Phase changes in solids during shock loading and unloading

Polymorphic phase transformations have been found in many substances by shock compression. The transition pressures observed by dynamic and static compression are generally in good agreement, provided that the kinetics of phase changes, such as those which are displacive or electronic in origin, is sufficiently fast to be completed within the short time interval of a shock process. Molecular dynamical calculations of rutile under uniaxial stress showed that the phase transition to high pressure fluorite phase could be completed within a few picoseconds, much shorter than the shock rise time within the shock front. The fact that the phase transition pressures of rutile single crystals depends on the shock loading directions can also be explained, given the uniaxial nature of the plane shock loading within the shock front. On the other hand, there are also many examples in which the recovered materials cannot be considered as due to the shock-induced high pressure phase itself, but metastably formed during the unloading process. To apply the shock wave technique for materials synthesis, the importance of the knowledge on the shock-induced phase transformations as well as pressure-temperature history of shock process is mentioned.