Solid Phase Transformations Under High Dynamic Pressures

4 Solid Phase Transformations Under High Dynamic Pressures 

The fast-moving boundary between the compressed and undisturbed matter, known as the shock front, is not a geometrical surface but has a finite depth which is inversely related to the shock wave velocity (in solids its is of the order of 100 A). Every physical parameter of the substance (medium) experiences a discontinuity on this boundary. The laws of the mass, momentum and energy conservation must hold on both sides of the shock front. From this conditions the pressure P on the front can be calculated as 

Now let us consider specific features of these transformations. The pressure of phase transition, Pn, depends on the direction in which the shock wave front propagates through a crystal. Thus, the transitions from a NaCl to a CsCl type structure occur at lower pressures if the crystallographic axis 111 is normal to the shock wave front, i.e. the unit cell is compressed along its spacial diagonal, because this is 

The studies of the shock transitions by the X-ray pulsed analysis have shown [211-214] that at low pressure the compression of a lattice is mono-axial, at medium pressure the compression evolves toward isotropic, and at high pressure the compression is strictly isotropic, being of a purely hydrostatic character. It results in similar values of the pressure for phase transitions under static and dynamic regimes, which in fact is the case. Generally speaking, the dynamic values usually must exceed the static characteristics because the sample under shock loading is heated, whereas 

Let us consider now the results of investigations of the substructure in nonmetal-lic compounds. The density of dislocations of shocked single crystals in this case also increases by several orders of magnitude, disorientation of blocks increases by 2- 5° and microhardness by several times. It is interesting to note that an increase of the dislocation density in many shocked solids is accompained by retaining the dislocation configurations which existed prior to shock loading. Only in KBr a new dislocation picture was observed, and that because the reversible phase transition had completely rebuilt the dislocation structure. 

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Batsanov SS (2004) Solid phase transformations under high dynamic pressures. In Katrusiak A, McMiUan P (eds) High-pressure crystallography. Kluwer, Dordrecht... 

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