The effect of prior cold work on the shock response of tantalum has been investigated via plate impact. As-received and 50% cold-rolled material has been studied to determine the Hugoniot Elastic Limit (HEL), shear strength evolution behind the shock front, and spall strength. Results show that there is a significant drop in both HEL and shear strength due to cold-rolling, but as the thickness of the target (or time) increases, results converge between the two states. Results suggest that this is due to the cold-rolling process moving dislocations away from the surrounding interstitial solute atoms that collect there, thus reducing the initial stress to initiate yield. In other words, the main contribution of cold-rolling is to increase the population of mobile dislocations within the microstructure rather that just increase the dislocation density as a whole. In contrast, the spall strength in both states appears almost identical. It is suggested that the high Peierls stress prevents a large increase in dislocation density during rolling and hence reduces any post rolling strengthening that might be observed in the spallation response. Finally, we observe a significant change in spall response below a pulse width of 150 ns. We believe that this represents a change from a nucleation and growth of ductile voids type mechanism to one based on ductile fracture of atomic planes. The fact that at these low pulse durations, results appear to trend towards the theoretical strength of tantalum would lend support to this hypothesis.
The mechanical response of niobium and molybdenum during one dimensional shock loading in the weak shock regime is investigated in terms of the Hugoniot elastic limit (dynamic yield) and spall (tensile) strengths. Results indicate that although both metals have high elastic limits of ca. 2 GPa, their responses are very different. Deformation in the weak shock regime in niobium is controlled by both the motion and generation of dislocations, resulting in high spall (dynamic tensile) strengths and ductility. In contrast, molybdenum has low spall strength and ductility, which suggests lower dislocation mobility in this metal. We have also shown that the strain-rate in the rising part of the shock front is related to the stress amplitude by the fourth power, as first shown by Swegle and Grady. Although we have not been able to elucidate further on the power relation, we believe that the scaling factor A is related to a materials ability to accommodate shock imposed plasticity via slip and dislocation generation. Overall, we have used arguments about the Peierls stress in body centred cubic metals to explain these results, with niobium (low Peierls stress) having a high dislocation mobility, resulting in behaviour showing some similarities to face centred cubic metals. Molybdenum, with its much higher Peierls stress has a much lower dislocation mobility, and hence lower spall strengths and ductility.
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