Visualizing Stacking Fault Formation in Shocked Diamond by Femtosecond X-Ray Radiography

Since plastic deformation of crystals is primarily driven by the motion of dislocations (line defects), it is necessary to understand how fast they can propagate in order to describe the deformation dynamics of solids compressed at highest strain rates. Although many models predict the upper bound of dislocation velocities to lie between the longitudinal and transverse acoustic velocities, experimentally capturing the ultrafast dislocation motion under high strain-rate deformation remains elusive. We present our recent experimental results of <i>in-situ </i>X-ray radiography of laser-shocked single crystal diamonds. By using the ultra-bright femtosecond-duration pulses available at the SACLA X-ray Free Electron Laser facility, we imaged inelastic shock waves in diamond that propagated with a velocity comparable to the bulk sound velocity of diamond (≥ 11 µm/ns). For these waves, we observed shock-induced stacking faults form along diamond’s {111} slip planes via imaging, at velocities that indicate that the dislocations leading the stacking fault formation propagate transonically (<i>i.e</i>., with a speed between the transverse and longitudinal wave velocities of the crystal). Our advanced characterization of transonic dislocation motion gives new insights into the fundamental behavior of the dislocation propagation in crystals.