Materials Science

Making Mg Magnificent

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Science  06 Sep 2013:
Vol. 341, Issue 6150, pp. 1045
DOI: 10.1126/science.341.6150.1045-b

When crystalline materials are stressed, defects in the crystal planes become mobile once a critical stress is reached. The stress required for movement of these dislocations along different slip planes can vary considerably, leading to poor ductility. Magnesium is an example of a material with an extreme anisotropy: The critical stress required for deformation along nonbasal planes is 100 times larger than along basal ones. Yu et al. postulated that even though materials are known to be stronger when they are smaller, there are upper bounds to this enhancement, so that the critical anisotropy should decrease. They tested single-crystal Mg samples ranging from 850 to 80 nm in size inside a quantitative electron microscope. At sizes between 200 and 400 nm, significant strengthening of the samples was seen, but the ductility remained poor. Below 100 nm, there was a shift in the deformation behavior. As the local flow stresses approached 2 GPa, there was increasing activation of the nonbasal planes, leading to a large amount of plastic deformation. These size effects could be employed to make better use of other high plastically anisotropic materials. The use of grain boundaries could allow for larger overall samples, because the boundaries will act as stress concentrators and preferred sources for the nucleation and emission of dislocations, which is important during plastic deformation of a sample.

Proc. Natl. Acad. Sci. U.S.A. 110, 13289 (2013).

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