Dislocations in solids

Abstract

Shock-wave compression is an extreme regime of deformation characterized by strain rates on the order of 106-1010 s-1, pressures ranging from a few to hundreds of GPa, and shock and residual temperatures that can exceed the melting point. These shock waves can be generated by several means of rapid energy deposition at the material surface. The most common are detonation of explosives in contact with surface, impact of a flyer plate with surface (accelerated by explosives, by compressed gases in gun, or by lasers), or direct laser irradiation. The principal dislocation structures observed after shock-wave compression are illustrated and the principal mechanisms of dislocation generation in shock compression are discussed, with their relative merits and limitations. The effect of polycrystallinity on the shock-wave configuration and on the defects generated is presented. Polycrystallinity creates shock-front irregularities which can contribute to greater dislocation generation. Recent work, in which both shock or isentropic compression was applied to FCC metals, is reviewed. The microstructure changes from cells to stacking faults and eventually to deformation twins as the pressure is increased. An analytical model for the transition from slip to twinning under shock and isentropic compression is presented; a model for the transition from cells to stacking faults is also presented. The results of atomistic simulations are presented and compared with experimental measurements. Interestingly, the dislocation spacing predicted by molecular dynamics (MD) is orders of magnitude lower than transmission electron microscopy measurements on recovered specimens. MD computations reveal that a significant portion of the dislocations generated in shock compression is annihilated when the pressure decays back to the ambient value. Hence, one can postulate that most dislocations generated at the shock front are annihilated upon unloading. The reflections of shock waves at free surfaces generate tensile pulses that can initiate spalling and fragment the back of the specimens if of sufficient amplitude. A mechanism for the early growth of these voids through the emission of shear loops from the void surface and their expansion, is presented. ?? 2009 Elsevier B.V. All rights reserved.