A molecular dynamics study of dislocation density generation and plastic relaxation during shock of single crystal Cu

The molecular dynamics simulation method is used to investigate the dependence of crystal orientation and shock wave strength on dislocation density evolution in single crystal Cu. Four different shock directions 〈100〉, 〈110〉, 〈111〉, and 〈321〉 are selected to study the role of crystal orientation on dislocation generation immediately behind the shock front and plastic relaxation as the system reaches the hydrostatic state. Dislocation density evolution is analyzed for particle velocities between the Hugoniot elastic limit (upHEL) for each orientation up to a maximum of 1.5 km/s. Generally, dislocation density increases with increasing particle velocity for all shock orientations. Plastic relaxation for shock in the 〈110〉, 〈111〉, and 〈321〉 directions is primarily due to a reduction in the Shockley partial dislocation density. In addition, plastic anisotropy between these orientations is less apparent at particle velocities above 1.1 km/s. In contrast, plastic relaxation is limited for shock in the 〈100〉 orientation. This is partially due to the emergence of sessile stair-rod dislocations with Burgers vectors of 1/3〈100〉 and 1/6〈110〉. The nucleation of 1/6〈110〉 dislocations at lower particle velocities is mainly due to the reaction between Shockley partial dislocations and twin boundaries. On the other hand, for the particle velocities above 1.1 km/s, the nucleation of 1/3〈100〉 dislocations is predominantly due to reaction between Shockley partial dislocations at stacking fault intersections. Both mechanisms promote greater dislocation densities after relaxation for shock pressures above 34 GPa compared to the other three shock orientations.