Transonic dislocation propagation in diamond

The motion of line defects (dislocations) has been studied for more than 60 years, but the maximum speed at which they can move is unresolved. Recent models and atomistic simulations predict the existence of a limiting velocity of dislocation motion between the transonic and subsonic ranges at which the self-energy of dislocation diverges, though they do not deny the possibility of the transonic dislocations. We used femtosecond x-ray radiography to track ultrafast dislocation motion in shock-compressed single-crystal diamond. By visualizing stacking faults extending faster than the slowest sound wave speed of diamond, we show the evidence of partial dislocations at their leading edge moving transonically. Understanding the upper limit of dislocation mobility in crystals is essential to accurately model, predict, and control the mechanical properties of materials under extreme conditions. The speed of sound is often a limit of a sort on how fast something can move through a system. For dislocation motions that play a large part in plastic deformation, the limits are very poorly constrained from experiments. Katagiri et al . used x-ray radiography of shocked single-crystal diamond to track dislocation motion during plastic deformation (see the Perspective by Knudson and Seagle). They found that the dislocation speed can be higher than the bulk sound speed of the material. This observation is important for refining deformation models at extreme conditions because these ultrafast dislocation motions have previously only been predicted with theory. —Brent Grocholski Femtosecond x-ray imaging visualized dislocation propagation speed in diamond, exceeding the sound wave barrier.