A Hard(ness) Question Cleared Up: New “Nano-Stabilities” for Nanostructured Al/Pb alloys .

1. NIRT: Reduced Degree of Freedom Predictive Methods for Control & Design of Interfaces in Nanofeatured SystemsDonald W. Brenner, North Carolina State University, DMR 0304299 • A Hard(ness) Question Cleared Up: New “Nano-Stabilities” for Nanostructured Al/Pb alloys. • Experiments by Sheng et al. showed addition of Pb to nano-Al increases microhardness. They report 5-30 nm Pb clusters • Experiments by Koch et al. measured decreased hardness w/Pb addition. HRTEM showed Pb wetting grain boundaries. • Using first principles, we discovered new “nano-phases” for Pb in Al that explain the apparent experimental discrepancies • We predict a crossover in cluster stability and grain boundary wetting, where clusters containing less than ~4800 atoms are unstable with respect segregation to a grain boundary. • We explain minimum Pb cluster sizes observed in Al-Pb • Processing methods can alter dispersion of Pb in nc-Al, resulting in very different nanostuctures for similar systems. • Molecular simulations in our group show that Pb on grain boundaries lowers yield strength, while clustering increases hardness, consistent with experiment. Clusters less than ~4800 atoms prefer to wet grain boundaries Pb in a cluster and at a grain boundary in Al

2. NIRT: Reduced Degree of Freedom Predictive Methods for Control & Design of Interfaces in Nanofeatured SystemsDonald W. Brenner, North Carolina State University, DMR 0304299 Broader Impact: In conventional materials, with grain sizes of microns or larger, increases in hardness almost always result in a decrease in ductility, making hard materials brittle and often difficult to process. In nanocrystalline materials, on the other hand, the deformation dynamics are dominated by the properties of the grain boundaries rather than defect motion in the grains, giving these structures potentially unique combinations of hardness and ductility. The discovery that grains in aluminum can be stabilized by trace lead impurities, which normally have very small solubilities in conventional aluminum, is leading to new strategies for engineering nanocrystalline materials that are more stable and able to withstand temperatures than are currently possible. Illustration of a simulated three-dimensional aluminum nc with lead at the boundaries. Lead atoms are red, remaining atoms are colored by local symmetry.