UC San Diego

The combination of high-energy pulsed-laser experiments and molecular dynamics simulations yields an improved understanding of the deformation and failure mechanisms under extreme loading conditions. Coordination of these techniques improves as the characteristic spatial and temporal scales of state-of-the-art laser experiments overlap increasingly with massive atomistic simulations. SPaSM and LAMMPS molecular dynamics codes were employed to simulate atomic systems containing up to one billion atoms. The extreme stress states generated by laser-driven shocks persist for incredibly short durations (picoseconds to nanoseconds). Over severely reduced time scales, extreme stresses activate novel phase transformations and defect mechanisms. An overarching theme of the present thesis is the role of limited time and superimposed stresses and strains via laser-generated shocks on the resulting deformation mechanisms occurring during the passage of the pulse. At the reduced time scales (picoseconds to nanoseconds), novel phase transformations and defect mechanisms are activated.

This dissertation focuses on two representative materials: tantalum, a body-centered cubic metal; and silicon, a diamond-cubic (covalently bonded) semiconductor. Significant structural changes were obtained experimentally and by molecular dynamics. In tantalum, the competition between dislocations and twinning is shown to be determined, inter alia, by the strain rate. Additionally, an unexpected phase transition to hexagonal at large compressive strains and to face-centered cubic at large tensile strains is revealed. The strain-rate dependence of spalling is modeled and successfully compared with experimental results; the effect of grain boundaries is established. An estimate of the ultimate tensile strength of tantalum, obtained by extrapolation of the strain-rate dependent spall strength to the Debye frequency of atoms as well as by an evaluation of the equation of state, is shown to be 33 GPa. In silicon, we identified supersonic dislocations bursts, which have durations of fractions of picoseconds; bulk and shear-induced amorphization; and intermediate phase changes to higher-coordinated structures.