High-strain-rate response of ultra-fine-grained copper

The high-strain-rate response of ultra-fine-grained (UFG) copper processed by equal channel angular pressing (ECAP) was characterized by three different dynamic testing methods: reverse Taylor impact, cylindrical compression specimens, and hat-shaped specimens in Hopkinson bar experiments. Upon recovery after impact, the specimens were found to undergo dynamic recrystallization at a calculated temperature of 360 K, indicating that the UFG copper is thermally unstable. Reverse Taylor tests were conducted on as-received oxygen-free high-conductivity copper rod and ECAP specimens with 2 and 8 sequential deformation passes. The dynamic deformation of the samples was modeled using AUTODYN-2D, and a modified Johnson-Cook constitutive equation was found to closely capture the dynamic response. Both the dynamic experiments and analysis from the reverse Taylor tests indicate enhanced strain-rate sensitivity in comparison with conventional polycrystalline copper, in agreement with predictions of reduced activation volume. The shear band thickness, as obtained in forced localization tests, showed a marked decrease, in comparison to conventional polycrystalline copper; this effect is interpreted as due to an accelerated thermal softening and inherent instability exhibited by the UFG structure. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.