Time-resolved x-ray diffraction experiments to examine the elastic-plastic transition in shocked magnesium-doped LiF

Time-resolved x-ray diffraction measurements were used to examine the lattice deformation during elastic-plastic deformation in Mg-doped (approximately 100 ppm) LiF single crystals shocked along $[100]$⁠. The magnesium impurities significantly increase the elastic limit of the LiF crystals, as compared to the low values observed for ultrapure LiF crystals, leading to a large amplitude elastic wave and significant stress relaxation behind the elastic wave. The objective of the current work was to examine lattice deformation throughout this wave profile using time-resolved, x-ray diffraction methods (2 ns resolution) for plate impact experiments to gain insight into time-dependent, elastic-plastic deformation at the microscopic level. The diffraction data were analyzed using an x-ray model coupled to an existing wave propagation code that incorporated dislocation mechanisms for elastic-plastic deformation including stress relaxation. All experimental results revealed a uniaxial lattice compression at the elastic wave front followed by a rapid transition toward isotropic unit cell compression during stress relaxation. Furthermore, comparison between the experimental data and the calculated streak records indicated that the lattice transition proceeds at a faster rate than predicted by the model. Further implications of these results are discussed.