Effect of grain size and pressure on twinning and microbanding in oblique shock loading of copper rods

Abstract

High-purity copper rods in a range of grain sizes—29 μm, 141 μm and 375 μm—were shock loaded in an cylique, cylindrical shock fixture which simultaneously and linearly strained the rods from zero to 5% at peak pressures calculated to range from 11 to 30 GPa at the top and bottom of the rods, respectively. These pressures correspond to the minimum and maximum shock deformation. Light metallography and transmission electron microscopy revealed that the density of deformation twins was measurably and observably increased with both increasing grain size and deformation (pressure). In addition, microbands coincident with traces of {111} planes were observed to be intermixed with the deformation twins. The microbands were almost exclusively associated with a single (or primary) {111} glide plane. The tendency for microband formation, isolated or intermixed with twins, was qualitatively observed to increase with increasing grain size. There appears to be a direct, shear-related connection between microbands and deformation twin/twin-faults in oblique shock loaded copper. The obliquity of the shock wave seems to suppress the critical twinning pressure of copper, since twinning was observed at pressures of only 11 GPa at the top of the rods in contrast to an established critical twinning pressure of approximately 20 GPa for plane-wave shock loading of copper.