Local icosahedral order in metallic glasses

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Posted July 16, 2013
(A) Experimental procedure of Angstrom-beam electron diffraction of a icosahedral cluster. The coherent electron beam has a diameter of 0.36 nm. (B) Simulated ABED patterns of an ideal icosahedron. Credit: (c) Science DOI: 10.1126/science.1232450

(A) Experimental procedure of Angstrom-beam electron diffraction of a icosahedral cluster. The coherent electron beam has a diameter of 0.36 nm. (B) Simulated ABED patterns of an ideal icosahedron. Credit: (c) Science DOI: 10.1126/science.1232450

Metallic glasses are essentially a frozen, supercooled liquid. They are amorphous metals, often alloys, which are non-crystalline and therefore have a highly disordered atomic arrangement. They are true glasses in the sense that they soften and flow upon heating. The ability to easily process these materials has led to their use in many products, like for example, those made by injection molding. Over a half century ago, it was proposed that metallic liquids, and perhaps even glasses possess icosohedral clusters. Despite attempts using neutron or x-ray scattering to reveal this icosohedral order, only average structural information has been obtained. A new paper published in Science, describes development of an Angstrom-beam electron diffraction (ABED) method which is able to probe local atomic structure with a 0.4 nm electron beam. The authors were able to characterize local icosohedral order in ZrPt (Zr80-Pt20)glass, and found that it is in close agreement with that predicted by computational simulations.

The researchers determined that a distorted icosahedron with partical fcc (face-centered-cubic) symmetry is representative of the local structure of the Zr-Pt glass. They further conducted a bond orientational order analysis based on a molecular dynamics simulation containing 12000 atoms to confirm this observation. This distortion of the icosahedra had been suggested by bond length variation resulting from a combination of atomic size disparities and kinetic fluctuations occurring during glass formation.

The finding that the distorted icosahedral always possessed fcc symmetry derives from a phenomenon known as “geometric frustration.” The origin of the frustration comes from competition between the low energy icosohedral or fcc states, and dense atomic packing. The concept of geometric frustration in metalic glasses derives from certain features of magnetism, where it is associated with the topological arrangement of spins. It also has been applied to ice formation.

Read more at: Phys.org



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