  Nanopowder consolidation under high strain rate shock compression is a
potential method for synthesizing and processing bulk nanomaterials. A thorough
investigation of the shock deformation of powder materials is of great
engineering significance. Here we combine nonequilibrium molecular dynamics
(NEMD) simulations and X-ray diffraction (XRD) simulation methods to
investigate the deformation twinning and pore compaction in shock-compressed
np-Mg. Significant anisotropy and strong dependence on crystallographic
orientation are presented during shock-induced deformation twinning. During the
shock stage, three typical types of twins were firstly induced, namely {11-21}
twin (T1), {11-22} twin (T2) and {10-12} twin (T3). Most of them were generated
in grains with a larger angle between the impact direction and the c-axis of
the lattice. With the increase in strain rate, the types and quantities of
twins continued to enrich, but they did not occur when the strain rate was too
high. We also discussed the deformation mechanisms of the three types of twins
and found that the coupling of slip and shuffle dominated twin deformation. In
addition, void filling occurred due to the interaction of twinning and other
plastic deformations, leading to the densification of np-Mg. During the release
stage, an interesting reverse change was observed, where the twins produced by
the impact receded, and twins were produced in grains that were previously
difficult to produce.
