Lattice dynamics of laser-driven compressed Al studied with ultrafast electron diffraction*

Understanding the lattice dynamics of shock-compressed materials is of great interest to many areas including planetary formation, aeronautics and spacecraft. Here we report the study on elastic-to-plastic strain transition in dynamically compressed Al using the technique of ultrafast-electron-diffraction (UED). The targets employed in our experiments are 200-nm-thick free-standing single-crystal Al thin films. The compression wave was launched normal to the sample surface by ablating the sample with a focused 800nm, 20ps, <12 mJ laser pulse. The lattice response of the sample was probed by MeV electrons coming from the opposite side at 45^o incident angle. As the sample is compressed, we first observed the elastic response and then the plastic relaxation, indicated by shifts of the Laue peaks reflected from the normal and transverse lattice planes respectively. The dependence of this elastic-plastic transition on the pump fluence and crystal orientation were studied and the results will be presented. We also performed Molecular Dynamics simulations incorporated with hydrodynamics simulations to understand the underlying physics of the experimental observations and the results will be presented.