Session B5: First-Principles and MD I: Polycrystalline and Porous Materials

Large-scale molecular dynamics simulations of the Richtmyer-Meshkov instability in aluminum

Several embedded atom method (EAM) potentials for aluminum were evaluated looking at the shock behavior. A potential was selected to study the characteristics of the growth of a Richtmyer-Meshkov instability in shock melted state with varying the amplitude of the surface while shocking into a vacuum. A brief look at the shock behavior of the potential will also be discussed. The evolution of the growth of the bubble and the spike as a function of time and $kh_0$ will be compared with the existing empirical models of G. Dimonte, et. al. [J. Appl. Physics \textbf{113}, 024905 (2013)] and K. Mikaelian, et. al. [Phys. Rev. Lett \textbf{80}, 508, (1998), J. Fluid Mech. \textbf{703}, 60 (2012)]. Molecular dynamics was selected for this study, in part, because the atomic representation is able capture secondary hydrodynamic effects such as viscosity and surface tension.   

Theoretical study of the porosity and temperature effects on the shock response of graphitic materials

The response of graphite, and graphite-like materials, to shock compression have been the subject of numerous experimental studies over a few decades, showing a substantial dependence of the shock properties (Hugoniot curves, transition to diamond, ...) on the initial porosity and granularity of the polycrystalline samples. Theoretical studies of these processes have been enabled only recently, thanks to the development of computationally efficient empirical potentials such as LCBOPII which reproduce accurately the various phases of carbon (graphene, graphite, diamond, liquid carbon) and the few available ab initio data for shock compression of graphite. These studies are restricted to monocrystalline samples which, in the case of graphite, represent a serious approximation to the actual experimental set-ups and may explain the large over-estimation of the graphite/diamond transition pressure ($\sim$ 60 GPa vs. 15-25 GPa). In this paper we present a theoretical study on the shock compression of porous graphite by means of Molecular Dynamics and Monte Carlo simulations using the LCBOPII potential. The results are compared to the available experimental data and the role of porosity and temperature on the shock properties and graphite/diamond transition is discussed.   

Simple Scaling Laws for the Role of Pre-existing and Shock-induced Microstructure on Spall Strength

Failure of ductile metals has long been attributed to void nucleation, growth, and finally coalescence leading to fracture. Under extreme loading conditions, experimental investigations have demonstrated a strong rate-dependence in the dynamic tensile strength of such metals, which may be attributed to the fact that voids are constrained to grow at finite rates. Here we show that bounds on these void growth rates may be derived analytically by considering the constraints imposed by micro-inertia and relativistic dislocation drag. We then make use of these bounds to derive simple scaling laws for predicting the role of pre-existing microstructure, e.g. second-phase particle spacing and grain size, on the rate-dependent spall strength of metals. Under typical loading conditions, we find that the spall is governed by this pre-existing microstructure with void growth governed primarily by micro-inertia. However, under the most extreme loading conditions, we find that the spall strength is governed instead by the shock-induced microstructure with growth mediated by dislocation emission. Lastly, we demonstrate how the scaling laws may be utilized to optimize the pre-existing microstructure, e.g. grain size considering the Hall-Petch effect, of a material for a particular application.   

Grain dynamics in compressed polycrystalline Al interfaces sliding at high velocities

We discuss the relationship between grain structure and the frictional force for polycrystalline Al interfaces with grain sizes of 13, 20 and 50 nm as seen in large scale NonEquilibrium Molecular Dynamics (NEMD) simulations at nominal pressures of 15 GPa. Simulation sizes were 138 M atoms for the 13 and 20 nm grain size samples and 1.8 B atoms for the 50 nm samples with times to 30 ns. We find that the frictional force in the steady state is independent of the initial grain size and that the grain distribution evolves to a dynamical steady state characterized by a sequence of grain growth and refinement events at very large local plastic strains and strain rates. Based upon these simulations, a meso/macro-scale model has been developed that reproduces the NEMD results for over two orders of magnitude in sliding velocity encompassing both solid and fluid regimes.   

Atomistic modeling of the dynamic behavior of single and nanocrystalline SiC under plane shock loading

The dynamic behavior of SiC in single and nanocrystalline samples is investigated by classical molecular-dynamics simulations of plane shock loading. The generation of elastic shock induced compaction, plastic deformation, and pressure induced structural phase transformation is characterized as a function of impact crystallographic direction ([001], [011], and [111]) and temperature (10 K {\&} 300 K). Shock profiles are calculated in the wide range of particle velocity from 0.1 km/s to 6 km/s. The predicted Hugoniot curves agree well with available experimental data. Results indicate the generation of elastic waves for shocks below 2 km/s. In the intermediate range of velocities between 2 km/s and 5 km/s the generated shock wave splits into an elastic precursor and a zinc blend-to-rocksalt structural transformation wave, which is triggered by the increase in shock pressure to over 90 GPa and results in increase of density to $\sim$ 4 g/cm3. A plastic wave is generated ahead of the transformation wave for shocks on all crystallographic with clear signs of twinning. For particle velocity greater than 4-5 km/s a single overdriven transformation wave is generated. We further examined the effects of grain boundaries in nanocrystalline samples on the generated shock Hugoniot curves. In particular, we discuss the generation of plastic deformation and the absence of shock wave splitting.   

Molecular dynamics modeling of ejecta production on shocked Pb surface

Ejecta emitting from the shocked metal surface is of current focus to engineering application and experimental diagnostics. This work is mainly dedicated to ejecta production of Pb surface with multiple grooves subjected to a triangular wave profile loading using molecular dynamics simulation. The solid and melted states upon shock or release for metal Pb are included. The simulations provide the ejecta volume distribution, total amount of ejected particles and ejecta size distribution, being in qualitative agreement with the experimental data and photon radiography images. These results will greatly contribute to the further understanding of physical mechanism and analyzing the experimental findings of ejecta process.