Dislocation density in copper and tantalum after shock loading up to 20-100 GPa

Dislocation density was investigated by X-ray radiography method in recovered copper samples having grain sizes of 0.5 and 30$\mu $m after loading by shock waves with amplitudes of 30-60GPa, and in tantalum after loading up to 20-100~GPa. The highest value was recorded in 30~$\mu $m copper loaded by pressure of 50\textit{GPa}, namely, it was 5$\cdot$10$^{11}$\textit{cm}$^{-2}.$ In ultrafine-grain copper (UFG) with grain size of 0.5$\mu $m, which had the highest initial dislocation density (1.8$\cdot $10$^{11}$~\textit{cm}$^{-2})$, it was possible to observe relatively small increment of dislocation density up to the values of $\sim $3$\cdot $10$^{11}$~\textit{cm}$^{-2}$ at \textit{P$\sim $}40-50~\textit{GPa}. In all samples, the maximum was revealed in the range of 40-50\textit{GPa} followed by drop. The authors explain this drop by annealing of defects at adiabatic heating caused by compression. The sharpest drop of dislocation density was observed in UFG copper, since the most deformed and nonequilibrium initial structure took place in it. In tantalum, monotonous growth of dislocation density was recorded up to the value of $\sim $~3$\cdot $10$^{10}$~\textit{cm}$^{-2}$ as pressure grew in shock wave. Comparison of the obtained data with results of the earlier measurements of density of twins in copper shows that strength depends on presence of the both types of defects in a structure.