Real-time microstructure of shocked LiF crystals: Use of synchrotron x-rays

We describe the use of a third generation synchrotron facility to obtain in situ, real-time, x-ray diffraction measurements in plate impact experiments. Subnanosecond duration x-ray pulses were utilized to record diffraction data from pure and magnesium-doped LiF single crystals shocked along the [111] and [100] orientations. The peak stresses were 3.0 GPa for the [111] oriented LiF and between 3.0 and 5.0 GPa for the [100] oriented LiF. For these stresses, shock compression along [111] results in elastic deformation and shock compression along [100] results in elastic-plastic deformation. Because of the quality of the synchrotron x-ray pulses, both shifting and broadening of the diffraction data were obtained simultaneously. As expected, shifts for elastic compression and elastic-plastic compression in shocked LiF were consistent with uniaxial and isotropic lattice compression, respectively. More importantly, diffraction patterns from crystals shocked along [100] exhibited substantial broadening due to elastic-plastic deformation. The broadening indicates that the shocked LiF(100) crystals developed substructure with a characteristic size for coherently diffracting domains (0.1-10 mu m) and a distribution of (100) microlattice-plane rotations (similar to 1 degrees wide). In contrast to the LiF(100) results, broadening of the diffraction pattern did not occur for elastically deformed LiF(111). Another important finding was that the amount of lattice disorder for shocked LiF(100) depends on the loading history; the broadening was larger for the magnesium-doped LiF(100) (large elastic precursor) than for ultrapure LiF(100) (small elastic precursor) shocked to the same peak stress. The data are simulated by calculating the diffraction pattern from LiF(100) with a model microstructure consisting of coherently diffracting domains. The lattice orientation and longitudinal strain is assumed uniform within domains, but they vary from domain to domain with Gaussian distributions. Simulations using such a model are in good agreement with the measured diffraction patterns. The principal finding from the present work is that synchrotron x-rays can provide real-time data regarding microstructure changes accompanying shock-induced deformation and structural changes.