membrane is estimated to be 8.02 kJ mol−1, a value much lower than that of commercially available FO membranes. The study reveals that the membrane characteristics including the pore width, offset, interlayer separation distance and number of layers have significant effects on the desalination performance. Unlike monolayer nanoporous GE membranes, at an optimum layer separation distance, the stacked GE membranes with large pore widths and completely misaligned pore configuration can retain complete ion rejection and maintain a high water flux. Findings from the present study are helpful in developing GE-based membranes for seawater desalination via FO.

Graphene has great potential to be used in flexible electronics owing to its novel physical and electronic properties, and super-high mechanical strength. However, its small mechanical strain is unable to meet the needs for most flexible and stretchable device applications where large deformation occurs. Kirigami is one of the useful techniques to increase the flexibility of graphene under deformation. In this study, we investigate the shear flexibility of kirigami graphene under shear loading by using molecular dynamics simulations. We found that kirigami is very efficient in modulating the shear properties of graphene. With the trade-off between strength and flexibility, the shear flexibility of kirigami graphene can be improved significantly. The flexibility can be further improved by using longer interior cuts. Our results indicate that kirigami can be a promising technique to manipulate the shear properties of graphene and other two-dimensional nanomaterials.

To examine the effect of aspect ratio, atomistic simulations are performed on the tensile failure of Cu 50 Zr 50 metallic glasses (MGs) with aspect ratios from 2:1 to 12:1. All samples fail via a single shear band (SB), and the Young’s modulus and the tensile strength are independent of the aspect ratio. For long MGs, however, an abrupt unstable shear banding failure is observed, while a combined abrupt unstable shear banding and slow stable SB sliding is observed for short MGs.

The mechanical properties of a sandwich architecture composed of Cu 50 Zr 50 nanoglass (NG)/Cu 50 Zr 50 metallic glass (MG)/Cu 50 Zr 50 NG are investigated using molecular dynamics simulation. The plastic deformation along the glass–glass interfaces in NG layers and the compressive residual stress in the MG layer provide the enhanced ductility, manifested as improved strain localization and delayed shear band formation; meanwhile the strength remains high in the MG layer. We provide routes to obtain strong, ductile MGs and to protect MGs by NG surface coating.

In the growth of polycrystalline graphene via chemical vapor deposition, grain boundaries (GBs) were shown to be energetically favorable sites for hydrogenation. Thus, it is of both scientific interest and technological significance to understand how hydrogenation on GBs affects the mechanical properties of polycrystalline graphene. Here we perform molecular dynamics simulations to investigate the mechanical properties of hydrogenated polycrystalline graphene. Our simulations reveal that the fracture strength of hydrogenated polycrystalline graphene is significantly reduced by the combined weakening effect of bond prestraining in highly defective GBs and sp3 hybridization of hydrogenated GB atoms. In addition, this reduction in fracture strength due to GB hydrogenation is observed in polycrystalline graphene samples of different grain sizes ranging from 2.5 to 10 nm. Our findings show that the loss of mechanical strength due to GB hydrogenation must be taken into account in the application of polycrystall...

We perform molecular dynamics simulations on large-area polycrystalline graphene containing a pre-existing circular notch with focus on the notch effect on tensile strength and failure pattern. Our results show that the failure of polycrystalline graphene becomes notch-sensitive if there is an overlapping of stress concentration zones induced by the notch and grain boundaries (GBs); otherwise, the failure becomes notch-insensitive. More specifically, when the notch diameter is larger than the average grain size, the failure is generally notch-sensitive. However, if the notch size is smaller than the grain size, whether the failure is notch-sensitive or not depends on the notch location. These observations can be well explained by following attributes: (1) Both GBs and circular notch can create stress concentration; (2) The stress concentration created by the notch is generally weaker than that by GBs; and (3) The strength of GBs is weaker than that of the grain interior. Our work provides useful guideline for designing polycrystalline graphene for structure and device applications.

We perform non-equilibrium molecular dynamics calculations to study the heat transport in crystalline-core amorphous-shell silicon nanowires (SiNWs). It is found that the thermal conductivity of the core-shell SiNWs is closely related to the cross-sectional area ratio of amorphous shell. Through shell amorphization, an 80% reduction in thermal conductivity compared to crystalline SiNWs with the same size can be achieved, due to the non-propagating heat diffusion in the amorphous region. In contrast to the strong temperature-dependent thermal conductivity of crystalline SiNWs, the core-shell SiNWs only show weak temperature dependence. In addition, an empirical relation is proposed to accurately predict the thermal conductivity of the core-shell SiNWs based on the rule of mixture. The present work demonstrates that SiNWs with an amorphized shell are promising candidates for thermoelectric applications.

Tensile-loading molecular dynamics simulations show that nanocrystalline SiC not only becomes ductile, but can be superplastically deformed at room temperature when grain sizes are reduced to d ∼ 2 nm. The calculated strain rate sensitivity, 0.67, implies a superplastic ceramic able to attain strains of up to 1000% at room temperature and typical strain rates (∼10−2 s−1). The origin of the superplasticity is linked to an unusually steep rise in creep rate to 106 s−1 for d = 2 nm. The results explain recent observations in SiC nanowires and suggest novel opportunities for structural ceramics.

Borophene, a new two-dimensional (2D) material with metallic characteristic, is promising for use as electrodes and interconnects in flexible nanodevices. Here, we study the mechanical properties of single-layer borophene using molecular dynamics simulations based on a newly developed interatomic potential. It is found that the hexagonal borophene exhibits highly anisotropic mechanical properties. The Young’s modulus and fracture strength in the zigzag direction are much lower than those in the armchair direction. The simulated fracture strength and fracture strain of borophene at 10 K are in good agreement with those predicted by first-principles calculations. We further reveal that the fracture properties of borophene are very sensitive to temperature. When the loading temperature increases from 10 to 700 K, the fracture strength and fracture strain decreases by 50% and 60%, respectively. In contrast, the fracture properties show a relatively weak sensitivity to the strain-rate. When the strain rate changes from 0.00001 to 0.01 ps−1, the fracture strength and fracture strain increase only by 6.8–8.6% and by 11–15%, respectively. Our observations on the temperature and strain-rate dependent fracture strength can be rationalized by the kinetic theory of fracture. The present study provides valuable insights into the deformation and failure behavior of borophene, which are of importance for the design and application of borophene-based nanodevices. ASTAR (Agency for Sci., Tech. and Research, S’pore)

The structure of diamond-like carbon (DLC) features an amorphous topology with a high fraction of sp3 bonds, which increases with density and reaches over 80% at 3.0 g/cm3. To perform large-scale simulations of DLC using interatomic potentials one should be able to generate this structure using a melting and quenching procedure. In the present work, we evaluate the accuracy of different bond order interatomic potentials developed for carbon and hydrocarbons to describe the properties of DLC, in particular its structure. The potentials include the reactive empirical bond order (REBO) and variants of the Tersoff potentials (standard, ZBL, extended cutoff). Results indicate that Tersoff potential with extended cutoff of 2.45 A is the most accurate in reproducing the experimental structure and mechanical properties of DLC and DLCH (hydrogenated diamond-like carbon). The calculated sp3 fraction as a function of density, pair correlation functions, and Young’s modulus as a function of density show excellent agreement with experimental data. Our findings suggest that the Tersoff potential with extended cutoff is a reliable interatomic potential for large-scale atomistic simulations of both DLC and DLCH.

Due to low formation energies, it is very easy to create atomic defects in phosphorene during its fabrication process. How these atomic defects affect its mechanical behavior, however, remain unknown. Here, we report on a systematic study of the effect of atomic vacancies on the mechanical properties and failure behavior of phosphorene using molecular dynamics simulations. It is found that atomic vacancies induce local stress concentration and cause early bond-breaking, leading to a significant degradation of the mechanical properties of the material. More specifically, a 2% concentration of randomly distributed mono-vacancies is able to reduce the fracture strength by ∼40%. An increase in temperature from 10 to 400 K can further deteriorate the fracture strength by ∼60%. The fracture strength of defective phosphorene is also found to be affected by defect distribution. When the defects are patterned in a line, the reduction in fracture strength greatly depends on the tilt angle and the loading direction. Furthermore, we find that di-vacancies cause an even larger reduction in fracture strength than mono-vacancies when the loading is in an armchair direction. These findings provide important guidelines for the structural design of phosphorene in future applications.

Owing to the superior thermal properties of graphene, graphene-reinforced polymer nanocomposites hold great potential as the thermal interface materials (TIMs) dissipating heat for electronic packages. However, this application is greatly hindered by the high thermal resistance at the interface between graphene and polymer. In this paper, some important aspects of the improvement of the thermal transport across the interface between graphene and epoxy in graphene-epoxy nanocomposites, including the effectiveness of covalent and noncovalent functionalization, isotope doping, and acetylenic linkage in graphene are systematically investigated using molecular dynamics (MD) simulations. The simulation results show that the covalent and noncovalent functionalization techniques could considerably reduce the graphene-epoxy interfacial thermal resistance in the nanocomposites. Among different covalent functional groups, butyl is more effective than carboxyl and hydroxyl in reducing the interfacial thermal resistance. Different noncovalent functional molecules, including 1-pyrenebutyl, 1-pyrenebutyric acid, and 1-pyrenebutylamine, yield a similar amount of reductions. Moreover, it is found that the graphene-epoxy interfacial thermal resistance is insensitive to the carbon isotope doping in graphene, while it can be reduced moderately by replacing the sp(2) bonds in graphene with acetylenic linkages.

Graphyne is an allotrope of graphene with both sp and sp 2 hybridized carbon atoms. In this paper, molecular dynamics simulations are carried out on four different graphynes (α, β, γ, and 6,6,12-graphenes) to explore their thermal conductivity (TC). The difference between the four graphyne structures lies in the different percentages of the acetylenic linkages (triple-bonded carbon linkages). The presence of the acetylenic linkages in graphynes is found to cause a significant reduction in the TC as a result of the associated low atom density in the structures and weak single bonds in the acetylenic linkages. Among the four graphynes considered, the 6,6,12-graphyne shows the most obvious directional anisotropy in the TC. Beside the structure, external strain and temperature are also found to affect the TC of graphynes.

In this Letter, the thermal conductivity of defective graphene is investigated by using non-equilibrium molecular dynamics simulations. It is found that various defects including single vacancy, double vacancy and Stone–Wales defects can greatly reduce the thermal conductivity of graphene. The amount of reduction depends strongly on the density and type of defects at small density level. However, at higher defect density level, the thermal conductivity of defective graphene decreases slowly with increasing defect density and shows marginal dependence on the defect type. The thermal conductivity is found to become less sensitive to temperature with increasing defect density.

We investigate the thermal conductivity of fluorinated graphene using molecular dynamics simulations. We find that fluorination reduces the thermal conductivity of graphene, with the amount of reduction strongly depending on both distribution and coverage of fluorination. The thermal conductivity shows a somewhat U-shaped change with coverage from 0% to 100%. At the same coverage, random fluorination results in a larger reduction in thermal conductivity than patterned fluorination, and the patterned strips oriented perpendicular to heat flux cause a stronger reduction than the strips parallel to heat flux. We also find that fluorination makes the thermal conductivity less sensitive to strain.

We investigate the thermal conductivity of hydrogenated graphene using non-equilibrium molecular dynamics simulations. It is found that the thermal conductivity greatly depends on the hydrogen distribution and coverage. For random hydrogenation, the thermal conductivity decreases rapidly with increasing coverage up to about 30%. Beyond this limit, however, the thermal conductivity is almost insensitive to the coverage. For patterned hydrogenation with stripes parallel to the heat flux, the thermal conductivity decreases gradually with increasing coverage from 0% to 100%. In contrast, when the stripe direction is perpendicular to the heat flux, a small (5%) coverage causes a sharp (60%) drop of thermal conductivity. The deterioration of thermal conductivity is due to the sp 2 -to-sp 3 bonding transition upon hydrogenation, which softens the G-band phonon modes. Percolation theory can be used to explain the variation of thermal conductivity at different hydrogenation distributions and coverages. The applicability of the rule of mixtures in predicting the thermal conductivity is also discussed. The work suggests that hydrogenation is a possible route to tune graphene thermal conductivity and manage heat dissipation in graphene-based nanoelectronic devices.

Molecular dynamics simulations of ultrananocrystalline diamond (UNCD), with random distribution of grain sizes and grain boundaries (GBs), are performed to investigate the effect of grain size and temperature on the mechanical properties and failure mechanisms under tensile loading. Results show that when the grain size of UNCDs decreases from 4.1 nm to 2.26 nm, the Young's modulus decreases from 891 GPa to 840 GPa, while the obtained intrinsic fracture strength, 113 GPa, is insensitive to the grain size. Elastic softening is attributed to the increased volume fraction of amorphous-like atoms. Our analysis reveals that at room temperature, UNCD fails via sliding along a grain boundary with a large shear stress. Such sliding triggers crack initiation at an adjacent triple junction and subsequent propagation along an adjacent grain boundary with a large normal stress. With increasing temperature, a crossover from grain sliding to a direct intergranular fracture is observed. The crossover is caused by a different dependence of GB shear and tensile strength on temperature. The present work provides information that may be useful to the design and optimization of the mechanical properties and failure behavior of UNCDs.

Using molecular dynamics simulations, we investigate the oscillatory behaviors of carbon nanotube oscillators containing sp 3 -hybridized defects formed by hydrogen chemisorption. It is found that the presence of these defects significantly affects the kinetic and potential energies of the nanotube systems, which in turn affects their oscillation periods and frequencies. We have also studied the oscillatory characteristics of the oscillators containing sp 3 -hybridized Stone–Wales defects. Our results show that it is possible to control the motion of the inner nanotube by introducing sp 3 -hybridized defects on the outer nanotube, which provides a potential way to tune the oscillatory behavior of nanotube oscillators.

The direct nanoimprinting process between a diamond mold and a copper substrate is studied using molecular dynamics simulations with embedded atom method potential. The deformation behavior, dislocation movement and imprint force are investigated. For the three imprint surfaces of (0 0 1), (1 1 0) and (1 1 1), it is found that the (0 0 1) surface results in the lowest imprint force, while the (1 1 1) surface results in the highest one. When imprinting on the (0 0 1) surface, the dislocations glide in an angle of about 45° to the imprint direction. In contrast, the dislocations move in parallel or normal to the imprint direction when imprinting on the (1 1 0) surface. For imprinting on the (1 1 1) surface, the dislocations glide in a small angle or normal to the imprint direction. The mold taper angle has little effect on the dislocation moving directions, though it has strong influence on the substrate deformation and imprint forces.

Molecular dynamics simulations have been performed to investigate the mechanical properties of hydrogen functionalized graphene for H-coverages spanning the entire range from graphene (H-0%) to graphane (H-100%). We find that the Young’s modulus, tensile strength, and fracture strain of the functionalized graphene deteriorate drastically with increasing H-coverage up to about 30%. Beyond this limit the mechanical properties remain insensitive to H-coverage. While the Young’s modulus of graphane is smaller than that of graphene by 30%, the tensile strength and fracture strain show a much larger drop of about 65%. We show that this drastic deterioration in mechanical strength arises both from the conversion of sp 2 to sp 3 bonding and due to easy-rotation of unsupported sp 3 bonds. Our results suggest that the coverage-dependent deterioration of the mechanical properties must be taken into account when analyzing the performance characteristics of nanodevices fabricated from functionalized graphene sheets.

The paper reviews recent research and developments on simulated indentation tests at micron and nanometer levels. For indentation at the maximum depth of several microns or hundreds of nanometer, classical continuum plasticity framework incorporating Taylor dislocation model via strain gradient plasticity embedded in the constitutive equation may be adopted to take care of the size effect. As higher-order stress components and higher-order continuity requirements can be made redundant, only C 0 finite elements incorporating strain gradient plasticity have to be formulated. This results in the significant ease and convenience in finite element implementation requiring minimal additional computational effort and resources. Alternatively, when the indentation depth is lower at nanometer level, either a large scale molecular dynamics model or a hybrid finite element and molecular dynamics simulation has to be adopted. The article includes certain results from the former approach on nanoindentation based on combination of both Morse potential and embedded-atom model potential.

This paper presents molecular dynamics (MD) simulations of DNA oligonucleotide and water molecules translocating through carbon nanotube (CNT) channels. Induced pressure difference is applied to the system by pushing a layer of water molecules towards the flow direction to drive the oligonucleotide and other molecules. This novel MD simulation investigates the flow behaviour of oligonucleotide and water molecules in nanochannel while controlling the temperature and volume of the system in canonical ensemble. The results show that the oligonucleotide is unable to translocate through the (8, 8)–(12, 12) CNT channel under the induced pressures applied. However, the oligonucleotide can transport through the (10, 10)–(14, 14) CNT channel easily under the same induced pressures. It is observed that less water molecules permeate through the center of the (8, 8)–(12, 12) CNT channel as the strength of the induced pressure is increased. In contrast, more water molecules flow through the (10, 10)–(14, 14) CNT channel at a higher induced pressure. The conformational energy of the oligonucleotide in the CNT channels has been shown to be affected by both the strength of the induced pressure and the size of the nanotube. Although the interactive force between oligonucleotide and CNT channel is dependent on their distance apart, the induced pressure within the (8, 8)–(12, 12) nanotube channel acts as an external factor that affects the distance between the oligonucleotide and the CNT junction. The insertion depth of the oligonucleotide in the (8, 8)–(12, 12) CNT channel relies on the magnitude of the induced pressure. Both the velocity of oligonucleotide and the interactive force between oligonucleotide and nanotube wall are shown to increase when the oligonucleotide is travelling through the narrower part of the (10, 10)–(14, 14) CNT channel.

In this paper, a three-dimensional finite element analysis is used to study the strain, stress and stain energy density distributions in quantum dots (QDs) arrays. Two different QD geometries are simulated: truncated-pyramidal and lens-shaped. The effect of the material anisotropy and the cap layer thickness on the elastic fields is studied. The simulation results show that the material anisotropy has significant influence on the strain distribution. The average compressive strain e xx in the QDs increases as the anisotropy ratio A increases from 1.0 to 4.0, while it decreases as A is reduced from 1.0 to 0.25. When the anisotropy ratio A >1, [1 0 0] and [1¯ 0 0] are the “elastic soft” directions with the strain e xx decaying rapidly in these directions. However, these lattice directions become “elastic hard” when A

A new method on examining the micro cracks of monocrystalline silicon during nano indentation is proposed. It is established based on a study of the increasing rate of absorbed energy in nano indentation. This method provides a simple approach in understanding whether cracks on the silicon surfaces occur, while it is tedious in conventional method.

Nanometric machining involves removal of materials at the order of a few nanometers or less. At such a small length scale, molecular dynamics (MD) simulation is an important tool in studying the nanometric machining mechanism and process. In this study, a series of large scale MD simulations with the model size of more than four-million atoms have been performed to study the nanometric machining of copper. The dislocations at finite temperature during the cutting processes are identified and their nucleation and movement are studied. The effects of cutting depth, cutting speed, crystal orientation and cutting direction on the material deformation, lattice defects and cutting forces are investigated. The simulation results show that a smaller cutting depth results in less plastic deformation and fewer dislocations in the workpiece and thus result in a smoother machined surface. It is found that as the cutting depth decreases, the specific cutting force increases rapidly, which shows that the “size effect” exists in nanometric machining. It is observed that a higher cutting speed results in more lattice defects at the cutting region and higher cutting forces. It is revealed that the crystal orientation and cutting direction have a strong effect on material deformation, dislocation movement and cutting forces.

The experimental researches on Equal Channel Angular Extrusion (ECAE) process of commercial available Al-6061 alloy were conducted and the grain refinement after ECAE processing was investigated. Sixteen passes of ECAE processing at room temperature were conducted and the relationship of grain refinement with extrusion pass was established. The property enhancements after ECAE processing including ultimate tensile strength and Vickers microhardness were investigated to determine the effects of the number of ECAE passes on the mechanical properties of the extruded samples. The research presents a whole picture of ECAE processing of the alloy for up to 16 passes.

Molecular dynamics simulations using the embedded atom method (EAM) potential are carried out to study the nanoimprint of copper single crystal. The effects of mould taper angle, mould tooth spacing and crystal orientation on the deformation behaviour, dislocation movement and imprint force are investigated. A four-stage deformation is observed for the mould with a small taper angle, while it cannot be clearly identified for the mould with a large taper angle. The four-stage deformation is: (1) a purely elastic deformation with a rapid increase of imprint force, (2) dislocation nucleation with a small drop of imprint force, (3) local pile-up of dislocations with a second rapid increase of imprint force, and (4) active dislocation movement with almost a plateau of imprint force. Among the three imprint surfaces of (0 0 1), (1 1 0) and (1 1 1), it is found that the (0 0 1) surface results in the lowest imprint force and the (1 1 1) surface results in the highest one. It is also observed that the pattern transfer on the (1 1 1) surface is less accurate than on the (0 0 1) and (1 1 0) surfaces. It is shown that the smaller the mould tooth spacing, the stronger is the interaction of the stress fields in the substrate, which results in more obvious influence on the material deformation, the dislocation movement and the imprint force. (Some figures in this article are in colour only in the electronic version)

Since no physical approach can be employed to study the mechanism in micro cutting, the molecular dynamics simulation is becoming more and more important. In this study, the results of molecular dynamics modeling and analysis on the nanometric machining on silicon surface are presented. According to the simulation, some phenomena in the nanometric cutting process are found. First, surface elastic rebound happens on the cut surface after cutter moving away. The value of the surface elastic rebound is calculated in the simulation. Second, the atoms near the corner of work piece swirl up following the cutter moving direction at the initial stage of removing atoms from the work piece. Third, the simulation results show that no matter how small material removal is, the burr is always formed at the edge of work piece.

Nanoscale machining involves changes in only a few atomic layers at the surface. Molecular dynamics (MD) simulation can play a significant role in addressing a number of machining problems at the atomic scale. In this paper, we employed MD simulations to study the nanometric cutting process of single crystal copper. Instead of the widely used Morse potential, we used the Embedded Atom Method (EAM) potential for this study. The simulations were carried out for various tool geometries at different cutting speeds. Attention was paid to the cutting chip formation, the cutting surface morphology and the cutting force. The MD simulation results show that both the tool geometry and the cutting speed have great influence on the chip formation, the smoothness of machined surface and the cutting force.

Severe plastic deformation (SPD) is an efficient approach for producing ultrafine or nanostructured bulk materials. Equal channel angular extrusion (ECAE) is the most effective SPD solution for material nanostructuring, as material billet undergoes severe and large deformation and the grains are efficiently broken up in the process. To improve material nanostructuring, the ECAE die design and process configuration are critical. The deformation behavior study through FE simulation in ECAE process provides basic and useful information for optimizing die design and process determination. In this research, the deformation behavior for three different die design scenarios is studied and the related deformation mechanisms and nanostructuring performance are investigated via FE simulation. Through multi-pass simulation, the optimal design scenario is then identified. The simulation results reveal deformation phenomena, and nanostructuring performance of the designs and the corresponding process can be recommende...

The crystallization process of a Ti–Al amorphous alloy during isothermal annealing was studied with molecular dynamics simulations. The structural development and phase transformation were analysed based on the variations of the internal energy, cell volume, radial distribution function, bond pairs, and atomic configuration. The crystal nucleation, grain growth, and grain coarsening during the crystallization process were studied. The three-stage feature of the crystallization process was identified. The simulation results also show that there are transformations from a metastable crystal phase to a more stable crystal phase during the crystallization.

Thermo-mechanical finite element analysis was carried out to study the deformation behavior and temperature distribution during equal channel angular pressing (ECAP). The material model used is the Johnson-Cook constitution model that can consider the multiplication effect of strain, strain rate, and temperature on the flow stress. The effects of pressing speed, pressing temperature, workpiece material and die geometry on the temperature rise and flow behavior during ECAP process were investigated. The simulated temperature rise due to deformation heating was compared with published experimental results and a good agreement was obtained. Among the various die geometries studied, the two-turn die with 0° round corner generates the highest and most uniform plastic strain in the workpiece.

In recent years, bulk nanostructured materials produced by methods of severe plastic deformation have attracted growing research interest. This article presents a coupled thermo-mechanical finite element analysis of the severe plastic deformation in the workpiece materials during equal channel angular pressing (ECAP) to optimize the ECAP process and the die design.

The rapid solidification of Ti3Al was studied with the constant-pressure and constant-temperature molecular dynamics (NPT-MD) technique to obtain an atomistic description of glass formation and crystallization in the alloy. The embedded atom method (EAM) potential for the Ti?Al binary system recently developed by Zope and Mishin (2003?Phys.?Rev.?B?68?024102) was applied in the simulations. The effects of different cooling rates on the glass formation and crystallization of liquid Ti3Al were studied. In addition, the crystallization of the amorphous Ti3Al as a function of increasing temperature was also studied. The calculated internal energy change and radial distribution function during cooling and heating processes provided a good picture of the structural transformations, and the results were consistent with the results obtained experimentally.

The temperature rise and temperature distribution in the workpiece during equal channel angular pressing were investigated by using the finite element method for Al–1%Mg and Al–3%Mg at different pressing speeds. The simulated temperature rise was compared with published experimental and analytical results.

Ab initio simulations on the basic clusters in the best glass formers of Cu–Zr metallic glasses (MGs) provide the most straightforward evidence that a gap in the density of states (DOSs) at the Fermi level is observed. We establish a direct connection between the electronic structure of the basic clusters in MGs and the glass-forming ability (GFA) of MGs, providing a new avenue to examine the GFA of MGs. And our findings provide a check for the atomic structural models of MGs, and have implications for understanding the formation and properties of MGs.

We investigate the in-plane and cross-plane thermal conductivities of molybdenum disulfide (MoS2) using non-equilibrium molecular dynamics simulations. We find that the in-plane thermal conductivity of monolayer MoS2 is about 19.76 W mK(-1). Interestingly, the in-plane thermal conductivity of multilayer MoS2 is insensitive to the number of layers, which is in strong contrast to the in-plane thermal conductivity of graphene where the interlayer interaction strongly affects the in-plane thermal conductivity. This layer number insensitivity is attributable to the finite energy gap in the phonon spectrum of MoS2, which makes the phonon-phonon scattering channel almost unchanged with increasing layer number. For the cross-plane thermal transport, we find that the cross-plane thermal conductivity of multilayer MoS2 can be effectively tuned by applying cross-plane strain. More specifically, a 10% cross-plane compressive strain can enhance the thermal conductivity by a factor of 10, while a 5% cross-plane tensile strain can reduce the thermal conductivity by 90%. Our findings are important for thermal management in MoS2 based nanodevices and for thermoelectric applications of MoS2.

This work investigates the nanoscale friction between diamond-structure silicon (Si) and diamond via molecular dynamics simulation. The interaction between the interfaces is considered as strong covalent bonds. The effects of load, sliding velocity, temperature and lattice orientation are investigated. Results show that the friction can be divided into two stages: the static friction and the kinetic friction. During the static friction stage, the load, lattice orientation and temperature dramatically affects the friction by changing the elastic limit of Si. Large elastic deformation is induced in the Si block, which eventually leads to the formation of a thin layer of amorphous Si near the Si-diamond interface and thus the beginning of the kinetic friction stage. During the kinetic friction stage, only temperature and velocity have an effect on the friction. The investigation of the microstructural evolution of Si demonstrated that the kinetic friction can be categorized into two modes (stick-slip and smooth sliding) depending on the temperature of the fracture region. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore)

The easy formation of vacancy defects and the asymmetry in the two sublayers of phosphorene nanotubes (PNTs) may result in brand new mechanical properties and failure behaviour. Herein, we investigate the mechanical properties and fracture behaviour of defective PNTs under uniaxial tension using molecular dynamics simulations. Our simulation results show that atomic vacancies cause local stress concentration and thus significantly reduce the fracture strength and fracture strain of PNTs. More specifically, a 1% defect concentration is able to reduce the fracture strength and fracture strain by as much as 50% and 66%, respectively. Interestingly, the reduction in the mechanical properties is found to depend on the defect location: a defect located in the outer sublayer has a stronger effect than one located in the inner layer, especially for PNTs with a small diameter. Temperature is also found to strongly influence the mechanical properties of both defect-free and defective PNTs. When the temperature is increased from 0 K to 400 K, the fracture strength and fracture strain of defective PNTs with a defect concentration of 1% are reduced further by 71% and 61%, respectively. These findings are of great importance for the structural design of PNTs as building blocks in nanodevices.

Because of the complex interactions among the energy beam, the powder bed, and the material phase transformations, powder-bed fusion additive manufacturing is very sensitive to process parameters, such as beam power and scan speed. As a result, the process window to produce fully-dense, ASTM-grade components is narrow. In such scenario, envisioning further control of mechanical properties is very challenging. As a departure from traditional attempts to control microstructure by changing the process parameters, the authors propose the introduction of a thermoelectric module (TEM) as an active device inside the build chamber. Using process modeling, the authors show that by altering the heat flow into the material through the TEM device, the volume fraction of martensite can be controlled in its entire range. In particular, the authors show that modern TEM modules can deliver sufficient thermal power to block the formation of martensite. As a result, microstructure can be controlled locally while retaining the beam power and scan speed optimal for part density and surface finish. While the results are demonstrated for Ti6Al4V and the electron beam melting process, the concept is general and, in principle, applicable to other materials and machines systems such as IN781 and selective laser melting.

Van der Waals heterostructures based on graphene and other 2D materials have attracted great attention recently. In this study, the mechanical properties and failure behaviour of a graphene/silicene/graphene heterostructure are investigated using molecular dynamics simulations. We find that by sandwiching silicene in-between two graphene layers, both ultimate tensile strength and Young's modulus of the heterostructure increase approximately by a factor of 10 compared with those of stand-alone silicene. By examining the fracture process of the heterostructure, we find that graphene and silicene exhibit quite different fracture behaviour. While graphene undergoes cleavage through its zigzag edge only, silicene can cleave through both its zigzag and armchair edges. In addition, we study the effects of temperature and strain rate on the mechanical properties of the heterostructure and find that an increase in temperature results in a decrease in its mechanical strength and stiffness, while an increase in strain rate leads to an increase in its mechanical strength without significant changes in its stiffness. We further explore the failure mechanism and show that the temperature and strain-rate dependent fracture stress can be accurately described by the kinetic theory of fracture. Our findings provide a deep insight into the mechanical properties and failure mechanism of graphene/silicene heterostructures.

Notched metallic glasses (MGs) have received much attention recently due to their intriguing mechanical properties compared to their unnotched counterparts, but so far no fundamental understanding of the correlation between failure behavior and notch depth/sharpness exists. Using molecular dynamics simulations, we report necking and large notch strengthening in MGs with symmetric sharp-and-deep notches. Our work reveals that the failure mode and strength of notched MGs are strongly dependent on the notch depth and notch sharpness. By increasing the notch depth and the notch sharpness, we observe a failure mode transition from shear banding to necking and also a large notch strengthening. This necking is found to be caused by the combined effects of large stress gradient at the notch roots and the impingement and subsequent arrest of shear bands emanating from the notch roots. The present study not only shows the failure mode transition and the large notch strengthening in notched MGs, but also provides significant insights into the deformation and failure mechanisms of notched MGs that may offer new strategies for the design and engineering of MGs.

Nanoindentation has been recently used to measure the mechanical properties of polycrystalline graphene. However, the measured failure loads are found to be scattered widely and vary from lab to lab. We perform molecular dynamics simulations of nanoindentation on polycrystalline graphene at different sites including grain center, grain boundary (GB), GB triple junction, and holes. Depending on the relative position between the indenter tip and defects, significant scattering in failure load is observed. This scattering is found to arise from a combination of the non-uniform stress state, varied and weakened strengths of different defects, and the relative location between the indenter tip and the defects in polycrystalline graphene. Consequently, the failure behavior of polycrystalline graphene by nanoindentation is critically dependent on the indentation site, and is thus distinct from uniaxial tensile loading. Our work highlights the importance of the interaction between the indentation tip and defects, and the need to explicitly consider the defect characteristics at and near the indentation site in polycrystalline graphene during nanoindentation.

Understanding the grain size-dependent failure behavior of polycrystalline graphene is important for its applications both structurally and functionally. Here we perform molecular dynamics simulations to study the failure behavior of polycrystalline graphene by varying both grain size and distribution. We show that polycrystalline graphene fails in a brittle mode and grain boundary junctions serve as the crack nucleation sites. We also show that its breaking strength and average grain size follow an inverse pseudo Hall-Petch relation, in agreement with experimental measurements. Further, we find that this inverse pseudo Hall-Petch relation can be naturally rationalized by the weakest-link model, which describes the failure behavior of brittle materials. Our present work reveals insights into controlling the mechanical properties of polycrystalline graphene and provides guidelines for the applications of polycrystalline graphene in flexible electronics and nano-electronic-mechanical devices.

We perform molecular dynamics simulations to investigate the nanoscale frictional behavior of a perfluoropolyether (PFPE) film sandwiched between two diamond-like-carbon (DLC) coatings. We show that the PFPE films behave like a solid and can perform either a motion-station movement or a continuous motion with fluctuating velocities. The former movement is caused by the alternating stick and slip at the two individual interfaces, while the latter is due to the dynamic sliding motions simultaneously occurring at both interfaces. We reveal that these motion characteristics are governed by the competition between the two interfacial adhesion energies, which are strongly affected by the thermal vibrations and interface roughness fluctuations. We also find that the Amonton's law modified by incorporating the adhesion effect can be used to describe the mean friction traction vs normal pressure relation, but large fluctuations are present at low contact pressures. The magnitude of atomic level friction forces at the interface is found to be highly nonuniform. The directions of atomic level friction forces can even be opposite. With increasing the normal pressure, the nonuniformity of atomic level friction forces decreases first and then increases again. This change can be explained by the concurrent effects from the large difference in material stiffness and the changes in surface roughness under normal pressure. The present work reveals interesting insights into the sliding mechanisms in sandwiched structures and provides useful guidelines for the design of nanoscale lubricant systems.

Vertical integration of 2D materials has recently appeared as an effective method for the design of novel nano-scale devices. Using non-equilibrium molecular dynamics simulations, we study the interfacial thermal transport property of graphene/phosphorene heterostructures where phosphorene is sandwiched in between graphene. Various modulation techniques are thoroughly explored. We found that the interfacial thermal conductance at the interface of graphene and phosphorene can be enhanced significantly by using vacancy defects, hydrogenation and cross-plane compressive strain. By contrast, the reduction in the interfacial thermal conductance can be achieved by using cross-plane tensile strain. Our results provide important guidelines for manipulating the thermal transport in graphene/phosphorene based-nano-devices.

Beam-based additive manufacturing (AM) is an innovative technique in which parts are built layerwise, starting from the material in powder form. As a developing manufacturing technique, achievement of excellent mechanical properties in the final part is of paramount importance for the mainstream adoption of this technique in industrial manufacturing lines. At the same time, AM offers an unprecedented opportunity to precisely control the manufacturing conditions locally within the part during build, enabling local influence on the formation of the texture and microstructure. In order to achieve the control of microstructure by tailoring the AM machine parameters, a full understanding and modeling of the heat transfer and microstructure evolution processes is needed. Here, we show the implementation of the non-equilibrium equations for phase formation and dissolution in an AM modeling framework. The model is developed for the Ti6Al4V alloy and allows us to show microstructure evolution as given by the AM process. The developed capability is applied to the cases of electron beam melting and selective laser melting AM techniques to explain the significantly different microstructures observed in the two processes.