Scientists from the Max Planck Institute of Metals Research in Stuttgart/Germany and Stanford University have recently discovered that deformation can be passed through materials faster than the speed of sound (Science, Feb. 12, 1999). This finding is very surprising, since the sound velocity has so far been considered the upper limit for the velocity of all mechanical processes including deformation. However, atomistic simulations have shown that this is not the case, and that a stress concentrator like a crack tip or a sharp indenter may induce deformation at supersonic velocities. These results are not only important for the understanding of high-speed and low-temperature deformation of engineering materials but may also play an important role in the understanding of the dynamics of geological shear faults.
The irreversible plastic deformation of crystalline materials like metals or semiconductor crystals is mainly carried by the motion of dislocations, which are line defects of the crystal lattice. The velocity of these dislocations is limited by lattice friction and by drag effects from the interaction with lattice vibrations and electrons. At low temperatures and high stresses, dislocation velocities can reach sizeable fractions of the shear wave speed. Conventional wisdom, based on elasticity theory, is that high-speed dislocation motion should be described relativistically and that dislocations can therefore never reach the velocity of the shear waves, because the energy required to drive the dislocations becomes infinite at this speed. Above this sound barrier, the elastic field equations have a second set of solutions for supersonic motion. This has long been known but has been regarded as a mathematical curiosity rather than a physically relevant state of motion.
Using atomistic computer simulations, researchers at the Max Planck Institute in Stuttgart have now discovered that it is not necessary to accelerate a subsonic dislocation through the sound barrier, but that the dislocation can be "born" as a supersonic dislocation. This requires a strong stress concentration such as provided by a crack tip or a sharp indenter. Once the supersonic dislocations are generated, their properties can easily be studied. The scientists have found that sustained supersonic motion is possible in an elastically loaded body. Even the sound barrier at the shear wave velocity has been investigated by decelerating the supersonic dislocation until it drops through the sound barrier. During this process, the dislocation stops completely before it restarts again as a subsonic dislocation. (This process can be viewed as an animated sequence of images at http://finix.mpi-stuttgart.mpg.de/~gumbsch/warp.html). While this temporary stopping can be reconciled with continuum elasticity theory, many of the properties of the supersonic dislocations, like their nucleation and radiation, cannot be reconciled and are clearly connected to the non-linear nature of the atomic interaction.
The practical importance of these supersonic dislocations appears to be somewhat limited by the difficulty of nucleation and by the rather high stresses required for sustained motion. However, the necessary conditions can certainly be met in the compression of "brittle" materials like rocks and even in the low-temperature deformation of metals. Whether supersonic dislocations may be relevant for geophysical phenomena is as yet unclear. On the one hand, the increasing temperature in the deeper crust makes thermally activated competing deformation processes more likely; on the other hand, low-symmetry crystal structures and the limited amount of slip systems certainly allow the build-up of large stresses which may reach the required order of magnitude. Finally, supersonic dislocations may help to understand some of the deformation phenomena which are usually considered collective. The dynamics of mechanical twinning and martensitic transformation may therefore have to be reinvestigated considering these results.
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Science