Stefan J. Turneaure

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 e...

Compressive shock wave response of a Zr-based bulk amorphous alloy

Plane shock wave experiments were performed at peak stresses up to 13 GPa on Zr-based bulk amorphous alloy (BAA) samples. A velocity interferometer was used to measure the particle velocity history either at the impact surface or at the rear surface of the BAA samples. From the measured particle velocity histories, the Hugoniot elastic limit (HEL) was determined to be 7.1±0.3 GPa, corresponding to an elastic strain of approximately 4%. For experiments in which the peak stress exceeded the HEL, a clear two-wave structure consisting of an elastic precursor followed by a plastic wave was observed. Measurements of the transmitted wave profiles, along with direct determination of the longitudinal stress and particle velocity at the impact surface, suggest that the shear strength of the Zr-based BAA is reduced as it is shocked above the elastic limit.

Real time synchrotron x-ray diffraction measurements to determine material strength of shocked single crystals following compression and release

We present a method to use real time, synchrotron x-ray diffraction measurements to determine the strength of shocked single crystals following compression and release during uniaxial strain loading. Aluminum and copper single crystals shocked along [111] were examined to peak stresses ranging from 2 to 6 GPa. Synchrotron x rays were used to probe the longitudinal lattice strains near the rear free surface (16 and 5 μm depths for Al and Cu, respectively) of the metal crystals following shock compression and release. The 111 diffraction peaks showed broadening indicating a heterogeneous microstructure in the released state. The diffraction peaks also shifted to lower Bragg angles relative to the ambient Bragg angle; the magnitude of the shift increased with increasing impact stress. The Bragg angle shifts and appropriate averaging procedures were used to determine the macroscopic or continuum strength following compression and release. For both crystals, the strengths upon release increased with increasing...

Response of a Zr-based bulk amorphous alloy to shock wave compression

Plane shock wave experiments were performed on bulk amorphous alloy (BAA) samples having a nominal composition of Zr56.7Cu15.3Ni12.5Nb5.0Al10.0Y0.5. Peak compressive stresses ranged from 4to16.4GPa. Piezoelectric pins and a velocity interferometer were used to measure elastic shock speeds and particle velocity histories, respectively. The elastic Hugoniot curve was determined and the Hugoniot elastic limit (HEL) was measured to be approximately 7.0GPa, a value significantly higher than expected from quasistatic uniaxial stress data. Impact loading beyond the HEL results in a distinct two wave structure due to elastic-plastic deformation. Our data also show clear evidence for strength loss under shock loading above the HEL. Unlike most metals, the present data show distinct elastic response during unloading. We present a continuum model to describe the deformation response of the BAA to shock loading. Simulations using this time-dependent, strain-softening strength model were able to successfully match the...

Inelastic deformation and phase transformation of shock compressed silicon single crystals

Silicon crystals were shock compressed along the [100] and [111] orientations to stresses between 15.9 and 21.7GPa. Transmitted wave profiles exhibited considerable orientation dependence for elastic and inelastic waves but very little orientation dependence for the phase transformation wave. Following the phase transformation wave, the silicon was compressed ∼23%. This compression is significantly greater than the previously reported compressions for silicon shocked to similar stresses, and the present data are consistent with a completed phase transformation. The measured mechanical impedance of the transformed silicon matches the bulk impedance of the simple hexagonal phase of silicon reported in static high pressure studies.

Material strength determination in the shock compressed state using x-ray diffraction measurements

Analytic developments are presented to determine the strength of shock compressed single crystals from real-time x-ray diffraction (XRD) measurements. Both linear elastic and nonlinear elastic analysis methods are considered. Material strength in the shocked (constant) state may be determined using one of two approaches: from measurements of longitudinal and lateral lattice strains; or from measurements of longitudinal lattice strains and longitudinal wave profiles. The second approach is demonstrated for aluminum single crystals following shock compression along [100] to peak impact stresses of 5.5–12.7 GPa and partial release (reflection from the window material) to final stresses of 3.5–7.1 GPa. The material strength of the Al(100) in the final state was found to increase with peak stress or plastic strain. The material strength at the Hugoniot elastic limit was 0.025 GPa and the material strength in the final state was 0.52 GPa for the highest stress experiment. Because of the large final stresses, in...

Shock-wave induced tension and spall in a zirconium-based bulk amorphous alloy

Dynamic tensile response and fracture of a Zr-based bulk amorphous alloy (BAA) were examined by subjecting samples to uniaxial tensile strain in plate-impact experiments. Following elastic compressive loading to peak stresses ranging between 3.9 and 6.1 GPa, wave interactions produced tensile loading resulting in spallation in the BAA samples. Rear-surface velocity histories, obtained using laser interferometry, provided a real-time measure of the tensile response including spallation. The initial tensile loading was elastic (loading rates approximately 8×105 s−1) and the data were analyzed to obtain a nonlinear, tensile stress-strain relation. Tensile fracture or spall, observed in all experiments, was initiated at a tensile stress of 3.8±0.3 GPa; this initiation value was independent of the impact stress and is significantly higher than that observed for crystalline metals. A phenomenological tensile fracture model was incorporated into one-dimensional wave propagation simulations to gain insight into t...

X-ray diffraction and continuum measurements in silicon crystals shocked below the elastic limit

The shock wave response of silicon, compressed along [100] and [111], was examined at both the lattice and continuum scales. Peak stresses were below the Hugoniot elastic limit and ranged between 2.8 and 6.9GPa. X-ray diffraction measurements provided the interplaner spacing changes along the shock loading direction. The continuum response was determined by using laser interferometry to measure the rear surface velocity histories. In contrast to earlier results, both the lattice and continuum results were consistent with the known nonlinear elastic constants of silicon. Additionally, the diffracted intensity in the shocked state was considerably larger than the intensity in the ambient state.

Real-time x-ray diffraction at the impact surface of shocked crystals

X-ray diffraction (XRD) measurements at the impact surface, rather than the rear surface, of a shocked crystal have two important advantages: time-dependent material response can be directly monitored and the shocked crystal may be examined in a constant state over a continuous range of peak stresses. Methods for obtaining XRD data at the impact surface of crystals impacting an x-ray window are presented. Quantitative accuracy of the impact surface XRD method was established using Si(100) shocked elastically to 5.4 GPa. For Si(100) shocked inelastically to 12.2 GPa, partial strength loss occurred and the lattice strain and orientation became quite heterogeneous.

Shock induced phase change in KCl single crystals: Orientation relations between the B1 and B2 lattices

The relative orientations between the lattices of the low pressure (B1) and high pressure (B2) phases of shock compressed KCl single crystals were examined using plate impact loading along the [111] and the [110] directions. The B2-phase lattice planes, perpendicular to the loading direction, were determined from transient x-ray diffraction measurements. Two closely spaced diffraction peaks were observed for the [111] loading direction. The lower Bragg angle peak is consistent with a 200 peak of a cubic B2-phase unit cell giving the orientation relation [111]B1 is parallel to [100]B2. The higher Bragg angle peak is not consistent with any peak from either a cubic B1 or a cubic B2 unit cell; the origin of this peak is unknown. Other experiments found no orientation relations; for the [111] loading direction, [111]B1 is not parallel to [211]B2 and for the [110] loading direction, [110]B1 is not parallel to [100]B2. The orientation relation determined for the [111] loading in this work is incompatible with a...