Shock-hardened polycrystalline copper was subjected to high strains (y -5) at high strain rates (q -104 s-1) using a stepped specimen in a Hopkinson bar. Microstructural analysis by transmission electron microscopy revealed that the highly deformed region consisted of very small grains (-0.1 pm) with a low dislocation density. These results are discussed in terms of dynamic recrystallization, which is enabled by the adiabatic temperatures rise associated with plastic deformation. This dramatic microstructural refinement enables a thermomechanical response that may lead to a stable plastic deformation in tension.
Copper pre-shocked to 50 GPa underwent dynamic recrystallization, when subsequently plastically deformed to high strains (γ~2-5) at high strain rates. The recrystallized structure generated at high strain rates consisted of small, equiaxed grains (0.1-0.3 µm). Low-strain-rate experiments on pre-shocked (and pre-heated specimens) revealed a recrystallized grain size of ~2-3 µm, with profuse annealing twins ; this is suggestive of different mechanisms operating at low and high strain rates. The effect of grain size (9.5, 25, 117, 315 µm) on high-strain, high-strain-rate deformation of copper was also investigated. Pre-shocking of the specimens generated a high dislocation density and deformation twins. Twinning was highly dependent on grain size, being profuse for 177 and 315 µm, and virtually absent for 9.5 µm specimens. This was observed to have a profound effect on the subsequent mechanical response of the larger grain-sized specimens, which underwent considerably larger shock-wave hardening than the smaller grain-sized ones
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