Shocked LiF Research Articles

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 μm) and a distribution of (100) microlattice-plane rotations (∼1° 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.

Accuracy limits and window corrections for photon Doppler velocimetry

Symmetric, plate-impact experiments were performed to validate photon Doppler velocimetry (PDV) with established shock wave diagnostics. Impact velocity measurements using shorting pins demonstrated that the velocity accuracy of PDV can be 0.1% or better. Shock velocities and refractive indices were also measured with PDV (at 1550 nm) and velocity interferometer system for any reflector (VISAR) (at 532 nm) to obtain window corrections for single crystal LiF (100), c-cut sapphire, and z-cut quartz. Time-dependent, free-surface velocity histories for shocked LiF(100) provide a direct comparison between PDV and VISAR diagnostics and illustrate the benefits and shortcomings of the new diagnostic. Further implications of these results are presented.

Ab initiostudy of the optical properties of shocked LiF

The equation of state and optical properties of shocked LiF are investigated using quantum molecular dynamics (QMD) in a wide range of pressures and temperatures along the principal Hugoniot. These simulations are bridging the gap between shock experiments up to 2 Mbars and recent laser shock experiments beyond 6 Mbars. We find that the melting of LiF corresponds to an increase in absorption which is later followed by an increase in reflectivity. The reflectivities computed at high pressures are in close agreement with the experimental measurements but with Hugoniot conditions at much lower temperatures and pressures than measured experimentally. Finally, we provide a simple fit to the variation of the QMD index of refraction for pressures above 1.5 Mbars.

Real-time x-ray diffraction to examine elastic–plastic deformation in shocked lithium fluoride crystals

An experimental method was developed to obtain quantitative, x-ray diffraction data in laboratory plate impact experiments. This method was used to examine lattice compression accompanying elastic and elastic–plastic deformation in shocked LiF single crystals by imposing well-characterized, macroscopic uniaxial strain along the [111] and [100] axes, respectively. The [111] results reveal that the unit cell undergoes a one-dimensional compression; in contrast, the [100] results reveal an isotropic compression of the unit cell. The implications of the present results and the ability to make quantitative measurements are discussed.

Shock Wave Response of Materials at Different Length Scales

AbstractAfter a brief introduction to shock wave propagation in solids, recent experimental developments to understand the real time response of shocked materials at various length scales are outlined. Results from two representative studies are summarized: xray diffraction to examine elastic-plastic deformation in shocked LiF crystals; and picosecond optical absorption to examine structural transformation in shocked CdS crystals. Issues related to examining material processes at different length scales in shock wave experiments are discussed briefly.

Orthogonal strains and onset of plasticity in shocked LiF crystals.

We report experimental determinations of the elastic component of the transverse strain during passage of a shockwave. A 1 ns laser pulse drove an \ensuremath{\approxeq}60 kbar shock into an aluminum coated, radiation hardened, pure LiF single crystal. By diffracting a 0.25 ns pulse of x rays from a nearby high brightness, time-synchronized laser plasma, we measured the simultaneous compression of the lattice both parallel and transverse to the shock propagation direction with subnanosecond resolution. Definite compression was recorded in the inertially confined transverse direction, indicating the onset of plastic lattice response. Peak strains were of comparable magnitude in the orthogonal directions, but were not equal.

Dislocation mechanisms for stress relaxation in shocked LiF

A study of shock wave propagation along the 〈100〉 direction in LiF single crystals is presented. Plate impact experiments were conducted to produce elastic impact stresses of approximately 29 kbar. Stress time profiles at the impact surface and rear surface for thicknesses up to 3.2 mm were observed. Experiments were done for two impurity concentrations and three different heat treatments. Material characterization to supplement the shock data was provided by quasistatic yield stress measurements, dielectric relaxation data, initial dislocation density counts, and spectrographic analysis. Elastic wave attenuation is strongly influenced by both Mg++ impurities and heat treatments. Impurity clustering generally reduces the rate of precursor decay. The plastic strain rate at the elastic shock front was computed from the data by a near−exact method which incorporates material nonlinearities. Beyond the first 1 or 2 mm of propagation, significant contributions to stress decay arise from overtaking by relief waves. Application of dislocation theory reveals dislocation densities to be approximately 3 orders of magnitude larger than grown−in dislocations, at least in the region of rapid stress decay. Present analysis contradicts the idea of regenerative multiplication of dislocations causing this large increase in density. A model for heterogeneous nucleation of dislocation based on an energy criterion is proposed which appears to be well suited for explaining large increases in dislocation densities. The present data suggest an applied shear stress of 3−5 kbar as the lower bound at which dislocations can nucleate at heterogeneities present in our crystals. Better material characterization concerning impurity clusters is needed to consider the quantitative aspects of rate and magnitude of heterogeneous nucleation. The mechanism for stress decay in the very soft LiF crystals is still not well understood.

Elastic compression to 30 kbar along 〈111〉 in shocked LiF

Experimental data are presented for shock propagation along the 〈111〉 crystallographic direction in single−crystal LiF. Within experimenal bounds, an elastic response to 30 kbar is observed. This is in marked contrast with identical 〈100〉 crystals and earlier studies on other single crystals. The elastic behavior along the 〈111〉 direction is in good agreement with low stress and strain rate studies but raises interesting questions regarding crystal strength and dislocation mechansim operative under shock loading.