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Graphene is a two-dimensional material that can be folded into diverse and yet interesting nanostructures like macro-scale paper origami. Folding of graphene not only makes different morphological configurations but also modifies their mechanical and thermal properties. Inspired by paper origami, herein we studied systemically the effects of creases, where sp2 to sp3 bond transformation occurs, on the thermal properties of graphene origami using molecular dynamics (MD) simulations. Our MD simulation results show that tensile strain reduces (not increases) the interfacial thermal resistance owing to the presence of the crease. This unusual phenomenon is explained by the micro-heat flux migration and stress distribution. Our findings on the graphene origami enable the design of the next-generation thermal management devices and flexible electronics with tuneable properties.

Interfacial adhesion between polymer matrix and reinforcing silica nanoparticles plays an important role in strengthening polypropylene (PP) composite. To improve the adhesion strength, the surface of silica nanoparticles can be modified by grafted functional molecules. Using atomistic simulations, we examined the effect of functionalization of silica nanoparticles by hexamethyldisilazane (HMDS) and octyltriethoxysilane (OTES) molecules on the deformation and failure of silica-reinforced PP composite. We found that the ultimate tensile strength (UTS) of PP composite functionalized by OTES (28 MPa) is higher than that of HMDS (25 MPa), which is in turn higher than that passivated only by hydrogen (22 MPa). To understand the underlying mechanistic origin, we calculated the adhesive energy and interfacial strength of the interphase region, and found that both the adhesive energy and interfacial strength are the highest for the silica nanoparticles functionalized by OTES molecules, while both are the lowest by hydrogen. The ultimate failure of the polymer composite is initiated by the cavitation in the interphase region with the lowest mass density, and this cavitation failure mode is common for all the examined PP composites, but the cavitation position is dependent on the tail length of the functional molecules. The present work provides interesting insights into the deformation and cavitation failure mechanisms of the silica-reinforced PP composites, and the findings can be used as useful guidelines in selecting chemical agents for surface treatment of silica nanoparticles.

Graphene-based layered materials have attracted particular interests recently owing to their excellent mechanical and thermal properties, which are attributed to their layer-by-layer hierarchical structures with intralayer strong sp2 bonds combined with interlayer crosslinks for efficient load and heat transfer. They have many potential applications such as high thermal conductive films in flexible electronics. In this paper, we firstly develop an analytical model to calculate the thermal conductivity of graphene layered materials. Then, extensive molecular dynamics simulations are performed to validate the analytical model. A good agreement is obtained between the analytical and simulation results. Besides, the effects of geometry factors such as graphene sheet size and crosslink density on the temperature distribution and thermal conductivity of graphene layered materials are investigated. It is found that the thermal conductivity of graphene layered materials can be effectively tuned by the graphene sheet size and crosslink density. Our works are of practical importance for the design and application of graphene layered materials as thermal film materials.

Using molecular dynamics (MD) simulations, we investigate the mechanical properties of hexagonal BCN monolayer, a newly synthesized two-dimensional material with an atom ratio of B/C/N = 1:1:1. The Tersoff potential is modified to get good agreement between predicted and measured fracture strengths of graphene. With this modified Tersoff potential, we perform extensive MD simulations to study the effect of temperature, strain rate and vacancy defect on the mechanical properties of h-BCN. It is found that h-BCN is a strong material with fracture strength of 81.4–93.5 GPa, albeit ∼35% lower than that of graphene. Similar to graphene, temperature has strong effect on the mechanical properties of h-BCN. As the temperature increases from 10 K to 1300 K, the fracture strength and strain of h-BCN drops by 55% and 62%, respectively. The strain rate is found to have a moderate effect. When the strain rate increases from 0.00002 to 0.0125 ps−1, the fracture strength and strain of h-BCN increases 6.1% and 12%, respectively. As for the atomic defect, a very small concentration (0.028%) of vacancy in h-BCN is able to cause a 28% reduction in fracture strength and a 35.5% reduction in fracture strain. These findings have significance for its future applications in nanodevices.

We investigate the mechanical behavior of polypropylene/silica composites under mode I and II loading by using molecular dynamics simulations, focusing on the effects of two typical surface coupling agents of HMDZ and APTES on the interface deformation and failure. It is found that under mode I loading, the silica surface functionalized with APTES can enhance the tensile strength of polypropylene/silica composites effectively. In contrast, the silica surface functionalized with HMDZ has little effect on the tensile strength. The simulation results are supported by experiments. Under mode II loading, it is found that both HMDZ and APTES can enhance the shear strength of polypropylene/silica composites with APTES being more effective. The underlying mechanisms for the enhancement of interface strength are analyzed. Our work provides a deep understanding of the interface structure, deformation and failure and will be useful for selecting suitable surface coupling agents in enhancing the interface strength of polymer composites.

Pentadiamond is a newly predicted 3D carbon allotrope by first-principles calculations. Using molecular dynamics simulations, we show that pentadiamond can keep structure stable at high temperatures up to 1100 K, but its fracture strength and fracture strain are very sensitive to temperature. As the temperature increases from 100 K to 900 K, the fracture strength and fracture strain decreases by 68.4% and 69.1%, respectively. We also find that the mono-vacancy and di-vacancy related to C2 position lead to stronger reduction in the fracture strength and fracture strain than the atomic vacancies at other positions. To be surprising, the mono-vacancy at C2position has stronger effect than the di-vacancy at C2-C3positions. Besides, we find that hydrogen functionalization has strong effect on the mechanical properties of pentadiamond. As the H-coverage increases from 0.0 to 1.0, the fracture strength and fracture strain drop by 75.7% and 76.6%, respectively.

Three-dimensional hexagonal boron nitride aerogels (hBNAGs) are novel porous materials with many promising applications such as energy storage, thermal insulation and sensing. However, the structure–property relationships of hBNAGs in complicated thermo-mechanical coupled environments are still not clear. In this study, we employed a binary phase-field crystal (PFC) model to construct the atomic structures of hBNAGs, upon which the mechanical and thermal behaviors of hBNAGs were systematically investigated using large-scale atomistic simulations. It is found that the hBNAG geometry and topological defects strongly affect the mechanical and thermal properties. For example, the Young's modulus and tensile strength follow the scaling laws of mass density with a power factor of about 1.4 and 1.2, respectively, indicating that the stretching and bending combine toward tensile deformation. In addition, cracks nucleate around the octagon defects, indicating that the tensile strength is also influenced by the topological defects. Under compression, complicated crumpled deformations and ridges in the entire region are observed and the compression strength follows the scaling law of mass density with a power factor above 2.0, which means that a large portion of the hBNAGs do not contribute to the compression load bearing. We find that hBNAGs have a very low thermal conductivity of about two orders of magnitude lower than that of a hBN sheet. Also, the thermal conductivity of hBNAGs increases with increasing mass density, which also follows a scaling law. The power of the scaling law is about 0.5, indicating that the thermal conductivity has a strong nonlinear dependence on the mass density. Our work provides a deep understanding of the structure–property relationships of hBNAGs, which is useful for the engineering applications of hBNAGs.

The recently fabricated monolayer MoS2–WSe2 lateral heterostructures are promising for many interesting applications, such as p–n diodes, photodetectors, transistors, sensors, light-emitting diodes and thermoelectric and flexible nanodevices. In this work, we study the mechanical and thermal properties of MoS2–WSe2 lateral heterostructures by using molecular dynamics (MD) simulations based on the recently parameterized Stillinger-Weber (SW) potential. It is found that the fracture strength and fracture strain of MoS2–WSe2 lateral heterostructures are dictated by the mechanical properties of MoS2, and are very sensitive to temperature. However, when a crack is introduced in the MoS2–WSe2 heterostructures, failure may occur either in MoS2 or WSe2, depending on the crack length and location. Interestingly, the fracture strengths obtained from our MD simulations are in agreement with those obtained from the Griffith theory. Our MD simulations further reveal that, in addition to the low thermal conductivities of MoS2 and WSe2, the MoS2–WSe2 heterojunctions exhibit a very low interfacial thermal conductance, which is about one order of magnitude lower than that of graphene–hBN heterojunctions.

Graphene and hexagonal BCnN (n = 0, 1, 2), possessing a similar honeycomb crystal structure, can easily form in-plane BCnN/graphene heterojunctions, which exhibit many unique physical properties. I...

Process consistency and control are bottleneck issues to wider insertion of powder-bed fusion additive manufacturing in the industrial shopfloor. Of particular interest is the porosity of the components, which remains the limiting factor to high-cycle fatigue performance. Recent experiments have shown that, with increasing energy density, a surge in porosity is seen in selectively laser melted metals. In this high-energy density regime, porosity must originate from mechanisms that are different from the well-known incomplete melting in the low energy density regime. To shed light on this interesting phenomenon, this paper first discusses the mechanism of bubble formation in the melt pool and possible trapping during the solidification, and then formulates a predictive model for porosity in this regime. To compare with experimental results, we perform computer modeling and simulations which have been fully validated by experiments to determine the parameters in the model. We show that the model predictions are in good qualitative and quantitative agreement with the experimental measurements. Hence, the proposed model can be used as a tool to predict the porosity, and further to control and possibly reduce porosity in laser powder-bed fusion additive manufacturing, paving the way for its wider adoption in manufacturing shopfloors.

Ultrafast laser-induced damage and ablation of graphene is the one of the most critical parts of precise nanopatterning of graphene by using laser ablation. In this article, we have studied the loc...

Atomic defects can be generated relatively easily in phosphorene due to their low formation energies. How these defects affect the buckling behavior of phosphorene nanotubes (PNTs) remains unexplored. Using molecular dynamics simulations, we investigate the effect of vacancies on the buckling properties of PNTs. We show that compared to a pristine PNT, the defective one exhibits a much lower buckling strength and strain. Remarkably, 1% concentration of vacancies in a PNT is able to cause a 30% reduction in buckling strength. Interestingly, for long PNTs, the buckling occurs via column or global buckling. As a result, the buckling strength decreases significantly with the increase (decrease) in the tube length (diameter) for both the pristine and defective PNTs, consistent with the Euler buckling theory. For short PNTs with small slenderness ratio (L/D), however, buckling occurs via shell or local buckling. As a result, the buckling strength increases with decreasing the tube diameter, consistent with shell buckling theory. Finally, with the increase in temperature, the buckling strength and strain can be reduced significantly for both the pristine and defective PNTs. These findings may provide important guidelines for the design and applications of PNTs-based nanodevices.

We study the wetting behavior of water droplets on pristine and defective phosphorene using molecular dynamics simulations. It is found that unlike prototypical two-dimensional materials such as graphene and MoS2, phosphorene exhibits an anisotropic contact angle along armchair and zigzag directions. This anisotropy is tunable with increasing the number of layers and vacancy concentration. More specifically, the water contact angles decrease with increasing the number of layers, indicating the importance of water–substrate interactions. The contact angles along both armchair and zigzag directions increase with the increasing vacancy concentration, and the anisotropy disappears when the defect concentration is high. For an in-plane pristine-defective phosphorene heterostructure, when the junction is zigzag-oriented, a spontaneous diffusion of water droplets from the defective region to the pristine region occurs; when the junction is armchair-oriented, however, the spontaneous motion is suppressed. The ene...

In recent years, short fiber reinforced thermoplastic composites (SFRTPCs) have attracted significant interest due to their low density, high strength and toughness, environmental resistance and low cost. Currently, an important task is to optimize their internal structures so as to maximize their mechanical properties. Interestingly, a previous experimental study showed that by adding a small amount of a stronger material in the matrix to link up short fibers, the strength of SFRTPCs was enhanced. We note, however, that a micromechanical study on the effect of fiber linkage on the mechanical properties is still lacking. In this work, we have developed a finite element model to conduct such micromechanical analysis. In this model, the polymer matrix is taken as elastic-plastic material, the fiber as a brittle material following a brittle damage model, while the interphase and linkage as the same polymer following a progressive damage model. Our simulations show that for a SFRTPC with a given short fiber weight fraction, by increasing the weight fraction of the linkage material, the Young's modulus, yielding strength, ultimate tensile strength, and tensile toughness of the SFRTPCs are all increased. By increasing the strength of the linkage material, the ultimate tensile strength and tensile toughness of the SFRTPCs are increased. Modified rules of mixing have been proposed to consider the fiber crosslinking effect. The present study provides a useful reference for the optimal design of SFRTPCs to maximize their mechanical performance.

The potential application of metallic glasses (MGs) as structural materials demands accurate measurements of their toughness and the understanding of the underlying factors affecting it. Currently, it is challenging to precisely measure the toughness of MGs, especially ductile MGs. Measured toughness values are widely scattered, even for MGs with identical compositions. That is attributed to the combined effect of intrinsic and extrinsic factors including processing, sample geometries, and loading conditions. A fundamental understanding of the influences of these elements is thus of great significance. In the present study, molecular dynamics simulations are performed to investigate the influence of intrinsic and extrinsic effects on the fracture toughness of CuZr MGs. In particular, focused is placed on the effects of cooling rate, notch shape and size, strain rate, and temperature. The results indicate that the fracture toughness of a MG scales with the cooling rate used to prepare it. This is attributed to increased free volume content generated at high cooling rates, which enhances plastic deformation and amplifies the associated energy dissipation during plastic deformation events. Fracture toughness also increases with the strain rate, arguably due to strain rate-induced crack extension delay. Overall, the results demonstrate that the largest fracture toughness are achieved when MG samples are fabricated at high cooling rates and subjected to high strain rate deformation. In addition, results suggest that the fracture toughness decreases with increasing temperature, due to the significant decrease in strength. The correlations revealed between these crucial intrinsic and extrinsic parameters and the calculated MG fracture toughness support the development of a framework to understand the root of the discrepancies in the measurement of the toughness of MGs and provide insights into the design of tough MGs for structural applications.

We perform molecular dynamics simulations to investigate the motion of phosphorene nanoflakes on a large graphene substrate under a thermal gradient. It is found that the atomic interaction between the graphene substrate and the phosphorene nanoflake generates distinct rates of motion for phosphorene nanoflakes with different orientations. Remarkably, for square phosphorene nanoflakes, the motion of zigzag-oriented nanoflakes is 2-fold faster than those of armchair-oriented and randomly-oriented nanoflakes. This large diffusion anisotropy suggests that sorting of phosphorene nanoflakes into specific orientations can be realized by a temperature gradient. The findings here provide interesting insights into strong molecular diffusion anisotropy and offer a novel route for manipulating two-dimensional materials.

Pentadiamond is a newly predicted 3D carbon allotrope by first-principles calculations. Using molecular dynamics simulations, we show that pentadiamond can keep structure stable at high temperatures up to 1100 K, but its fracture strength and fracture strain are very sensitive to temperature. As the temperature increases from 100 K to 900 K, the fracture strength and fracture strain decreases by 68.4% and 69.1%, respectively. We also find that the mono-vacancy and di-vacancy related to C2 position lead to stronger reduction in the fracture strength and fracture strain than the atomic vacancies at other positions. To be surprising, the mono-vacancy at C2 position has stronger effect than the di-vacancy at C2-C3 positions. Besides, we find that hydrogen functionalization has strong effect on the mechanical properties of pentadiamond. As the H-coverage increases from 0.0 to 1.0, the fracture strength and fracture strain drop by 75.7% and 76.6%, respectively. Our work provides valuable insights into the mechanical behaviors of this new material.

Metallic glasses (MGs) possess large elastic limit and high strength, but unfortunately they are of limited commercial utility due to their macroscopic brittle nature. Here, we report that a chiral nanolattice can be used to design large-scale MGs with negative Poisson’s ratio, large elastic deformation capability, extensive hardening, and large ductility. Furthermore, the mechanical behaviors of the metallic glass chiral nanolattice (MGCN) can be significantly altered through changing the thickness and length of the ligaments in the nanolattice. An exceptional combination of high strength and ductility is observed for MGCN with thin and long ligaments, wherein the sample density is only twice that of water. The deformation mechanism that characterizes the chiral topology is a combination of rotation of the nodes with bending and extension of the ligaments, leading to the extraordinary mechanical behaviors. The present study not only offers a potential solution to mitigating the brittleness of MGs, but also provides some guidelines in designing large-scale MG meta-materials for prospective applications in the fields of acoustics and energy absorption.

Penta-graphene (PG) has recently been identified as a novel two-dimensional carbon allotrope comprising entirely of pure pentagons and possessing many unique properties beyond graphene. Herein we investigate the thermal conductivity of hydrogenated and oxidized PGs at different levels of coverage by using non-equilibrium molecular dynamics simulations. Our simulation results show that the oxidized PG possesses a higher thermal conductivity than the hydrogenated counterpart at the same coverage. Interestingly, both functionalized PGs exhibit an unusual down-and-up trend in the thermal conductivity with the increase of coverage level, and the fully functionalized PGs (at 100% coverage) possess almost the same thermal conductivity as the pristine PG. We further explore the underlying mechanisms for the coverage-dependent thermal conduction. Our work provides important insights into the effect of chemical functionalization on the thermal conductivity of this new carbon allotrope.

Penta-graphene (PG), a newly proposed two-dimensional material composed entirely of carbon pentagons, is believed to possess much lower failure stress and strain than those of graphene. An open question is whether and how these properties can be enhanced. Herein using molecular dynamics simulations, we examine the deformation and failure processes of PG functionalized with different functional groups. We reveal that complete chemical functionalization leads to remarkable increases in the failure stress and strain of PG by up to 86.6% and 82.4%, respectively. The underlying reason for this enhancement is that the buckled pentagonal rings in pristine and partially functionalized PGs can easily transform into planar polygon rings under stretching; in contrast, complete functionalization of PG strongly stabilizes its structure and prevents such transformation, thereby significantly increasing the failure stress and strain. Our findings suggest a possible route to enhance the mechanical properties of PG for potential applications in nanocomposites and nanodevices.

Phosphorene, a new two-dimensional material, has attracted tremendous attention in recent years due to its superior physical and electrical properties. Despite the fact that it may have a strong impact in future flexible electronics, there has been so far little or no atomic-level understanding of the failure process of phosphorene under nanoindentation. Here, we report a systematic study of the deformation and failure mechanism of phosphorene under nanoindentation by using molecular dynamics simulations. Three different regimes of deformation behaviors can be identified. The bump behavior in the first regime is different from that in the graphene and other 2D materials. Our simulations reveal the strong correlation between the normal load and the number of C–P pairs with repulsive force across the contact interface. Furthermore, the failure mechanism does not change with increasing tip size or defect concentration. However, it is found that the failure load decreases by 30% even though the defect concent...

Two-dimensional (2D) materials have attracted a great deal of attention recently owing to their fascinating structural, mechanical, and electronic properties. The failure phenomena in 2D materials can be diverse and manifested in different forms due to the presence of defects. Here, we review the structural features of seven types of defects, including vacancies, dislocations, Stone-Wales (S-W) defects, chemical functionalization, grain boundary, holes, and cracks in 2D materials, as well as their diverse mechanical failure mechanisms. It is shown that in general, the failure behaviors of 2D materials are highly sensitive to the presence of defects, and their size, shape, and orientation also matter. It is also shown that the failure behaviors originated from these defects can be captured by the maximum bond-stretching criterion, where structural mechanics is suitable to describe the deformation and failure of 2D materials. While for a well-established crack, fracture mechanics-based failure criteria are still valid. It is expected that these findings may also hold for other nanomaterials. This overview presents a useful reference for the defect manipulation and design of 2D materials toward engineering applications.

Intermetallic Ti-Al alloys exhibit attractive physical and mechanical properties at high temperatures relevant to turbine machinery applications. However, TiAl and Ti3Al alloys have low ductility and low fracture toughness, as well as anomalous hardening behaviour at high temperatures. Molecular dynamics (MD) simulations can directly model critical atomistic mechanisms responsible for these intriguing properties, but the validity of the underlying empirical/semi-empirical interatomic potentials for such complex alloys is not clear. Here, we examine four interatomic potentials (two EAM and two MEAM) developed for the Ti-Al material system and identify their respective strength and weakness in modeling the plastic and fracture properties of the L10 γ -TiAl and D019 α 2 -Ti3Al crystal structures. We compare the lattice constants, elastic constants and their temperature-dependence, cohesive energies, stacking fault energies, surface decohesion and thermal expansion coefficients against available experimental and density functional theory data. In addition, we test the deformation behaviours of these potentials under uniaxial tensile loading conditions at a wide range of temperatures. Our results show that these interatomic potentials can accurately reproduce some but not the full scope of the material properties of the γ and α 2 crystal structures. In particular, one MEAM potential reproduces some of the properties (some of the negative Cauchy pressure and stacking fault energies) comparable to DFT-calculated data. The results here provide guidance to select the appropriate interatomic potentials for specific applications. Our study also indicates that further optimization of the parameters in the MEAM formalism may lead to better interatomic potentials for the Ti-Al system.

Phosphorene, a new two-dimensional (2D) semiconducting material, has attracted tremendous attention recently. However, its structural instability under ambient conditions poses a great challenge to its practical applications. A possible solution for this problem is to encapsulate phosphorene with more stable 2D materials, such as graphene, forming van der Waals heterostructures. In this study, using molecular dynamics simulations, we show that the thermal stability of phosphorene in phosphorene/graphene heterostructures can be enhanced significantly. By sandwiching phosphorene between two graphene sheets, its thermally stable temperature is increased by 150 K. We further study the thermal transport properties of phosphorene and find surprisingly that the in-plane thermal conductivity of phosphorene in phosphorene/graphene heterostructures is much higher than that of the free-standing one, with a net increase of 20-60%. This surprising increase in thermal conductivity arises from the increase in phonon group velocity and the extremely strong phonon coupling between phosphorene and the graphene substrate. Our findings have an important meaning for the practical applications of phosphorene in nanodevices.

A hexagonal graphene-like boron-carbon-nitrogen (h-BCN) monolayer, a new two-dimensional (2D) material, has been synthesized recently. Herein we investigate for the first time the thermal conductivity of this novel 2D material. Using molecular dynamics simulations based on the optimized Tersoff potential, we found that the h-BCN monolayers are isotropic in the basal plane with close thermal conductivity magnitudes. Though h-BCN has the same hexagonal lattice as graphene and hexagonal boron nitride (h-BN), it exhibits a much lower thermal conductivity than the latter two materials. In addition, the thermal conductivity of h-BCN monolayers is found to be size-dependent but less temperature-dependent. Modulation of the thermal conductivity of h-BCN monolayers can also be realized by strain engineering. Compressive strain leads to a monotonic decrease in the thermal conductivity while the tensile strain induces an up-then-down trend in the thermal conductivity. Surprisingly, the small tensile strain can facilitate the heat transport of the h-BCN monolayers.

Depending on fiber size and volume fraction, polymer matrix type, processing temperature, injection speed, etc., short fiber reinforced composites produced by injection molding may exhibit complex fiber concentration, orientation and length distributions at different locations, causing large variations in their mechanical properties. In this work, a representative volume element (RVE) scheme is developed to describe the elastic properties of short glass fiber reinforced PA6,6 composites produced by injection molding. In the RVE formulation, fiber length is assumed to follow a two-parameter Weibull distribution, and a new concept, namely preferential angle, is introduced to account for the preferential alignment of fibers at different locations of injection molded composites. A random sequential insertion algorithm is developed to implement the fiber length distribution and fiber preferential alignment, and finite element method (FEM) is employed to analyze the relations of the elastic properties of RVE with three parameters, that is, the mean length, volume fraction and preferential angle of the fibers. Based on the FEM results, quantitative formulas for these relations are obtained. It is found that the variations in the mean length, volume fraction, and preferential angle of the fibers can cause large variations in elastic moduli and elastic anisotropy of the RVE.

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.

Using non-equilibrium molecular dynamics simulations, we investigate the thermal transport in van der Waals heterostructures consisting of alternating multilayer graphene and multilayer MoS2. It is found that the thermal conductance at graphene/MoS2 (G/M) interface is much lower than that at graphene/graphene (G/G) and MoS2/MoS2 (M/M) interfaces. This low interfacial thermal conductance is attributed to the low friction at G/M interface, which significantly reduces the contribution of shearing modes to the thermal conductance. It is also found that there is no thermal rectification at the G/M interface as the thermal conductance is independent on the heat flux direction. Moreover, the interfacial thermal conductance can be effectively tuned by cross-plane strain. More specifically, a 5% tensile strain is able to reduce the interfacial thermal conductance by 70%; while a 5% compressive strain is able to increase the thermal conductance by 150%. Unexpectedly, the G/M interfacial thermal conductance is found to increase with increasing the defect density near the interface, which is in strong contrast to the in-plane thermal conductivity. This unexpected increase in thermal conductance can be explained by the enhanced phonon coupling at the G/M interface arising from the enhanced interface friction caused by the defects.

As a new two-dimensional (2D) material, phosphorene has drawn growing attention owing to its novel electronic properties, such as layer-dependent direct bandgaps and high carrier mobility. Herein we investigate the in-plane and cross-plane thermal conductivities of single- and multi-layer phosphorene, focusing on geometrical (sample size, orientation and layer number) and strain (compression and tension) effects. A strong anisotropy is found in the in-plane thermal conductivity with its value along the zigzag direction being much higher than that along the armchair direction. Interestingly, the in-plane thermal conductivity of multi-layer phosphorene is insensitive to the layer number, which is in strong contrast to that of graphene where the interlayer interactions strongly influence the thermal transport. Surprisingly, tensile strain leads to an anomalous increase in the in-plane thermal conductivity of phosphorene, in particular in the armchair direction. Both the in-plane and cross-plane thermal conductivities can be modulated by external strain; however, the strain modulation along the cross-plane direction is more effective and thus more tunable than that along the in-plane direction. Our findings here are of great importance for the thermal management in phosphorene-based nanoelectronic devices and for thermoelectric applications of phosphorene.

We perform non-equilibrium molecular dynamics calculations to study the coupling effects of surface, defects, and strain on heat transport in silicon nanowires (SiNWs). It is found that, for single-crystalline SiNWs, rippled surface and vacancy defects can significantly reduce the thermal conductivity κ of SiNWs. In addition, through surface amorphization, the thermal conductivity can be further remarkably reduced, due to strong phonon scattering at the interface as well as the non-propagating diffusion of phonons in the amorphous region. However, the κ value of amorphous-surface SiNWs is not sensitive to the roughness and vacancy in amorphous surface. Besides surface and defects, it has been observed that stress-induced defects/deforms can introduce additional mechanism of phonon scattering, which decrease the thermal conductivity. The present work demonstrates that, through phonoic engineering to tune the thermal conductivity, SiNWs can be promising candidates for thermoelectric applications.

Cellular metallic glasses (MGs) can be good candidates for structural and functional applications due to their light weight, enhanced ductility and excellent energy absorption performance. Motivated by the size-dependent mechanical properties of MGs, we perform molecular dynamics simulations of uniaxial compression testing on MGs with nanoscale cellular structures, focusing on the influence of cell size, shape, pattern, and ratio of the cell size to the spacing on the deformation behaviors. With increasing cell size, a clear transition in failure mode from localized plastic deformation to homogeneous plastic deformation is observed. Furthermore, we show that MGs with circular cellular structures exhibit higher strength and energy absorption capacity than those with hexagonal cellular structures due to the reduction of the stress concentration in the honeycomb's joints, in agreement with recent experimental studies. Finally, with decreasing the cell size to spacing ratio, shear band (SB) formation is observed when the shortest distance between the neighbouring cells in the diagonal direction reaches a value comparable to the SB thickness. Our present results not only provide an in-depth atomic understanding of the deformation and failure mechanisms of MGs with nanoscale cellular structures, but also propose an effective way to attain optimized material properties through controlling the cell features.

We investigate the thermal conductivity of interlayer-bonded bilayer graphene, trilayer graphene, and pyrolytic graphite using molecular dynamics simulations. We find that interlayer sp 3 bonding greatly reduces the thermal conductivity, with the reduction up to 80 %, depending on the distribution and coverage of sp 3 bonds. Besides, we find that tensile strain reduces the thermal conductivity of interlayer-bonded graphene further by up to 50 %. Our findings suggest the possibility of using interlayer sp 3 bonds and tensile strain to tune and manipulate the thermal conductivity of multi-layer graphene, which may be useful in thermal management of graphene-based nanodevices and in thermoelectric applications of graphene.

Monolayer molybdenum disulfide (MoS2), a new two-dimensional material beyond graphene, has attracted tremendous attention recently. Its applications in nanoelectronic and thermoelectric devices usually require manipulating the thermal transport properties. Using nonequilibrium molecular dynamics simulations, we investigated the effects of lattice defects and mechanical strain on the thermal conductivity of MoS2. We found that the thermal conductivity of monolayer MoS2 can be effectively tuned by introducing even a small amount of lattice defects. For example, a 0.5% concentration of mono-Mo vacancies is able to reduce the thermal conductivity by about 60%. Remarkably, the thermal conductivity of the defected sample can further be tuned by mechanical strain. For example, a 12% tensile strain is able to reduce the thermal conductivity by another 60%. We also found that the tensile strain exerts almost the same impact on the thermal conductivity of both pristine and defective MoS2, which signifies that there...

Monatomic metallic glasses (MGs), being the simplest glass formers, have been obtained successfully in recent experiments, offering unique possibilities for studying the local atomic structures in glasses. In this study, the atomic-level structures in tantalum (Ta) liquid and Ta monatomic MG have been analyzed using ab initio molecular dynamics simulations. A comparison of atomic structures in monatomic MGs and in multicomponent MGs has been made. It is found that the dominant Voronoi polyhedra in Ta monatomic MG is the distorted icosahedra 〈0, 1, 10, 2〉 rather than the full icosahedra 〈0, 0, 12, 0〉, which has been identified as the basic local structure in many multicomponent MGs. Our results reveal that the fundamental process of Ta MG formation is the formation of icosahedral type clusters, including the distorted and full icosahedra. The global atomic packing in Ta MG can be characterized by the spherical-periodic order combined with the local translational symmetry of atomic arrangements in MGs, in good agreement with previous theoretical and experimental studies on multicomponent MGs. Our findings have implications in understanding the formation of MGs, and provide a check for the existing atomic structural models proposed over the years for MGs.

Increasing demand for high-quality additive manufactured parts in the aerospace, automotive, medical, and oil and gas industries requires careful control of the part microstructure, residual stress, and density homogeneity. In order to improve part quality, partial remelting of the as-built material during subsequent beam scans is desirable. Here, we make use of computer simulations to explicitly study remelting in laser- or electron beam-melting additive manufacturing. By explicitly implementing phase transformations between the powder, the liquid, and the bulk, we track the amount of material that is subject to remelting. The influence of the beam parameters, such as the beam size, scan speed and power, are investigated and both the cases of an exponential as well as a linear beam absorption profile are considered. We find that, at constant beam cross section, there is an optimal beam shape for remelting. Further, we show how the optimal shape changes when the scan speed is changed and in this case interlayer remelting is also specifically addressed. Calculations are presented for the model case of AISI 316L stainless steel but can be extended to a wide class of metals.

We investigate the mechanical properties of polycrystalline graphene deforming under uniaxial tension by using molecular dynamics simulations, focusing on the effects of grain size, temperature and strain rate. It is found that polycrystalline graphene with smaller grain size shows a larger drop of Young’s modulus and fracture strength. For the polycrystalline graphene with grain size of 7.5 nm, the Young’s modulus and fracture strength is about 80% and 40% that of single-crystalline graphene, respectively. Our simulation results also reveal that the Young’s modulus and fracture strength of polycrystalline graphene are more sensitive to the changes of temperature and strain rate than that of single-crystalline graphene. When temperature increases from 100 to 1200 K, the fracture strength of polycrystalline graphene reduces by around 45%. At room temperature (300 K), the fracture strength of polycrystalline graphene increases by 10% as the strain rate increases from 5 × 10−5 to 5 × 10−3 ps−1. Furthermore, the strain rate has a stronger influence on the fracture strength of polycrystalline graphene at a higher temperature than at a lower temperature; while the temperature has a stronger influence at a lower strain rate than at a higher strain rate. Our study thus provides a comprehensive understanding of the mechanical properties of polycrystalline graphene.

We investigate the thermal conductivity of chemically functionalized bilayer graphene sheet by using molecular dynamics simulations, focusing on the effects of different functional groups and tensile strain. Our simulation results show that chemical functionalization leads to the reduction in the thermal conductivity of bilayer graphene sheet and the reduction is proportional to the coverage of functional groups. Unlike the pristine graphene sheet, the tensile strain can enhance the thermal conductivity of functionalized bilayer graphene sheet. The underlying mechanisms for the thermal conductivity change are elaborated via the vibrational density of states of the graphene sheets.

The study of structural flaws is of paramount importance in engineering applications. However, the effect of flaws on both the macroscale and nanoscale metallic glasses (MGs) is a debatable topic, drawing from conflicting reports on notch strengthening, notch weakening, and notch insensitivity for MGs. In the present study, molecular dynamics simulations have been performed on nanoscale notched MGs to systematically investigate the influence of notch geometrical features on their fracture strengths and failure mechanisms. It is found that the symmetric double-edge notched MGs always induce a notch strengthening effect, which becomes more pronounced with increasing notch depth or sharpness. It is reasoned that the constrained growth of the plastic zone is responsible for the notch strengthening behavior. Accompanied by this notch strengthening, the deformation mode shifts from shear banding to homogeneous deformation within the un-notched ligament, when the un-notched ligament length is less than twice the shear band width. Furthermore, our simulations reveal that notch strengthening effect vanishes for asymmetric double-edge notched MGs and single-edge notched MGs. The present study provides significant insights into the deformation and failure mechanisms of nano-sized notched MGs, as well as useful guidelines for the design of MGs-based nanostructures.

The strength-ductility tradeoff has been a common long-standing dilemma in materials science. For example, superplasticity with a tradeoff in strength has been reported for Cu50Zr50 nanoglass (NG) with grain sizes below 5 nm. Here we report an improvement in strength without sacrificing superplasticity in Cu50Zr50 NG by using a bimodal grain size distribution. Our results reveal that large grains impart high strength, which is in striking contrast to the physical origin of the improvement in strength reported in the traditional nanostructured metals/alloys. Furthermore, the mechanical properties of NG with a bimodal nanostructure depend critically upon the fraction of large grains. By increasing the fraction of the large grains, a transition from superplastic flow to failure by shear banding is clearly observed. We expect that these results will be useful in the development of a novel strong and superplastic NG.

It is believed that the basic local cluster in metallic glasses (MGs) is the key to understand the origin of glass-forming ability (GFA) of MGs. Our ab initio simulations on the basic cluster in the binary Cu–Zr MGs as well as in the ternary Cu–Zr–Al(Ag) MGs provide the most straightforward evidence that the electronic stability of the basic cluster in MGs is the origin of the GFA. Our calculations reveal that the stability of the basic cluster is determined by its electronic states near the Fermi level. For the basic clusters in the best glass formers of Cu–Zr MGs, a gap near Fermi level is observed, indicating the stability of the basic clusters. With substitution of one Al(Ag) atom into the basic cluster in Cu50Zr50 MGs, a strong sp-d coupling of Al(Ag) with Cu and Zr is observed, which results in a stronger bonding and thus more stability of the basic cluster. Our findings provide a check for the atomic structural models proposed over the years for MGs, and have implications for understanding the formation and properties of MGs.