* 02 April 2005
* NewScientist.com news service
* Catherine Zandonella
"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 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. In the 1960s, Pol Duwez at the California Institute of Technology in Pasadena came up with an answer. By pouring molten metal onto a cold, rapidly rotating copper cylinder, he could make sheets of "superfrozen" amorphous metal. The problem was that the sheets he made were only a few nanometres thick. If he slowed the rotation of the cylinder to try to make a thicker sheet it left enough time for crystals to form.
It is only recently that researchers at Liquidmetal Technologies in Lake Forest, California, have improved on this. The company was founded by William Johnson, a former student of Duwez. For a long time, Johnson saw no way to make metallic glasses thicker than a millimetre or so. Then he heard about the work of Akihisa Inoue of Tohoku University's Institute for Materials Research in Sendai, on the Japanese island of Honshu. Inoue had found that adding big, bulky metal atoms such as lanthanum to an alloy dramatically slows its crystallisation rate. The huge atoms disrupted crystal formation, making it possible to freeze alloys into glasses without going to extreme lengths to cool them quickly.
Inoue argued that all kinds of metallic alloys could be made glassy in this way. All that was needed was the right combination of large and small atoms. Get it right, and as the molten alloy cools, the smaller atoms will cluster around the larger ones. Other small atoms fill in the spaces between the clusters, and the result is a morass of disordered atoms. The resulting material makes an incredibly stiff alternative to normal metal.
In the early 1990s Johnson and his colleague at Caltech, Atakan Peker, finally made an alloy based on this method and used it to launch Liquidmetal. They called their material Vitreloy. It contained large atoms of zirconium, titanium, copper and nickel, and small atoms of beryllium. Vitreloy is springier than steel and what's more, it becomes malleable at temperatures as low as 400 °C, compared with over 1000 °C for steel. This made it potentially much cheaper to manufacture.
But things did not go exactly to plan. The company's first Vitreloy product was a golf club, and the material's springiness meant that the club could hit balls further than anything else on the market. There was a problem, however. Although metallic glasses are super-strong, they can be incredibly brittle. Like window glass they crack and shatter if hit with sufficient force. While ordinary metals fail slowly along grain boundaries, metallic glasses break without warning. So the golf clubs were never marketed. Early prototypes would often splinter into pieces after as few as 40 hits.
Liquidmetal's research showed that the brittleness is due to the formation of "shear bands" at stress points in the material. In a crystalline material, these bands extend a short way along a grain boundary then stop when they run into a crystal, but in amorphous substances these bands just keep on growing. Johnson and his group fixed the problem by creating a kind of intermediate. They mixed crystal particles into the Vitreloy, effectively placing barriers in the way of the shear bands and stopping them from extending.
This was not an ideal solution, however. Johnson wanted to find a metallic glass that would be tough without the need for crystals. Finding one involved guessing, heating various ingredients together, and then simply trying them out. "As a first test we throw the material on the ground," says Jan Schroers, a researcher at Liquidmetal. "The good ones throw sparks but they don't shatter".
Last year the search paid off. Johnson's group finally found a glassy mixture of platinum, copper, nickel and phosphorus that is not brittle. When they applied a force to it, lots of shear bands appeared, but each one was small and thin. These bands appear to increase overall toughness by interfering with each other so that no one band can extend into a long crack (Physical Review Letters, vol 93, p 255506). "This is the first time that a combination of these properties has been seen," says Schroers, "not only for bulk metallic glasses, but for all metal alloys".
Since the new glass is almost 60 per cent platinum it is too expensive for widespread use. But the numerous studies have hinted at a pattern that might help researchers find alternatives more quickly. It turns out that the springier a metallic glass is, the less likely it is to be brittle.
This is related to the degree of liquid-like behaviour of the material. Liquids flow so easily that they can immediately return to a former shape: dip a spoon in a liquid and then remove it and the hole fills instantly. The same property, of flowing away from pressure instead of snapping, makes it hard to shatter liquids.
Researchers have long known that alloys are less liquid-like when the atomic bonds in them are metallic, meaning that the electrons that glue the atoms together can flow easily from one part of the material to another. So researchers might be able to reduce brittleness by choosing elements that will increase the degree of metallic bonding. The problem is that no one knows how to do this, says Mark Eberhart, a geochemist at the Colorado School of Mines in Golden. "You can find it in a basic chemistry text: 'metallic bonding leads to ductile metals'," says Eberhart, "but where does it tell you how to change that? It doesn't." He is now trying to describe the precise relationship between the properties of a material and the distribution of electrons in its bonds. The more metallic the bond, the less brittle the material.
Another important advance is the ability to make metallic glass using cheap metals such as iron and copper. In 2003, Joseph Poon and Gary Shiflet at the University of Virginia in Charlottesville announced the first steel glass, containing carbon, iron and a little manganese. As manganese is not magnetic, the resulting material was one of the first non-magnetic steels. It could be a big breakthrough, because ships built of non-magnetic steel would be able to elude radar detection more easily.
Poon and Shiflet's material is still fairly brittle, but progress is rapid - Johnson is already preparing to report on a copper-based alloy that is ductile. In fact, because researchers now know that they are looking for liquid-like bonding, they have been able to produce a number of cheap new metallic glasses in the past few months. "Instead of two months of randomly adding materials, you can find out in a day or couple of days," says Poon.
Liquidmetal is already producing the platinum-based metallic glass for medical devices, scalpel blades and professional tennis rackets. Inoue in Japan has used metallic glass to build a miniature motor. And the strength of metallic glasses means that, in addition to aircraft parts and ships' hulls, the US Department of Defense is now considering them as non-toxic alternatives to depleted uranium on the tips of armour-piercing bullets. Liquidmetal has even signed a contract with Samsung to make cellphone parts. 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."