Simulation, Experiment Probe Shocked Nanocrystalline Metals

Using computer models and experiments with a high-power laser, a team of scientists at Lawrence Livermore National Laboratory in Livermore, Calif., in collaboration with Paul Scherrer Institut of Villigen, Switzerland, has demonstrated that the shock loading of nanocrystalline metals may yield ultrahard materials. Such materials may have applications as targets for inertial confinement fusion research and as improved types of armor.

In the work, which the investigators reported in the Sept. 16 issue of Science, the lab's cluster supercomputers, comprising 64 to 4000 processors, modeled the effects of 5- to 220-GPa shock waves on nanocrystalline copper. At a pressure of approximately 25 GPa, the system has reduced grain boundary sliding, which limits hardening in nanocrystalline materials subjected to lower stresses, and only small amounts of dislocation nucleation, which limits hardening at higher stresses. Materials subjected to this intermediate stress, the researchers found, should be twice as hard during loading as they were before the treatment.

They also examined the effects of laser-induced 20- to 40-GPa shock waves on 300-µm-thick foils of nanocrystalline nickel. Analysis of the shocked samples revealed signs of dislocations indicative of shocked hardening, in agreement with the models. This would lead to enhanced hardening after the shock treatment.