One crystal out of many

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Science  30 Nov 2018:
Vol. 362, Issue 6418, pp. 996-997
DOI: 10.1126/science.aav6733

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A large fraction of the “stuff” that surrounds us, whether natural (e.g., rocks) or artificial (e.g., most metals and ceramics), is polycrystalline—assemblies of smaller single crystals. The size and perfection of the crystals govern property optimization. For example, materials made up of nanocrystals can have exceptional strength and damping capability. However, in some cases, single crystals optimize performance. Substantial effort has gone into growing large single crystals of nickel superalloys because at this macroscopic scale, the grain boundaries between crystallites are a source of weakness at the high temperatures used in gas-turbine engines. Converting a sample of conventional polycrystalline material with typical grain sizes of 20 µm into coarse-grained material is not a simple task. The grain boundaries must overcome many physical barriers in order to migrate over distances comparable to the desired grain size. Indeed, it is accepted that thin films rarely will coarsen beyond the thickness of the film itself (1). Thus, the study by Jin et al. (2) on page 1021 of this issue is very surprising because it demonstrates that grain boundaries can be moved in foils of metals such as copper and platinum over distances on the order of centimeters.