Materials Science

Of Grains and Glasses

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Science  15 May 2009:
Vol. 324, Issue 5929, pp. 859
DOI: 10.1126/science.324_859b

Most crystalline materials do not naturally form single crystals, but instead form ordered regions separated by thin grain boundaries. Although the boundaries may occupy only a small fraction of the volume, they can substantially affect the mechanical and electrical properties. Zhang et al. use simulations to explore the boundaries at high temperatures, where less is known about them experimentally, and specifically probe how atom mobility changes with temperature. They find that the motion of the atoms resembles that in glass-forming liquids. In particular, both materials show the cooperative motion of strings of atoms, with similar behavior in the size and motion of the strings over a range of temperatures, including the formation at low temperature of cages that trap atoms into localized oscillations. The authors believe their model can explain why grain boundary behavior is dependent on the mode of an applied stress, something that is not captured by conventional grain boundary migration theories. The nature of the strain (tensile or compressive), for example, alters the average chain length, which in turn influences the mobility. Similarly, impurities can either disrupt or enhance the formation of atom chains, and thus render the boundary region either a stronger or weaker glass former.

Proc. Natl. Acad. Sci. U.S.A. 106, 10.1073/pnas.0900227106 (2009).

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