Shock Compression of Monocrystalline Copper: Atomistic Simulations BUYANG CAO, EDUARDO M. BRINGA, and MARC ANDRE ´ MEYERS Molecular dynamics (MD) simulations were used to model the effects of shock compression on [001] and [221] monocrystals. We obtained the Hugoniot for both directions, and analyzed the formation of a two-wave structure for the [221] monocrystal. We also analyzed the dis- location structure induced by the shock compression along these two crystal orientations. The topology of this structure compares extremely well with that observed in recent transmission electron microscopy (TEM) studies of shock-induced plasticity in samples recovered from flyer plate and laser shock experiments. However, the density of stacking faults in our simulations is 10 2 to 10 4 times larger than in the experimental observations of recovered samples. The difference between experimentally observed TEM and calculated MD results is attributed to two effects: (1) the annihilation of dislocations during post-shock relaxation (including unloading) and recovery processes and (2) a much shorter stress rise time at the front in MD (<1 ps) in comparison with flyer-plate shock compression (~1 ns). DOI: 10.1007/s11661-007-9248-9 Ó The Minerals, Metals & Materials Society and ASM International 2007 I. INTRODUCTION MOLECULAR dynamics (MD) is currently capable of simulating the large-scale shock compression of crystals; simulations with several hundred million atoms can be carried out in current supercomputers (e.g., Reference 1). The first MD simulations related to shock were conducted by Mogilevsky [2,3] who used static compression and observed the changes in structure in the compressed material. This early work revealed the generation of dislocations with an associated decrease in the deviatoric stresses during compression. These dislo- cation loops nucleated preferentially at point defects. Since then, a large body of work has been carried out, revealing the detailed nature of the defects generated. Most of these studies, pioneered by Holian and others, considered the cases in which shocks travel along the [100], [110], and [111] directions. [4–7] Holian and Lomdahl [3–5] found, for [100] fcc monocrys- tals, that plasticity occurred by stacking-fault nucleation. The nucleation threshold was extremely high for homoge- neous nucleation, but low in the presence of preexisting defects. There are several other studies of shocks in single crystals along the main symmetry directions, [8,9] and one along a nonsymmetric direction on NiAl. [10] Figure 1 shows, for conceptual clarity, how a shock front propagating along [001] interacts with the four slip systems; dislocation loops are generated in the slip planes. This is the schematic representation of the homogeneous nucleation mechanism. [11,12] Two situa- tions are shown: (1) perfect and (2) partial dislocation generation. As they expand, the edge components move toward and away from the front and the screw components parallel to the front. These loops will, upon expansion, interact and generate dislocation reactions. Thus, the mobility of dislocations is severely hampered by the interactions. Since it is possible to generate either partial or perfect dislocation loops, this has a profound effect on the shock-induced structure. Whereas the screw components of perfect dislocations can cross-slip, partial dislocations cannot cross-slip and the resultant structure is marked by a large density of stacking faults and characterized by planar features. Meyers et al. [12] and Schneider et al. [13,14] characterized laser-generated shock structures in copper and copper- aluminum alloys, respectively. Tanguy et al. [15] devel- oped a detailed analysis of the dislocation loop nucleation and growth in a shock wave. Nucleation was found to be thermally driven, whereas growth was the result of the relaxation of shear stresses. They obtained a critical diameter of the loop at which it expands as ~10 r, where r is the atomic radius. Germann et al. [16,17] demonstrated that the configura- tion of dislocations predicted by MD depends signif- icantly on the crystal orientation (at pressures just above the Hugoniot elastic limit); they compared shock propagation along [100] and [111]. For [100], loops of partial dislocations were observed (such as in Figure 1(b)), whereas for [111] both leading and BUYANG CAO, Postdoctoral Fellow, and MARC ANDRE ´ MEYERS, Professor, are with the Materials Science and Engineering Program, Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA, USA. Contact e-mail: bcao3@jhu.edu EDUARDO M. BRINGA, Physicist, is with the Materials Science Division, Lawrence Livermore National Labo- ratory, Livermore, CA, USA. This article is based on a presentation made in the symposium entitled ‘‘Dynamic Behavior of Materials,’’ which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee. Article published online July 11, 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 38A, NOVEMBER 2007—2681