A metallic glass might sound like an oxymoron, but such things do exist and are potentially useful materials. In this week’s Nature, Hong Wei Sheng and colleagues show how the packing of atoms in metallic glasses maintains a delicate balance between crystal-like and random structures.
Normal metals are crystalline: their atoms are arranged in regular, orderly arrays, like oranges stacked on a greengrocer’s stall. In window glass, on the other hand, the component atoms are disorderly, jumbled together like an unruly yet virtually static crowd. Metallic glasses also have a disordered arrangement of atoms — but they are neither ductile, like ordinary metals, nor brittle, like window glass. They are tough and springy, which is why they are being explored for uses ranging from golf-club heads to armour plating.
Although there is no ‘long-range’ order in metallic glasses they do have some degree of regularity over shorter scales. Not only are each atom’s immediate neighbours arranged rather similarly to those in a crystal, but even the next-nearest neighbours and the ones beyond may have some non-randomness, some predictability, in their positions.
Several different theoretical models have been proposed for the structure of metallic glasses, but only now do the experiments and computer-modelling studies of Sheng and colleagues subject them to scrutiny. The researchers show that groups of atoms tend to form clusters with geometric, polyhedral shapes in the short range; they also propose some fundamentally new packing schemes which constitute medium-range ordering, up to length scales of several ångströms.
From the Nature Press Release of the week…
January 30th, 2006 | General Science