Investigation on the Mechanical Properties of Polycrystalline Mg Using Molecular Dynamics Simulation

Magnesium(Mg)and its composites have been widely used in different fields,but the mechanical properties and deformation mechanisms of polycrystalline Mg(polyMg)at the atomic scale are poorly understood.In this paper,the effects of grain size,temperature,and strain rate on the tensile properties of polyMg are explored and discussed by theMolecular dynamics(MD)simulation method.The calculated results showed that there exists a critical grain size of 10 nm for the mechanical properties of polyMg.The flow stress decreases with the increase of grain size if the average grain size is larger than 10 nm,which shows the Hall-Petch effect,and the deformation mechanism of large grain-sized polyMg is mainly dominated by the movement of dislocations.When the average grain size is less than 10 nm,it shows the reverse Hall-Petch effect that the flow stress decreases with the decrease of grain size,and the deformation mode of polyMg with small grain-size is the movement and deformation of atoms at the grain boundary.Due to the more active motion of atoms as the system temperature increases,the material can easily reach the plastic stage under tensile loading,and the mechanical properties of polyMg decrease at high temperatures.The strain rate has a hardening effect on the properties of composite.Based on our calculated results,it can provide theoretical guidance for the applications of Mg metal and Mg matrix composites.

Molecular dynamics simulations of mechanical properties of epoxy-amine:Cross-linker type and degree of conversion effects

Molecular dynamics(MD)simulations are conducted to study the thermo-mechanical properties of a family of thermosetting epoxy-amines.The crosslinked epoxy resin EPON862 with a series of cross-linkers is built and simulated under the polymer consistent force field(PCFF).Three types of curing agents(rigidity1,3-phenylenediamine(1,3-P),4,4-diaminodiphenylmethane(DDM),and phenol-formaldehyde-ethylenediamine(PFE))with different numbers of active sites are selected in the simulations.We focus on the effects of the cross-linkers on thermo-mechanical properties such as density,glass transition temperature(T_(g)),elastic constants,and strength.Our simulations show a significant increase in the Tg,Young’s modulus and yield stress with the increase in the degree of conversion.The simulation results reveal that the mechanical properties of thermosetting polymers are strongly dependent on the molecular structures of the cross-linker and network topological properties,such as end-to-end distance,crosslinking density and degree of conversion.

Mechanical properties and thermal conductivity of pristine and functionalized carbon nanotube reinforced metallic glass composites:A molecular dynamics approach

This work uses the molecular dynamics approach to study the effects of functionalization of carbon nanotubes(CNTs)on the mechanical properties of Cu64Zr36 metallic glass(MG).Three types of functional groups,carboxylic,vinyl and ester were used.The effect of CNT volume fraction(Vf)and the number of functional groups attached to CNT,on the mechanical properties and thermal conductivity of CNT-MG composites was analysed using Biovia Materials Studio.At lower values of Vf(from 0 to 5%),the percentage increase in Young’s modulus was approximately 66%.As the value of Vf was increased further(from 5 to 12%),the rate of increase in Young’s modulus was reduced to 16%.The thermal conductivity was found to increase from 1.52 W/mK at Vf?0%to 5.88 W/mK at Vf?12%,thus giving an increase of approximately 286%.Functionalization of SWCNT reduced the thermal conductivity of the SWCNT-MG composites.

Molecular Dynamics Study of Collagen Fibrils: Relation between Mechanical Properties and Molecular Chirality

Collagen is a basic biopolymer usually found in animal bodies, but its mechanical property and behavior are not sufficiently understood so as to apply to effective regenerative medicine and so on. Since the collagen material is composed of many hierarchical structures from atomistic level to tissue or organ level, we need to well understand fundamental and atomistic mechanism of the collagen in mechanical response. First, we approach at exactly atomistic level by using all-atom modeling of tropocollagen (TC) molecule, which is a basic structural unit of the collagen. We perform molecular dynamics (MD) simulations concerning tensile loading of a single TC model. The main nature of elastic (often superelastic) behavior and the dependency on temperature and size are discussed. Then, to aim at coarse-graining of atomic configuration into some bundle structure of TC molecules (TC fibril), as a model of higher collagen structure, we construct a kind of mesoscopic model by adopting a simulation framework of beads-spring model which is ordinarily used in polymer simulation. Tensile or compression simulation to the fibril model reveals that the dependency of yield or buckling limit on the number of TCs in the model. Also, we compare the models with various molecular orientations in winding process of initial spiral of TC. The results are analyzed geometrically and it shows that characteristic orientational change of molecules increases or decreases depending on the direction and magnitude of longitudinal strain.

Molecular Dynamics Study on Mechanical Properties in the Structure of Self-Assembled Quantum Dot

Stress and strain in the structure of self-assembled quantum dots constructed in the Ge/Si(001) system is calculated by using molecular dynamics simulation. Pyramidal hut cluster composed of Ge crystal with {105} facets surfaces observed in the early growth stage are computationally modeled. We calculate atomic stress and strain in relaxed pyramidal structure. Atomic stress for triplet of atoms is approximately defined as an average value of pairwise (virial) quantity inside triplet, which is the product of vectors between each two atoms. Atomic strain by means of atomic strain measure (ASM) which is formulated on the Green’s definition of continuum strain. We find the stress (strain) relaxation in pyramidal structure and stress (strain) concentration in the edge of pyramidal structure. We discuss size dependency of stress and strain distribution in pyramidal structure. The relationship between hydrostatic stress and atomic volumetric strain is basically linear for all models, but for the surface of pyramidal structure and Ge-Si interface. This means that there is a reasonable correlation between atomic stress proposed in the present study and atomic strain measure, ASM.

Molecular Dynamics Simulation of Mechanical Properties for α-SiO₂Crystal

The mechanical properties of the α-SiO2 crystal are studied by molecular dynamics method with Tersoff potential function. The results show that the α-SiO2 crystal goes through elastic deformation, plastic deformation and fracture deformation in the process of uniaxial loading at room temperature. The α-SiO2 is from crystal phase transformation to amorphous phase in plastic deformation. And also by studying the influence of temperature on the tensile mechanical properties of α-SiO2, it finds that the yield strength and elastic modulus of α-SiO2 decrease gradually as the temperature increases. Moreover, the higher the temperature, the lower the fracture stress and fracture strain;the α-SiO2 crystal is easy to be broke under high temperature uniaxial loading. And it also finds that the crack is able to decrease the mechanical properties of α-SiO2 crystal.

Enhanced mechanical and thermal properties of two-dimensional SiC and GeC with temperature and size dependence

Two-dimensional materials with novel mechanical and thermal properties are available for sensors,photodetectors,thermoelectric,crystal diode and flexible nanodevices.In this investigation,the mechanical and thermal properties of pristine SiC and GeC are explored by molecular dynamics simulations.First,the fracture strength and fracture strain behaviors are addressed in the zigzag and armchair directions at 300 K.The excellent toughness of SiC and GeC is demonstrated by the maximal fracture strain of 0.43 and 0.47 in the zigzag direction,respectively.The temperature-tunable tensile strength of SiC and GeC is also investigated.Then,using non-equilibrium molecular dynamics(NEMD)calculations,the thermal performances of SiC and GeC are explored.In particular,the thermal conductivity of SiC and GeC shows a pronounced size dependence and reaches up to 85.67 W·m^(-1)-K^(-1)and 34.37 W·m^(-1)-K^(-1),respectively.The goal of our work is to provide a theoretical framework that can be used in the near future.This will enable us to design an efficient thermal management scheme for two-dimensional materials in electronics and optoelectronics.

Mechanical property and deformation mechanism of gold nanowire with non-uniform distribution of twinned boundaries:A molecular dynamics simulation study

The mechanical property and deformation mechanism of twinned gold nanowire with non-uniform distribution of twinned boundaries(TBs)are studied by the molecular dynamics(MD)method.It is found that the twin boundary spacing(TBS)has a great effect on the strength and plasticity of the nanowires with uniform distribution of TBs.And the strength enhances with the decrease of TBS,while its plasticity declines.For the nanowires with non-uniform distribution of TBs,the differences in distribution among different TBSs have little effect on the Young's modulus or strength,and the compromise in strength appears.But the differences have a remarkable effect on the plasticity of twinned gold nanowire.The twinned gold nanowire with higher local symmetry ratio has better plasticity.The initial dislocations always form in the largest TBS and the fracture always appears at or near the twin boundaries adjacent to the smallest TBS.Some simulation results are consistent with the experimental results.

Structure and Mechanical Behavior of Cellulose Nanofiber and Micro-Fibrils by Molecular Dynamics Simulation

Cellulose nanofiber (CNF) and CNF micro-fibrils (CNF-MFs) are computationally modeled by molecular dynamics with united atom (UA) methodology of polymers. Structural stability and mechanical properties of these materials are focused on. Diffusion coefficient decreases with increase of the number of shells in CNF-MF. The structure of CNF-MFs with crystalline alignment is totally stabilized with twist which is an accumulation of torsion angles at Glycosidic bonds between monomers inside CNFs. Unique fiber drawing simulation, where a single CNF fiber is taken out of CNF-MF structure, is first conducted. The CNF fiber which is drawn out stretches up to relatively large strain, with linear increase of tensile stress. The computation results show that, the larger the number of shell structure of CNF-MF is, the larger the stretch and the stress of drawn fibers are.

Mechanical Properties of Ni-Coated Single Graphene Sheet and Their Embedded Aluminum Matrix Composites

The effects of Ni coating on the mechanical behaviors of single graphene sheet and their embedded Al matrix composites under axial tension are investigated using molecular dynamics （MD） simulation method. The results show that the Young＇s moduli and tensile strength of graphene obviously decrease after Ni coating. The results also show that the mechanical properties of Al matrix can be obviously increased by embedding a single graphene sheet. From the simulation, we also find that the Young＇s modulus and tensile strength of the Ni-coated graphene/Al composite is obviously larger than those of the uncoated graphene/Al composite. The increased magnitude of the Young＇s modulus and tensile strength of graphene/Al composite are 52.27% and 32.32% at 0.01 K, respectively, due to Ni coating. By exploring the effects of temperature on the mechanical properties of single graphene sheet and their embedded Al matrix composites, it is found that the higher temperature leads to the lower critical strain and tensile strength.

Molecular dynamics simulation on mechanisms of plastic anisotropy in nanotwinned polycrystalline copper with{111}texture during tensile deformation

Based on molecular dynamics(MD)simulation,the mechanisms of plastic anisotropy in nanotwinned polycrystalline copper with{111}texture during tensile deformation were systematically studied from the aspects of Schmid factor of the dominant slip system and the dislocation mechanism.The results show that the Schmid factor of dominated slip system is altered by changing the inclining angle of the twin boundaries(TBs),while the yield stress or flow stress does not strictly follow the Schmid law.There exist hard and soft orientations involving different dislocation mechanisms during the tensile deformation.The strengthening mechanism of hard orientation lies in the fact that there exist interactions between the dislocations and the TBs during plastic deformation,which leads to the dislocation blocking and reactions.The softening mechanism of soft orientation lies in the fact that there is no interaction between the dislocations and the TBs because only the slip systems parallel to the TBs are activated and the dislocations slip on the planes parallel to the TBs.It is concluded that the plastic anisotropy in the nanotwinned polycrystalline copper with{111}texture is aroused by the combination effect of the Schmid factor of dominated slip system and the dislocation mechanism.

Simulation on microstructure evolution and mechanical properties of Mg-Y alloys: Effect of trace Y

The influence of trace Y on the microstructure evolution and mechanical properties of Mg_(100−x)Y_(x)(x=0.25,_(0.75),1.5,3,4,5,at.%)alloys during solidification process was investigated via molecular dynamics(MD)simulations.The results show that the Mg_(100−x)Y_(x) alloys are mainly characterized by a face-centered cubic(FCC)crystal structure;this is different from pure metal Mg,which exhibits a hexagonal close packed(HCP)structure at room temperature.Among these alloys,Mg_(99.25)Y_(0.75) has a larger proportion of FCC cluster structures,with the highest fraction reaching 56.65%.As the content of the Y increases up to 5 at.%(Mg95Y5 alloy),the amount of amorphous structures increases.The mechanical properties of the Mg_(100−x)Y_(x) alloys are closely related to their microstructures.The Mg_(99.25)Y_(0.75) and Mg_(97)Y_(3) alloys exhibit the highest yield strengths of 1.86 and 1.90 GPa,respectively.The deformation mechanism of the Mg−Y alloys is described at the atomic level,and it is found that a difference in the FCC proportion caused by different Y contents leads to distinct deformation mechanisms.

Mechanical properties of self-irradiated single-crystal copper

Molecular dynamics simulations are performed to investigate the influence of irradiation damage on the mechanical properties of copper. In the simulation, the energy of primary knocked-on atoms （PKAs） ranges from 1 to 10 keV, and the results indicate that the number of point defects （vacancies and interstitials） increases linearly with the PKA energy. We choose three kinds of simulation samples： un-irradiated and irradiated samples, and comparison samples. The un-irradiated samples are defect-free, while irradiation induces vacancies and interstitials in the irradiated samples. It is found that due to the presence of the irradiation-induced defects, the compressive Young modulus of the single-crystal Cu increases, while the tensile Young modulus decreases, and that both the tensile and compressive yield stresses experience a dramatic decrease. To analyze the effects of vacancies and interstitials independently, the mechanical properties of the comparison samples, which only contain randomly distributed vacancies, are investigated. The results indicate that the vacancies are responsible for the change of Young modulus, while the interstitials determine the yield strain.

THE COUPLED EFFECTS OF MECHANICAL DEFORMATION AND ELECTRONIC PROPERTIES IN CARBON NANOTUBES

Coupled effects of mechanical and electronic behavior in single walled carbon nanotubes are investigated by using quantum mechanics and quantum molecular dynamics.It is found that external applied electric fields can cause charge polarization and significant geometric deformation in metallic and semi-metallic carbon nanotubes.The electric induced axial tension ratio can be up to 10% in the armchair tube and 8.5% in the zigzag tube.Pure external applied load has little effect on charge distribution,but indeed influences the energy gap.Tensile load leads to a narrower energy gap and compressive load increases the gap.When the CNT is tensioned under an external electric field,the effect of mechanical load on the electronic structures of the CNT becomes significant,and the applied electric field may reduce the critical mechanical tension load remarkably.Size effects are also discussed.

Effects of tilt interface boundary on mechanical properties of Cu/Ni nanoscale metallic multilayer composites

The effect of tilt interfaces and layer thickness of Cu/Ni multilayer nanowires on the deformation mechanism are investigated by molecular dynamics simulations. The results indicate that the plasticity of the sample with a 45° tilt angle is much better than the others. The yield stress is found to decrease with increasing the tilt angle and it reaches its lowest value at 33°. Then as the tilt angle continues to increase, the yield strength increases. Furthermore, the studies show that with the decrease of layer thickness, the yield strength gradually decreases. The study also reveals that these different deformation behaviors are associated with the glide of dislocation.

Molecular Dynamics Study of RDX/AMMO Propellant

Molecular dynamics simulations have been performed to investigate well-known energetic material cyelotrimethylene trinitramine （RDX） crystal, 3-azidomethyl-3-methyloxetane （AMMO） and RDX/AMMO propellant. The results show that the binding energies on different crystalline surface of RDX changes in the order of （010）〉（100）〉 （001）. The interactions between RDX and AMMO have been analyzed by means of pair correlation functions. The mechanical properties of RDX/AMMO propellant, i.e. elastic coefficients, modulus, Cauchy pressure, and Poisson＇s ratio, etc., have been obtained. It is found that mechanical properties are effectively improved by adding some amounts of AMMO polymers, and the overall effect of AMMO on three crystalline surfaces of RDX changes in the order of （100）〉（010）〉（001）. The energetic properties of RDX/AMMO propellant have also been calculated and the results show that compared with the pure RDX crystal, the standard theoretical specific impulse of RDX/AMMO propellant decrease, but they are still superior to those of double base propellant.

Molecular dynamics simulation of mechanical properties of TATB/fluorine-polymer PBXs along different surfaces

The mechanical properties of PBXs composed of the well-known insensitive explo-sive TATB (1,3,5-Triamino-2,4,6-trinitrobenzene) crystal and four kinds of typical fluo-rine-polymers, i.e. poly (vinylidene difluoride) (PVDF), polychlorotrifluoroethylene (PCTFE), fluo-rine rubber (F2311) and fluorine resin (F2314) individually, have been simulated by molecular dy-namics (MD) method. The results show that the mechanical properties of TATB can be effectively improved by blending with a small amount of fluorine-polymers. Different mechanical properties of PBXs are obtained by putting fluorine-polymer binders parallel to different crystalline surfaces. The whole effect of improving mechanical properties is found to be (010)?(100)>(001).

Molecular dynamics investigation of mechanical properties of single-layer phagraphene

Phagraphene is a very attractive two-dimensional (2D) full carbon allotrope with very interesting mechanical, electronic, optical, and thermal properties. The objective of this study is to investigate the mechanical properties of this new graphene like 2D material. In this work, mechanical properties of phagraphene have been studied not only in the defect-free form, but also with the critical defect of line cracks, using the classical molecular dynamics simulations. Our study shows that the pristine phagraphene in zigzag direction experience a ductile behavior under uniaxial tensile loading and the nanosheet in this direction are less sensitive to temperature changes as compared to the armchair direction. We studied different crack lengths to explore the in fluence of defects on the mechanical properties of phagraphene. We also investigated the temperature effect on the mechanical properties of pristine and defective phagraphene. Our classical atomistic simulation results confirm that larger cracks can reduce the strength of the phagraphene. Moreover, it was shown the temperature has a considerable weakening effect on the tensile strength of phagraphene. The results of this study may be useful for the design of nano-devices using the phagraphene.

Mechanical properties of Fe-based amorphous–crystalline composite: a molecular dynamics simulation and experimental study

A new Fe-based amorphous–crystalline composite without non-metallic elements, Fe_(55)Cr_(15)Mo_(15)Ni_(10)W_(5), was prepared by melt-spinning. The formation ability and structure information were investigated by X-ray diffractometer(XRD), energy-dispersive spectrometer(EDS) and scanning electron microscope(SEM). The mechanical properties of the amorphous–crystalline composite were investigated by nanoindentation. A molecular dynamics simulation study was performed to simulate the formation of Fe_(55)Cr_(15)Mo_(15)Ni_(10)W_(5) amorphous alloy. The mechanical properties were obtained by compression simulations simultaneously. The results indicate that the Fe_(55)Cr_(15)Mo_(15)Ni_(10)W_(5) ribbon is an amorphous–crystalline composite structure with good ductility, and the hardness of the amorphous–crystalline composite is about 75%higher than that of master ingot. The simulation mechanical properties are in good agreement with the results of nanoindentation at the nanoscale.