Molecular Dynamics Investigations On The Mechanisms Of Plastic Deformation And Phase Transformation Of Nanocrystalline Metals Under Shock Compression

The mechanisms of plastic deformation and phase transformation of metals under shock compression have been important and difficult subjects in material sciences and shock wave physics for a long time. In this paper, we have studied the microscopic mechanisms of plastic deformation of the face-centered cubic (fcc) structured nanocrystalline (nc) aluminum and copper, and phase transformation of body-centered cubic (bcc) structured nc iron under shock compression by means of molecular dynamics (MD) simulations. Because the shock front is one of the most important macroscopic representations of the plastic deformation and phase transformation processes of materials under shock compression, in this paper we focus on the detailed structures of shock front, the influencing factors on the shock-front structure and the atomistic processes of plastic deformations or phase transformations occuring in the shock-front regions.First, we investigate the method to construct metallic nc samples for MD simulations. There are two procedures in this method. First, we construct the initial nc samples by Voronoi geometrical method. Secondly, the initial samples are relaxed to reduce the high energy and stress caused by the unphysical grain boundaries (GBs). The residual internal stress is employed to define the difference between the simulated and the experimental synthesized samples for the first time. The initial nc samples generated by Voronoi method can reach their local minimum energy states after quenching about 10ps, and there is no evidence that much longer quenching time can reduce the energy and stress further. Then the simulated annealing at ambient temperature and stress for 40 ~ 100ps is needed to approach the lowest global energy and the lowest residual stress state. It is not necessary to anneal the samples longer. We find that the higher annealing temperature is set, the less annealing time is needed, but the annealing temperature must not be higher than 65% of the melting point. Meanwhile, we monitor the structures of GBs, the temperature descending processes and the local distributions of averaged internal stresses and the energies of the samples during the relaxation processes, and calculate the elastic constants of the nc samples under various conditions at the end of relaxations. The results show that the energy, residual internal stress and elastic constants of the computer generated samples at final state are close to the experimental preparation of nc metals.Secondly, the effects of GBs on the shock-front structures and the mechanisms of plastic deformations of nc aluminum under shock compression are qualitatively investigated, and the quantitative relationship between shock stress and width of shock front of nc copper is also studied. The influences of GBs-related plastic deformation on the shock-front structures are obtained by observing the shock stress and particle velocity profile and the atomistic views of plastic deformation processes. The quantitative relationship between the shock-front width of nc copper and the shock stress can be calculated by a series of MD simulations at different shock compression conditions, and it is almost alike to other MD simulation results. The results show that: after the elastic wave generates, the GBs sliding and deformation dominated the early plastic deformation procedure, then the partial dislocations are nucleated at the deformed GBs and spread within grains, finally the process of stacking faults, deformation twins and full dislocations forming in grains dominated the latter part of the plastic deformation. The structural characteristics after the shock front swept over are that the stacking faults and the deformation twins are left in grains and the majority of the full dislocations are annihilated at the opposite GBs. We also find that the contribution of GB-mediated plasticity to the shock-front is comparable to dislocation-mediated plasticity for nc aluminum.Then, a comparative study of plastic deformation mechanisms and the shock-front structures between nc aluminum and copper with the same texture and grain size is carried out. By comparing the one-dimensional (1D), 2D shock wave profiles and the atomistic views of plastic deformation processes of aluminum and copper at the same shock stress and strain, we find that the rise rate of nc copper in the elastic deformation is higher than that of nc aluminum. The reason may be that the lattice constant of aluminum is slightly larger than copper, leading to a more extended deformation range before softening, it will need more time to compress aluminum atom than copper. We also find that the duration of GB-dominated plastic deformation of nc copper is shorter than that of nc aluminum, and that the contribution to shock-front width from the GB-dominated plastic deformation is smaller than that of nc aluminum. The overall width of the shock front of nc copper is less than the width of nc aluminum, and the dislocation densities of nc copper are higher than the densities of nc aluminum. The mechanisms of plastic deformation for nc copper and aluminum are also different slightly, that is to say, we have observed partial dislocations, full dislocations and deformation twins in nc aluminum, but only partial dislocations are observed in nc copper. On the other hand, we find some GB-related plastic mechanisms like GB sliding, GB thickening and GB bending in nc aluminum, but no GB bending is found in nc copper. The main reason for these differences of plastic mechanisms between these two nc ductile metals is that the intrinsic parameters, such as lattice constants, stacking fault energies and shear modulus and so on result in a higher critical resolved shear stress for the leading dislocation emission of aluminum than that of copper, and finally lead to less dislocations emitted and more GB-related mechanisms and wider shock-front widths in nc aluminum.Finally, the shock-induced phase transformations of nc irons with different grain sizes are investigated. The study shows that the critical shock stress for shock-induced phase transformation of nc irons is about 15 GPa. Under shock compression, the nc irons first experience elastic deformation, then plastic deformation purely caused by grain boundaries, after that phase transformation nucleated mostly at the grain boundaries, and finally nucleation areas expanding into the entire samples. These processes can be reflected by the stress and particle velocity profiles, and also be distinguished by local atomic structures analyses in the corresponding areas. Most of the bcc irons atoms transform to hexagonal-closed packed (hcp) structure except the GB atoms. The major microstructures of the nc iron samples at final state are GBs and hcp atoms with small amount of fcc atoms as the twin boundaries. There are more GB atoms for the larger grains in the range of grain sizes studied in this paper, and more bcc iron atoms transform to hcp iron atoms, and more energy are dissipated during phase change, and turn out to bring down the final shock stress.This study primarily discuss the effects of GB which is a type of prevalent defect in materials on the mechanisms of plastic deformation and phase transformation of ductile metals, and how to further affect the typical macroscopic response of materials subject to shock compression–– shock front. The results can improve the understanding of physical procedure of the plasticity and phase transformation, and also have reference significance for revealing the links between macro-micro processes at nanosecond- micrometer scale and building the physical images of micro-processes. Although some quantitative results presented in this study for nc metals are different to the results in shock compression experiments for traditional coarse-grained (cg) metals, but the understanding achieved in this paper is also helpful to investigate the dynamic responses of ordinary cg metals, such as interpretations of microscopic mechanisms of the shock-front structures.