Session T2: MD-5: Molecular Dynamics V

High Strain Rate Effects in Quasi-Isentropic Compression of Solids

We have performed large-scale molecular dynamics (MD) simulations of shock loading and quasi-isentropic compression in copper crystals, modeling the inter-atomic interactions with an embedded-atom method potential. We find that under high shear-stress conditions, disordering and amorphization competes with traditional (dislocation) plastic deformation mechanisms. Characteristic parameters for amorphization (stress relaxation and strain rate) are used in a modification of Eyring's theory for Couette shear flow in fluids, which can be compared with MD results for quasi-isentropic compressions in initially damaged, isotropic crystals. Above the yield stress (non-zero for a solid), the theory exhibits a linear normal-stress difference vs strain rate, which then bends over at high rates in a manner reminiscent of shear-thinning in fluids; the MD data shows remarkably similar behavior.   

MD study of femtosecond laser spallation of aluminum and gold

The ultrashort laser-matter interaction with femtosecond laser pulse (fsLP) and moderate incident fluence is important for fundamental physics of fast processes, warm dense matter physics as well as for a wide range of industrial applications. Fast heating of target material by fsLP results in formation of thermo-mechanically stressed zone of 10-100 nm thick with pressure 10-50 GPa. Its unloading may cause frontal cavitation and ablation of subsurface layer at a crater depth of 50 nm for aluminum and 100 nm for gold as our MD simulations and experiments indicate. The ablation thresholds of Al and Au obtained in simulations with the use of our new EAM potentials agree with experimental data as well. The compression wave propagating deep into material hits the rear side of a target with formation of a rarefaction wave. The last may produce cracks and spallation on rear side. MD simulations of spallation of Al and Au films induced by fsLP show that the used EAM potentials (Mishin et. al. and our new ones) predict the different spallation thresholds while the shock wave profiles are similar. Simulated spall strength of Al is 7.4 GPa, that is noticeably less than 10 GPa obtained from acoustic approximation with the use of velocity pullback on velocity profile of free rear surface.   

Comparison between experiments and molecular dynamic simulations of spallation induced by ultra-short laser shock on micrometric Tantalum targets

Shock wave propagation and the spallation within materials induced by laser shock have been investigated for roughly two decades. With the latest laser technologies evolution, one can access to shorter regimes in durations, going below the picosecond range. Shots performed with the LULI 100TW facility evidence the possibility to obtain spallation in a few microns thick metallic target. Such conditions provide an experimental data layout directly comparable with molecular dynamic simulations accessible to these scales. Molecular dynamic simulations on a single crystal of Tantalum have been performed with the CEA TERA 10 computer. First, the Hugoniot calculated by the equilibrium molecular dynamics has been compared with experimental data to check the potential (EAM) relevance to reproduce the shock wave propagation. Then, a large scale simulation on a micrometric target has been performed. We have observed the microscopic ductile damage process, the pore apparition and their time and space evolution. The results are compared with experimental results and classical one- dimensional hydrodynamic simulations.   

Molecular Dynamics Simulation of Dynamic Response of Beryllium

The response of beryllium to dynamic loading has been extensively studied, both experimentally and theoretically, due to its importance in several technological areas. Compared to other metals, it is quite challenging to accurately represent the various anomalous behaviors of beryllium using classical interatomic potentials. The spherically-symmetric EAM potential can not reproduce the observed $c/a$ ratio for $\alpha$-Be under ambient conditions, which is significantly smaller than the ideal HCP value. The directional-dependence of the MEAM potential overcomes this problem, but introduces additional complexity. We will compare predictions of these classical potentials to experimental measurements of beryllium at ambient conditions, and also to theoretical calculations at high temperatures and pressures. Finally, we will present initial results from non-equilibrium molecular dynamics simulations of beryllium under dynamic loading. This work is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories.   

Molecular dynamics simulation of thermodynamic and mechanical properties and behavior of Be when shock loading

Classical MD approach has been applied to modeling Be properties and behavior when dynamic loading. Special attention has been paid to calculation of melting curve and physical properties when melting. Hugoniostat MD technique was applied to obtain Hugoniot of beryllium taking melting into account. The results of calculations were compared to experimental data and the results of ab initio and quantum MD calculations. The results of direct MD simulation of shock wave loading of nano- polycrystalline beryllium (hcp grains, average grain size $\sim$10nm) and the data on dynamic yield stress as depended on shock stress were obtained. So as the length of Be samples used was about 0.2 micron only ultra-fast stage (time-scale $\sim$20 ps) of the relaxation process behind shock front has been investigated. Results of the simulation have been discussed and analyzed along with experimental data   

Atomistic simulation of dislocation nucleation in aluminum

Homogeneous and defect induced dislocation nucleation are considered by means of molecular dynamics simulation. Different mechanisms of dislocation loop nucleation and growth are revealed. Dislocation nucleation starts from the formation of partial dislocation loop by thermally fluctuations. The dependence of the loop energy on shear stress and radius is obtained. It is in a good agreement with dislocation theory. Homogeneous nucleation rates are calculated at different temperatures and stress. During the loop growth the twinning is observed rather than second partial dislocation nucleation. Defects (void, precipitate) in crystal result in concentration of shear stress under deformation. Dislocations appear in the sites with the largest shear stress by formation of the part of dislocation loop. The ends of the dislocation line come together in the defect. The loop forms by rounding the defect like mechanism of Frank-Read source. The activation shear stresses of these mechanisms are obtained at different temperatures. This work was supported by the RAS programs {\#} 11, 12 and SNL under the US DOE/NNSA ASC program.