IN THE movie Terminator 2, the villain is a robot made of liquid metal. He morphs from human form to helicopter and back again with ease, moulds himself into any shape without breaking, and can even flow under doorways.
Now a similar-sounding futuristic material is about to turn up everywhere. It is called metallic glass. 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…
