| Summary: | <p>Shock compression is a powerful tool with which to study the high pressure response of matter. Metals subjected to shock wave loading are well-known to behave elastically up to extraordinarily high stress states within a brief time-window after impact, beyond which the yield point quickly decays towards an equilibrium limit as the shock wave propagates further into the target. Despite six decades of study, the microstructural processes which are at the origin of this phenomenon remain obscure and highly material dependent. In this work, the transient states of the dynamic yield point are investigated through the evolution of the elastic precursor wave with propagation distance, enabling study of the time-dependence of dislocation mechanisms and plastic flow which cannot be captured by other means.</p>
<p>In the first of three experimental campaigns, highly textured extruded magnesium is shock loaded along two impact orientations. The dynamic strength is found to be highly dependent on the grain orientation distribution, caused by relative differences in slip system activity. Application of a novel orientation-based analysis framework shows that the yield point of the polycrystalline material can be predicted reasonably well, predicated upon the use of dynamic critical resolved shear stress (CRSS) values from single crystal data modified by a fitted strengthening factor.</p>
<p>Second, the effects of grain size are investigated in conventionally cast and melt-sheared samples of Mg AZ91, in which the latter displays significant grain refinement. Measurements indicate that grain boundaries are inconsequential during dynamic yielding, a result of the short timescales of shock loading inhibiting interactions between moving dislocations and grain boundaries.</p>
<p>In the final study, pure Al and Al 6082 is cold worked to increasing strains in order to study the effects of the initial dislocation density. It is found that a higher initial dislocation density results in a decrease in the dynamic strength, demonstrating for the first time conclusive empirical proof of a reversal in the role of dislocation density when increasing the applied strain rates past 10^4 s^−1. Both analytical and numerical precursor decay models applied to the experimental data show that increased stress relaxation is caused by higher initial mobile dislocation densities. On the whole, these results quantitatively establish the relationship between the yield point during high strain rate loading and the dynamic behaviour of dislocations, as determined by microstructural features such as slip system activity and initial dislocation density.</p>
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