The strength or hardness of a metal depends on the properties of the constituent crystallites—known as grains—that compose it. By reducing the grain size of a metal to the nanometer scale, we can produce nanostructured materials that are much harder than their coarse grain counterparts. This characteristic provides opportunities for developing new materials for applications ranging from protection shields for spacecrafts and military vehicles to millimeter-sized targets for inertial confinement fusion experiments.
However, this strengthening characteristic is limited by mechanisms occurring at grain boundaries. One effect is that, as the dimension of the grains is reduced, they begin to slide over each other—a softening mechanism that imposes a limit to the maximum strength the material can have.
Combining terascale atomistic simulations and novel shock-loading experiments on nanocrystalline metals, our team has uncovered new deformation mechanisms and new approaches to optimize high-strength materials. Results on nanocrystalline nickel and copper showed that, by applying shock waves traveling faster than the speed of sound, the softening effects do not occur and can be controlled by both high strain rate and high pressure.
Many Laboratory missions, including stockpile stewardship, require an understanding of how metals respond to shock waves and subsequent high-strain-rate deformations. To assess the behavior under these conditions, it is important to study the origin of deformation and strength, and how defects within a crystal structure (known as dislocations) affect the properties of materials.
Measuring the dynamic deformation process during high-strain-rate loading at the nanoscale is difficult using experimental methods alone. However, by combining simulations with experiments, we can use our improved understanding of these processes to build numerical models that accurately predict the materialís response under various temperatures and pressures over time. The information we glean will also bring us closer to the development of new materials with superior strength.
We carried out the largest atomistic simulations of shocked nanocrystalline samples of nickel and copper to date, employing up to 4000 processors on Livermore’s supercomputers. Shock waves move faster than the speed of sound and generate pressures nearly one million times larger than atmospheric pressure. The extremely short compression timescales (or very high strain rates) imply that dislocation formation and movement are the dominant plasticity mechanisms involved. Our simulations show that a large increase in pressure significantly reduces grain-boundary sliding, limiting the softening mechanism and doubling the flow stress.
|Figure 1. A transmission electron microscope image of a nickel sample with grains measuring 30–50 nm shows that the nanostructure remains intact after a shock pressure of 40 GPa. After the shock, some grains grow up to 300 nm, while others decrease in size (as in the grains marked by white lines in the right panel).|
In addition to the simulations, we also performed the first shock experiments on nanocrystalline metals at the same scale of our simulations. Because of experimental constraints, it is extremely difficult to directly measure the dynamic deformation process during high-strain-rate loading at the nanoscale. Using the Livermore JANUS laser, we shocked and recovered nickel samples, which were later studied with a transmission electron microscope (TEM) as shown in Figure 1. We observed clear experimental evidence of dislocation activity occurring inside grains, in agreement with our atomistic simulations. During loading at 50 GPa, our molecular dynamics simulations predict a dislocation density of approximately 1013/cm2.
Although the exact dislocation density in recovered samples is difficult to estimate, our high-resolution TEM images showed residual dislocations inside some nanograins. This is quite unusual in nanocrystalline materials and not typically achievable under normal deformation conditions. Our experiments also indicate an increase in hardness in the samples recovered after shock loading, as expected from the measured residual dislocation densities. By turning off the mechanism that softens the grains, we create a material that is harder during and following the shock-wave application.
|Figure 2. A molecular dynamics simulation of a 50-nm nickel crystal shows the effect after a 50-GPa shock wave. The spatially inhomogenous shock front leads to various types of dislocation activity (shown as various colors) with different orientations. The close-up shows perfect dislocations (narrow ribbons) and partial loops inside the same grain.|
Connecting atomistic models with shock experiments performed on nanocrystalline metals advances our understanding of how extreme pressure affects the behavior of metallic materials at the nanoscale. Specifically, our experiments on nickel confirm simulation results indicating that, at extremely short compression timescales, the dominant plasticity mechanisms are dislocation formation and movement (Figure 2). Our studies demonstrate that shock waves can be used to turn off the grain-boundary sliding that softens the metal, creating a much harder material. They also provide new insights that can be applied toward the construction of new computational models to better understand crystal plasticity.
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E. M. Bringa et al., “Ultra-Hard Nanocrystalline Metals by Shock Loading,” Science 309, 1838 (2005).
Y. W. Yang, E. M. Bringa, et al., “Deforming Nanocrystalline Nickel at Ultrahigh Strain Rates,” Appl. Phys. Lett. 88, 061917 (2006).