Towards a Lattice-Level Understanding of Shock Compression of Materials

When a material is subject to uniaxial shock compression above the so-called Hugoniot Elastic Limit it starts to deform plastically.  Complicated atomic rearrangements take place to accommodate and relieve the significant shear stresses, including dislocation generation and flow, twinning, and polymorphic phase transitions.  A detailed understanding of such basic phenomena at the lattice level has long been sought.  Over the past few years we have been using the x-ray pulses from free-electron-lasers to interrogate the lattice level response of materials shocked by optical laser-ablation.  The single shot diffraction patterns, obtained in less than one phonon period, have allowed us to watch lattice rotation and twinning in real time, providing insight into the fundamentals of plastic deformation both during compression and release [1,2]. We have also confirmed predictions [3] that a material that is compressed rapidly enough can reach the ultimate compressive elastic strength of a solid [4].   Much of this work has been made possible by the fact that the time and length scales of experiments and multi-million-atom molecular dynamics simulations have converged.  Beyond studying plasticity, we further show that even within the short nanosecond duration of a laser-driven shock, some of the most complex polymorphic phase transitions that exist within a single element (transformation to a host-guest structure) can be observed [5].