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Fractal atomic-level percolation in metallic glasses

Science  18 Sep 2015:
Vol. 349, Issue 6254, pp. 1306-1310
DOI: 10.1126/science.aab1233

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Percolating cluster, factal structure

Metallic glasses are appealing materials because they are strong and can bend without breaking. These materials are disordered but possess none of the defects seen in crystalline counterparts. Chen et al. developed a model for metallic glasses in which clusters of atoms float free in the liquid, begin to jam, and finally organize into a short-range fractal structure below the glass transition temperature. This model also accounted for the density and high strength characteristics of bulk samples.

Science, this issue p. 1306

Abstract

Metallic glasses are metallic alloys that exhibit exotic material properties. They may have fractal structures at the atomic level, but a physical mechanism for their organization without ordering has not been identified. We demonstrated a crossover between fractal short-range (<2 atomic diameters) and homogeneous long-range structures using in situ x-ray diffraction, tomography, and molecular dynamics simulations. A specific class of fractal, the percolation cluster, explains the structural details for several metallic-glass compositions. We postulate that atoms percolate in the liquid phase and that the percolating cluster becomes rigid at the glass transition temperature.

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