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they made an interesting observation: The copper got stronger as the space between the boundaries decreased from 100nm, ultimately reaching a peak of strength at 15nm. However, as the spacing decre- ased from 15nm, the metal got weaker. „This is very puzzling,“ Gao said. So Gao and Brown graduate stu- dent Xiaoyan Li dug a little further. The Brown scientists reproduced their collabora- tors‘ experiment in computer simulations involving 140 million atoms. They used a supercomputer at the Nati- onal Institute for Computati- onal Sciences in Tennessee, which allowed them to analyze the twin boundaries at the atomic scale. To their surprise, they saw an entirely new phenomenon: A highly ordered dislocation pattern controlled by nucleation had taken hold and dictated the copper‘s strength. The pattern was characterized by groups of atoms near the dis- location core and assembled in highly ordered, necklace- like patterns.

„They‘re not getting in each other‘s way. They‘re very organized,“ Gao said. From the experiments and the computer modeling, the researchers theorize that at the nanoscale, dislocation nucleation can become the governing principle to determining a metal‘s strength or weakness. The authors presented a new equation in the Nature paper to describe the prin-

10-04 :: April 2010

ciple. „Our work provides a concrete example of a source-controlled deformation mechanism in nano- structured materials for the first time and, as such, can be expected to have a profound impact on the field of materials science,“ Gao said.

Pictured are Brown University engineering professor Huajian Gao, right, and engineering graduate student Xiaoyan Li. © Lauren Brennan, Brown University

image left: A material science team led by Brown University engineers has found that the deformation of nanotwinned

metals is characterized by the motion of highly ordered, necklace-like patterns of crystal defects called dislocations. © Huajian Gao and Xiaoyan Li, Brown University

Xiaoyan Li, Yujie Wei, Lei Lu, Ke Lu, Huajian Gao: Dislocation nucleation governed softening and maxi- mum strength in nano-twinned metals, In: Nature, Vol 464(2010), Number 7290, April 08, 2010, DOI:10.1038/ nature08929:

http://dx.doi.org/10.1038/nature08929