Nanoscale view of shock-wave splitting in diamond
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6/30/04
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Nanoscale View of Shock-Wave Splitting in Diamond S.V. ZYBIN, M.L. ELERT, and C.T. WHITE Molecular dynamics (MD) simulations of shock wave propagation in the 110 crystallographic direction of diamond were carried out using a reactive empirical bond order potential. A split twowave shock structure due to a lattice transformation is observed above a piston velocity threshold of up 1.8 km/s corresponding to the Hugoniot elastic limit of 125 15 GPa. This transition is triggered while the temperature increase in the leading shock remains negligibly small in comparison to the melting temperature of diamond and is limited to moderate shock strengths.
I. INTRODUCTION
A nanoscale study of relaxation in shock-strained crystals and of the corresponding shock-induced lattice structural transformations is important for a better understanding of the initiation and development of these responses under rapid loading within the shock front. In particular, the constitutive model of shock propagation in solids requires a detailed description of the underlying atomistic processes within the shock layer where relaxation processes occur. Such structural transformations can be significantly influenced not only by the available slip systems for a given shock orientation in a crystal, but also by the nature of the interatomic forces. Molecular dynamics (MD) simulations using empirical potentials have proven to be able to include enough atoms for long enough times to investigate phenomena in detail at the atomic level, including plastic deformations,[1,2,3] melting,[4] amorphization,[5] and shock wave splitting caused by phase transitions.[6,7] Previous atomistic simulations of shock waves in Lennard–Jones solids and metals have already revealed that the mechanism of shock-induced plasticity in crystals shows a strong dependence not only on the direction of shock propagation[3,4] but also on the shock strength and the nature of the interatomic potential. Specifically, under certain conditions of shock loading, the plastic deformation can occur not as a consequence of emission of stacking fault arrays at the shock front but through alternative relaxation mechanisms, such as a martensitic transformation[8] or twinning.[9] Recently, the appearance of twinning-like “chevron band” patterns has been observed in a uniaxial Hugoniostat simulation[10] of a 100 shock wave in the Lennard–Jones crystal (at 26 pct compression) preceding a structural change from the fcc to the hcp crystal lattice. However, materials other than rare gas solids (Lennard– Jones potential) and metals (Morse and embedded atom method potentials) have not yet been extensively investigated. An important first step in extending these simulations to a S.V. ZYBIN, Postdoctoral Fellow, is with the Department of Chemistry, The George Washington University, Washington, DC 20052. M.L. ELERT, Professor, is with the Chemistry Department, United States Naval Academy, Annapolis, MD 21402–5026. C.T. WHITE, Senior Scientist, is with the Naval Research L
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