Nanostructured materials contain a high density of interfaces that lend them advantageous mechanical properties, such as ultrahigh strength. But the integrity of the interfaces in harsh conditions is closely tied to how the material is made. Metal heterostructures, for example, whose interfaces are nearly perfectly ordered thanks to epitaxial growth or some other near-equilibrium technique required to produce them, possess extraordinary thermal stability and radiation tolerance. But such techniques are labor intensive, expensive, and produce little material. Irene Beyerlein and her colleagues at Los Alamos National Laboratory have now shown that a bulk processing method that severely strains a bimetallic composite can produce similarly stable and ordered interfaces. By subjecting an alternating stack of millimeter-thick copper and niobium sheets to a metalworking technique known as accumulative roll bonding—repeatedly rolling the sheets thin and then cutting and restacking them like croissant dough—the researchers reduced the layers of Cu and Nb to as thin as 20 nm, as shown here. That's equivalent to stretching a nickel coin 2.2 km in length. Using neutron diffraction and transmission electron microscopy, they found that when the layers became thinner than about 700 nm, each interface spontaneously adopted one of a small number of crystallographic orientations. Molecular-dynamics simulations bore out the same result—the emergent orientations correspond to minima in the interface formation-energy landscape. (I. J. Beyerlein et al., Proc. Natl. Acad. Sci. 111, 4386, 2014.)
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An ordered nanomaterial from bulk processing
31 March 2014
When two immiscible metal sheets are squeezed, cut, and stacked together repeatedly, a remarkably ordered nanoscale structure can emerge at the interfaces between them.
© 2014 American Institute of Physics