J. Belak

High-pressure, high-strain-rate lattice response of shocked materials

Laser-based shock experiments have been conducted in thin Si and Cu crystals at pressures above the published Hugoniot Elastic Limit (HEL) for these materials. In situ x-ray diffraction has been used to directly measure the response of the shocked lattice during shock loading. Static film and x-ray streak cameras recorded x rays diffracted from lattice planes both parallel and perpendicular to the shock direction. In addition, experiments were conducted using a wide-angle detector to record x rays diffracted from multiple lattice planes simultaneously. These data showed uniaxial compression of Si (100) along the shock direction and three-dimensional compression of Cu (100). In the case of the Si diffraction, there was a multiple wave structure observed. This is evaluated to determine whether there is a phase transition occurring on the time scale of the experiments, or the HEL is much higher than previously reported. Results of the measurements are presented.

Shock Induced α‐ε Phase Change in Iron: Analysis of MD Simulations and Experiment

Multimillion atom non‐equilibrium molecular dynamics simulations for shock compressed iron are analyzed using Fourier methods to determine the long scale ordering of the crystal. By analyzing the location of the maxima in k‐space we can determine the crystal structure and compression due to the shock. This report presents results from a 19.6GPa simulated shock in single crystal iron and compare them to recent experimental results of shock compressed iron where the crystal structure was determined using in‐situ wide angle x‐ray diffraction.

Picosecond X‐Ray Diffraction from Laser‐Shocked Copper and Iron

In situ X‐ray diffraction allows the determination of the structure of transient states of matter. We have used laser‐plasma generated X‐rays to study how single crystals of metals (copper and iron) react to uniaxial shock compression. We find that copper, as a face‐centred‐cubic material, allows rapid generation and motion of dislocations, allowing close to hydrostatic conditions to be achieved on sub‐nanosecond timescales. Detailed molecular dynamics calculations provide novel information about the process, and point towards methods whereby the dislocation density might be measured during the passage of the shock wave itself. We also report on recent experiments where we have obtained diffraction images from shock‐compressed single‐crystal iron. The single crystal sample transforms to the hcp phase above a critical pressure, below which it appears to be uniaxially compressed bcc, with no evidence of plasticity. Above the transition threshold, clear evidence for the hcp phase can be seen in the diffracti...

X‐Ray Diffraction from Shocked Crystals: Experiments and Predictions of Molecular Dynamics Simulations

When a crystal is subjected to shock compression beyond its Hugoniot Elastic Limit (HEL), the deformation it undergoes is composed of elastic and plastic strain components. In situ time‐dependent X‐ray diffraction, which allows direct measurement of lattice spacings, can be used to investigate such phenomena. This paper presents recent experimental results of X‐ray diffraction from shocked fcc crystals. Comparison is made between experimental data and simulated X‐ray diffraction using a post‐processor to Molecular Dynamics (MD) simulations of shocked fcc crystals.

Picosecond X-ray diffraction studies of shocked crystals

We have used picosecond X-ray diffraction to gain information on the timescale of plastic flow in shock compressed single crystals of silicon (Si) and copper (Cu).