Dislocation mechanism and kinetics of shock-induced phase transformation in KCl

A high-speed dislocation mechanism of the phase transition in potassium chloride under shock loading is suggested. At shock pressures exceeding the equilibrium pressure of the phase transformation, the shock-induced shear stress provides the generation, motion and multiplication of partial dislocations of the initial lattice B1 (rocksalt-type; two face-centered cubic sublattices of ions of opposite sign). This results in the development of the intermediate B* structure (two ionic sublattices of like sign: unchanged, face-centered cubic, and transformed, base-centered orthorhombic) with further transformation into the final cesium chloride structure B2 (two simple cubic sublattices of ions of opposite sign). Since the unrelaxed shear stress favors an easy dislocation climb, the transformation rate is very high, varying from 1 to 3 nsec(-1), for different orientations of the shocked crystal. This rate is typical for the first stage of transformation, B1 --> B*. A comparatively low (5-25 mu sec(-1)) transformation rate at the second stage, B* --> B2, is due to the decrease of ability of dislocations to cross the parallel glide planes and to the closure of new phase regions impeding the forward motion of dislocations. (C) 1997 Elsevier Science Ltd.