Orientation and rate dependence in high strain-rate compression of single-crystal silicon

High strain-rate ($\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\varepsilon}}\ensuremath{\sim}{10}^{6}\ensuremath{-}{10}^{9}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$) compression of single crystal Si reveals strong orientation- and rate-dependent precursor stresses. At these high compression rates, the peak elastic stress, ${\ensuremath{\sigma}}_{\mathrm{E}_\mathrm{Peak}}$, for Si [100], [110], and [111] exceeds twice the Hugoniot elastic limit. Near the loading surface, the rate at which Si evolves from uniaxial compression to a three-dimensional relaxed state is exponentially dependent on ${\ensuremath{\sigma}}_{\mathrm{E}_\mathrm{Peak}}$ and independent of initial crystal orientation. At later times, the high elastic wave speed results in a temporal decoupling of the elastic precursor from the main inelastic wave. A rapid high-$\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\varepsilon}}$ increase in the measured elastic stress at the onset of inelastic deformation is consistent with a transition from dislocation flow mediated by thermal activation to a phonon drag regime.