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March 12, 2007

Atomic Design of Superstrong Materials

From MIT Technology Review, researchers have learned how to design the nanoscale features of materials to make them four times stronger without making them brittle. The new insight is the result of a significant improvement to an existing computer model that allowed researchers to, for the first time, simulate the complex mechanical behavior of nanostructures in metals. The advance is part of a larger ongoing effort to use software to discover new materials that would be impossible or impractical to discover using experiments alone.

The work "Interfacial plasticity governs strain rate sensitivity and ductility in nanostructured metals", described in the Proceedings of the National Academy of Sciences by researchers at MIT, Ohio State, and the Georgia Institute of Technology, could lead to more-durable materials for gears in microscale machines. Subra Suresh (MIT), Ting Zhu (Georgia Tech), Ju Li (Ohio State), Amit Samanta (Ohio State) and Hyoung Gyu Kim (Ohio State) were the authors. It could also lead to coatings that dramatically improve the performance of larger-scale structures, such as metal plating on artificial joints, says Subra Suresh, professor of materials science and engineering at MIT.

For decades, researchers have known that making the grains smaller--say, 10 nanometers across instead of a few micrometers--makes the material stronger. That's because there are more grains and, therefore, more boundaries that prevent the atoms from shifting.

A few years ago, researchers at the Shenyang National Laboratory for Materials Science in China synthesized a novel form of nanostructured metal, nano-twinned copper. The material was created by introducing controlled concentrations of twin boundaries within very small grains of the metal using a technique known as pulsed electrodeposition. The Shenyang group, working in collaboration with Suresh's group at MIT, demonstrated in the last two years that nano-twinned copper has many of the same desirable characteristics as nano-grained copper, and in addition resulted in a good combination of strength and ductility. By controlling the thickness and spacing of twin boundaries inside small grains to nanometer-level precision, they were able to produce copper with different "tunable" combinations of strength and ductility.

If too much force is applied, however, the dislocated atoms at these boundaries can cause the materials to break apart. This makes nanostructured materials more brittle.

The new version of copper contains another set of boundaries, called twins. These occur inside a grain when atoms on either side of an imaginary line are mirror images of each other. In the new copper, the grains are much larger than 10 nanometers, but they're divided into twins with boundaries about 10 nanometers apart. These boundaries are much more orderly than grain boundaries are.

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