The dynamics equation for each individual atom is established directly around the equilibrium state of the system of N atoms based on the inter-atomic potential energy of EAM model.Using the theory of lattice dynamics and periodical boundary condition,the 3N×3N stiffness matrix in eigen equations of vibration frequencies for a parallelepiped crystal is reduced to a 3n×3n matrix of eigen equations of vibration frequencies for a unit lattice.The constitutive relation of the crystal at finite temperature is extracted based on the quantum-mechanical principle.The thermodynamic properties and the stress-strain relationships of crystal Cu with large plastic deformation at different temperatures are calculated,the calculation results agree well with experimental data.
Based on the crystal plasticity theory and interatomic potential, in this paper a new thermo-elasto-plasticity constitutive model is proposed to study the behavior of metal crystals at finite temperature. By applying the present constitutive model, the stress-strain curves under uniaxial tension at different temperatures are calculated for the typical crystal Al, and the calculated results are compared with the experimental results. From the comparisons, it can be seen that the present theory has the capability to describe the thermo-elasto-plastic behavior of metal crystals at finite temperature through a concise and explicit calculation process.
The influence of specimen size on the mechanical behavior of Au pillars is studied by means of molecular dynamics (MD) simulations with the EAM potential.Under compression at 300 K,as the deformation of pillars is in the plastic stage,nucleation of partial dislocations is observed.The coupling effect of surface stress and thermal activation is considered when analyzing the size effect on the yield property of the Au pillars.It appears that both the tensile stress component and the temperature in the surface layer impart significant effect on the mechanical behaviors of the nano-sized Au pillars.
It has been widely accepted that silicene is a topological insulator, and its gap closes first and then opens again with increasing electric field, which indicates a topological phase transition from the quantum spin Hall state to the band insulator state. However, due to the relatively large atomic spacing of silicene, which reduces the bandwidth, the electron–electron interaction in this system is considerably strong and cannot be ignored. The Hubbard interaction, intrinsic spin orbital coupling(SOC), and electric field are taken into consideration in our tight-binding model, with which the phase diagram of silicene is carefully investigated on the mean field level. We have found that when the magnitudes of the two mass terms produced by the Hubbard interaction and electric potential are close to each other, the intrinsic SOC flips the sign of the mass term at either K or K for one spin and leads to the emergence of the spin-polarized quantum anomalous Hall state.
Bending-induced phase transition in monolayer black phosphorus is investigated through first principles calculations.By wrapping the layer into nanotubes along armchair and zigzag directions with different curvatures, it is found that phase transitions of the tubes occur when radius of curvature is smaller than 5 in bending along the zigzag direction, while the tubes remain stable along the armchair direction. Small zigzag tubes with odd numbered monolayer unit cells tend to transfer toward armchair-like phases, but the tubes with even numbered monolayer unit cells transfer into new complex bonding structures. The mechanism for the bending-induced phase transition is revealed by the comprehensive analyses of the bending strain energies, electron density distributions, and band structures. The results show significant anisotropic bending stability of black phosphorus and should be helpful for its mechanical cleavage fabrication in large size.
We find by ab initio simulations that significant overall tensile strain can be induced by pure bending in a wide range of two-dimensional crystals perpendicular to the bending moment, just like an accordion being bent to open.This bending-induced tensile strain increases in a power law with bent curvature and can be over 20% in monolayered black phosphorus and transition metal dichalcogenides at a moderate curvature of 2 nm-1but more than an order weaker in graphene and hexagon boron nitride. This accordion effect is found to be a quantum mechanical effect raised by the asymmetric response of chemical bonds and electron density to the bending curvature.
This paper presents a new continuum thermal stress theory for crystals based on interatomic potentials.The effect of finite temperature is taken into account via a harmonic model.An EAM potential for copper is adopted in this paper and verified by computing the effect of the temperature on the specific heat,coefficient of thermal expansion and lattice constant.Then we calculate the elastic constants of copper at finite temperature.The calculation results are in good agreement with experimental data.The thermal stress theory is applied to an anisotropic crystal graphite,in which the Brenner potential is employed.Temperature dependence of the thermodynamic properties,lattice constants and thermal strains for graphite is calculated.The calculation results are also in good agreement with experimental data.
In this study, molecular dynamics simulations were carried out to study the effect of machining velocities on the mechanism of chip formation in nano-metric copper. A wide range of cutting velocities was performed from 10 to 2000 m/s, and the microstructure's evolution from a crystalline state to an amorphous state was studied. At the low machining velocity, dislocations were generated from the surface in front of the tool, and the immobile dislocation deduced by the cross slip of dislocation was observed. At the high machining velocity, no crystal dislocation nucleated, but instead disorder atoms were found near the tool. Temperature near the tool region increased with the increasing machining velocities, and the temperature had an important effect on the phase transition of the crystal structure.
Molecular dynamics simulations are performed to investigate the deformation behavior of nanocrystalline Ni with pre-twin atom structure.The simulation sample is composed of four grains with average size 12 nm.The simulation technique of isobaric-isothermal ensemble(NPT) with high pressure is applied to obtain a sample with two circle twins.Under uniaxial tensile and shear loading,as well as different detwinning deformation behaviors are observed.Under uniaxial tension the detwinning deformation is induced by the event of grain growth,and it is supported by local energy analysis.Under the shear loading the detwinning deformation is related to the loading rate.The results show that there may be a critical shear rate.As the shear rate is sufficiently high the circle twin is found to be failed;as the shear rate is less than that rate,the size of circle twin become smaller and gradually approach a constant value.Our simulation results are in good agreement with experiment observation.
