Glass is such a familiar material that we may be hard pressed to think of it more abstractly than the clear, hard substance that lets light through a window. For a materials scientist, though, a ’glass’ is the material that forms when the atoms in a cooling liquid pack into a non-periodic arrangement. These atoms never stop ‘flowing’, but the process is so slow that the glass essentially behaves as a solid.

Metals with glassy structures are both very springy and strong. These technologically attractive characteristics have inspired Maozhi Li at the Renmin University of China in Beijing, in collaboration with scientists in China and the US,1 to better understand what causes the atoms in the copper–zirconium (CuZr) metal alloy to cluster, slow down and form a glass on cooling. Through molecular dynamics simulation, a powerful technique that models the positions of the copper and zirconium atoms as they are cooled, the researchers showed that they can relate the ‘slow’, glass-forming behavior of CuZr to the presence of clusters with a particular structure.

Fig. 1: An atomic cluster that forms in the metallic glass CuZr. This particular cluster — where copper and zirconium atoms (blue) form a distorted icoshedron (red lines) around a central copper atom (red) — contributes to the slow, glassy dynamics.

Characterizing the amorphous structure of glass — rather than the periodic lattice of a crystal — requires special nomenclature. In each simulation, the team counted the number of ‘Voronoi polyhedra’ formed by the atom clusters. A Voronoi polyhedron is the three-dimensional volume formed by putting each face of the polyhedron halfway between a central atom and its neighbors (red lines in Fig. 1). The polyhedra are classified according to the number of faces they have with three, four, five and six edges.

Li and his colleagues found that in addition to icosahedral polyhedra (those with 12 pentagonal faces), most of which are copper-centered, zirconium-centered clusters with many pentagonal faces seem to determine the stability and slow dynamics of the metallic glasses. Furthermore, adding copper increases the number of clusters with mainly five-edged faces, which also influences the alloy's glass-forming ability. “The strong spatial correlation and the relative population of these clusters make the atoms slow down and favor dense atomic packing,” explains Li.

These simulations may help scientists engineer better glass formers. “One could do this by tuning the composition or introducing other elements, like aluminium or silver, that favor the formation of the polyhedra that facilitate glass formation.”