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Metallic glass: a drop of the hard stuff

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In the past year, researchers have made metallic glass three times stronger than the best industrial steel and 10 times springier. Almost a match for the Terminator, in other words.

Metallic glass sounds like an oxymoron, and in a way it is. It describes a metal alloy with a chaotic structure. While metal atoms normally arrange themselves in ordered arrays, or crystals, the atoms in a metallic glass are a disordered jumble, rather like the atoms in a liquid or a glass. And although strictly speaking a metallic glass isn't a liquid, because the atoms are fixed in place, one company is already marketing the stuff as "liquid metal".

It is the unusual structure that makes metallic glass so promising. In crystalline metal alloys, the atoms are ordered within regions called "grains", and the boundaries between the grains are points of weakness in the material. Metallic glasses, however, have no grain boundaries, so they are much stronger. Hit a crystalline metal with a hammer and it will bend, absorbing some of the energy of the blow by giving way along grain boundaries. But the atoms in an amorphous metal are tightly packed, and easily bounce back to their original shape after a blow (see Diagram). These materials lack bulky crystalline grains, so they can be shaped into features just 10 nanometres across. And their liquid-like structure means they melt at lower temperatures, and can be moulded nearly as easily as plastics.

No wonder companies are interested. The trouble is, no one was able to make a useful metallic glass until very recently. That is because, when molten alloys are cooled down, they inevitably begin to crystallise, with ordered arrays of atoms growing from various points in the molten liquid. To make a metallic glass, crystallisation needs to be stopped in its tracks. This should happen if a liquid is cooled extremely fast, but just putting a cupful of molten metal into a freezer won't cool it fast enough.

But by pouring molten metal onto a cold, rapidly rotating copper cylinder, one could make sheets of "superfrozen" amorphous metal. The problem was that the sheets he made were only a few nanometres thick. If the rotation of the cylinder is slowed to try to make a thicker sheet it left enough time for crystals to form.

You won't know it to look at them, but before too long many of the metallic parts in everyday products will be the stuff of the Terminator.

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