By Zeeya Merali
A metal alloy masquerading as a glass is the first material to be fabricated that is as strong and as tough as the toughest steel. The feat could eventually see such materials replace steel in buildings, cars or bridges.
The terms 'strength' and 'toughness' may be used almost interchangeably in everyday life, but until now, no materials have been found that display both these characteristics. Some materials, such as glass, are strong -- that is, they are scratch-resistant and it is difficult to permanently bend them out of shape when you place a heavy load on them -- but they also tend to be brittle. Others, such as metal, are tough -- that is, they are more difficult to shatter -- but they are generally more malleable.
"The challenge has always been to achieve both high strength and toughness," says Marios Demetriou, a materials scientist at the California Institute of Technology in Pasadena. "But until now we have always had to compromise between the two."
Strength in weakness
Normal metals are weak and malleable because their atoms are lined up in a crystalline structure and easily slip past each other when placed under a heavy load. Amorphous metals are stronger. They are made by rapidly cooling molten metal, so that its atoms are stuck in a disordered arrangement -- resembling the structure of glass. It takes much more energy to permanently shift these atoms around.
Unfortunately, for a long time these 'glassy metals' also seemed to be inherently brittle. This is because they contain small defects that can clump together into bands, as the material begins to fail, says Demetriou. These bands can rapidly turn into cracks, and the material shatters.
However, materials scientists now know that, ironically, generating an extremely large number of these bands could have the opposite effect -- making the material tougher. "If you dramatically amplify the number of bands, they start to interlace and form networks," says Demetriou. "When new cracks evolve in the glassy metal, these networks cluster around the crack tips, blocking them from growing."
On their 109th attempt, the team finally struck on the right mix for a glassy metal that can generate a large number of these bands. The successful recipe was mostly palladium, with a small fraction of silver and a smattering of other elements. "We don't yet know why this particular composition works -- it will take a deeper investigation to find out what chemical properties have been altered that affect the material's toughness," Demetriou says.
The team tested the glassy metal's toughness by monitoring how quickly and deeply a crack could move through it. "We found that it is as tough as the toughest steel," he says.
"It's unique to see this combination of strength and toughness in a single material," says John Lewandowski, a materials scientist at Case Western Reserve University in Cleveland, Ohio. It's now important to investigate whether adding more elements to the mix -- to create small crystalline regions in the material -- could improve its toughness further, without sacrificing strength, he adds.
Lewandowski also notes that because palladium is a precious metal, it will be too expensive to use the material widely.
Demetriou acknowledges that problem. "It will be most suited for making strong and tough dental and medical implants, because their very high fabrication cost can often justify a high material cost."
However, the team plans to look for a cheaper version based on copper, iron or aluminium, says Demetriou. "If we find it, that material will take over from steel in large-scale engineering forever."