UC San Diego

Monocrystalline copper with orientations of [001] and [221] was subjected to shock/recovery experiments at shock pressures of 30 GPa and 57 GPa at 90 K. The microstructural evolution in both specimens was investigated by SEM and TEM. It was found that the residual microstructures were dependent on orientation, pressure, and heat generation and transfer during shock. At the same shock pressure, different post-shocked microstructures formed in samples with different crystalline orientations. This most likely is because they have different resolved shear stresses on their crystalline planes, due to the different geometric relationship between the shock propagation direction and the samples' crystalline orientations. The plate impact technique was compared with laser compression. They have varying effects on the defect substructure because of the differences in pulse duration which result in different amounts of heating during shock compression. Molecular Dynamics (MD) simulations have been conducted to model the plate impact of [001] and [221] monocrystalline copper at a wide range of shock pressures. The initiation of defects and different dislocation structures has been generated due to shock propagation in these two monocrystalline orientations. The orientation of the defects generated is consistent with the microstructure observations. However, there is a difference of several orders of magnitude between MD and experimental results. This striking difference is consistent with other results presented in the literature. One of the possible explanations is that the recovery observations do not reflect the true configuration during shock compression. The energetics of loop nucleation was analyzed, since they are the primary sources of dislocations in the Meyers model. Two types of shear dislocation loops were considered: perfect and partial dislocation loops. The calculations reveal a transition from perfect dislocation loops at low pressure to partial dislocation loops. This agrees with experimental results in shock compression, which show a clear transition from dislocation cells (enabled by cross slip and relaxation of perfect dislocation) to stacking- fault packet at higher pressures, produced from the expansion of partial dislocation loops