  Recent experiments by Kiritani et al. have revealed a surprisingly high rate
of vacancy production during high-speed deformation of thin foils of fcc
metals. Virtually no dislocations are seen after the deformation. This is
interpreted as evidence for a dislocation-free deformation mechanism at very
high strain rates.
  We have used molecular-dynamics simulations to investigate high-speed
deformation of copper crystals. Even though no pre-existing dislocation sources
are present in the initial system, dislocations are quickly nucleated and a
very high dislocation density is reached during the deformation.
  Due to the high density of dislocations, many inelastic interactions occur
between dislocations, resulting in the generation of vacancies. After the
deformation, a very high density of vacancies is observed, in agreement with
the experimental observations. The processes responsible for the generation of
vacancies are investigated. The main process is found to be incomplete
annihilation of segments of edge dislocations on adjacent slip planes. The
dislocations are also seen to be participating in complicated dislocation
reactions, where sessile dislocation segments are constantly formed and
destroyed.
