Shock-Wave-Induced Phase Transitions

Many solid substances exist in several crystalline forms, in which the arrangement of atoms or molecules is different. Under the action of shock waves, atoms or molecules in solids undergo high-speed motion that results in their rearrangement, that is, a phase transition takes place. In China, as in many other 

Boron nitride is an important superhard material. Preparation of superhard w-BN and c-BN by phase transition of g-BN induced by shock waves has been a hot research topic in the last decade. In China, early work in this field was done by Yun and colleagues [10]. Using an annular detonation wave generator to produce sliding detonation, about 20% of g-BN in a steel tube could be converted to w-BN. 

The phase transition of BN was also investigated by Li et al. [12]. Fine particles of h-BN with G.I. of 7 and purity higher than 98 % as the raw material were treated by shock waves with a pressure of 70-90 GPa produced by high-speed impact of a steel plate. The product obtained was w-BN, with a small amount of c-BN. A chair model was suggested for the mechanism of this phase transition, in which g-BN can be converted directly to c-BN without formation of w-BN as an intermediate. They suggested that the yield of c-BN can be increased significantly by increasing the pressure and duration of the shock wave. 

There are many works dealing with the phase transition of other solids. 

He and colleagues [13] treated a mixture of fiillerene and nickel powder by shock waves produced by high-speed impact of a steel plate. They found that below 11.3 GPa no phase transition of C60 occurs. Under shock waves with a 

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Duvall, G.E., Shock-Induced Phase Transitions in Solids, in Propagation of Shock Waves in Solids (edited by Varley, E.), the American Society of Mechanical Engineers, New York, 1976, pp. 97-114. 

Fig. 2.12. If solids undergo a shock-induced polymorphic transformation, the volume change at the transformation causes significant changes in the wave profile produced by shock loading. In the figure, is the applied pressure, Pj is the pressure of the phase transition, and HEL is the Hugoniot elastic limit.

The indicated transition pressure of 15 GPa is in agreement with the published data with shock-wave structure measurements on a 3% silicon-iron alloy, the nominal composition of Silectron. A mixed phase region from 15 to 22.5 GPa appears quite reasonable based on shock pressure-volume data. Thus, the direct measure of magnetization appears to offer a sensitive measure of characteristics of shock-induced, first-order phase transitions involving a change in magnetization. 

Shock wave compression cannot only induce deformation in the form of high density of defects such as dislocations and twins but can also result in phase transition, structural changes and chemical reaction. These changes in the material are controlled by different components of stress, the mean stress and the deviatoric stress. The mean stress causes pressure-induced changes such as phase transformations while the deviators control the generation and motion of dislocations. 

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