Molecular Dynamics Simulation Of Shock Wave Propagation In Nano-copper

The research of dynamic response of materials attracts intensive attention of researchers from different fields, and it has widely applications in industry, weapon and aircraft design. Shock loading is a common method to generate high strain rate deformation in materials. Experiments can easily observe the profiles of shockwave, which reflect the dynamic properties of materials, such as equation of states, variation of strength et al. The variation of strength undergoes plastical deformation. For ductile metals, the plasticity contains dislocation sliding, grain boundary (GB) movements and phase transition. The macroscopic profiles of shock front we observed interpret all contribution of microscopic structure evolution. More deeply we understand shock science; more important is it to study plasticity from microscopic view. Although we can observe microscopic structure of recovery sample by transmission electron microscope (TEM), the dynamic process of microscopic structure deformation is hardly detected by experiments as well as the relationship between the macroscopic profile of shockwave and the microscopic structure evolution.Molecular dynamics simulation (MD) has advantage to directly observe the detailed microscopic process of deformation of materials. There were many MD simulations on shockwave propagation in monocrystalline metals, revealing details of dislocation nucleation and emission behind shockwave front and atomistic movements during phase transition. Those microscopic deformation mechanisms in monocrystalline metals decide the characteristics of profiles of shockwave front. However, the metals usually have polycrystalline structure. The GB participates intensively in plastically deformation, which is hardly observed by experiments. Nanocrystalline metals become the best objects to investigate the behavior of GB by MD because of its small grain size and high fraction of GB atoms. GB sliding and dislocation emission are found to be the most important deformation mechanisms of nanocrystalline metals under dynamic loading by means of MD. But because of the complexity of deformation in nanocrystalline metals, there still lack reports to connect the macroscopic shockwave profile to the microscopic deformation mechanisms on shockwave front in nanocrystalline metals.In this paper, shockwave propagation in nanocrystalline copper is studied by MD. The characteristics of microscopic structure deformation on shockwave front and its connection to macroscopic shockwave profile are revealed for the first time. The initiate configuration of nanocrystalline copper sample is established by an improved Voronoi method. Conjugate gradient method is applied to minimize its energy until the most stable structure is obtained. Furthermore, a thermal relaxation process is taken to reduce the sample's residual internal stress. By pair analyze method, the structure is found to fit the configuration of nanocrystalline copper observed by TEM very well.The sample for shock loading contains 8,890,877 atoms within a 120nm×30nm×30nm box , and the average grain size is 10nm. Grains' crystalline orientations along shock direction are limited to 3 narrow regions in order to separate dislocation emission and GB sliding in space as well as in sequence. shockwave is generated in the sample by piston method. The profile is measured precisely and the microscopic structure under shock loading is depicted by a program based on pair analyse method.The simulation results first give a multi-yield phenomenon in nanocrystalline copper under shock loading and the corresponding multi-wave structure.. Because of the limitation of grains' orientation in sample, the grain boundary sliding and dislocation emission are obviously separated on the profile of shockwave. The shockwave front consists of an elastically deforming area, a plastically deformation area dominated by grain boundary sliding and a plastically deformation area dominated by dislocation emission. When the shockwave arrives, the sample firstly experiences an elastically deformation. Then grain boundaries begin to slide inducing the sample to yield. Subsequently dislocations are nucleated at the grain boundaries, and slide across the interior of grain, which causes the interior of grains to yield, where the sample deforms dominantly by dislocation movement. The GB sliding mechanism and dislocation emission mechanism can be observed from the flow stress profile of shockwave front. The width of elastic front is small because of the elastic wave's anisotropy in grains; the width of plastic front is much bigger, because the width is not just decided by the plastic wave's anisotropy in different grains but also the variation yielding in different grains during shock loading. The total width including both the elastic and the plastic waves becomes smaller as the shock pressure increases.For the first time, the results connect the characteristic of microscopic structure evolution in nanocrystalline metal under shock loading with a macroscopic profile of shockwave. It not just promote the research of dynamic mechanical properties of nanocrystalline metal, but also provide a solid support for studying elastic-plastic properties of metals.