Tough metallic glasses

16 January 2011
Posted by Chris Smith.

Tougher glasses have been developed than ever before

Metals are wonderful material and have revolutionised human life. They are all crystalline which means that their atoms are arranged in repeating patterns over a relatively large scale, but they are not nearly as strong or stiff as you would expect from the bonds between the atoms. One of the reasons for this is that tiny defects in the crystals can move through the crystal relatively easily allowing the crystal to deform at much lower forces than you would otherwise expect.

Secondary electron micrographs taken after a bending test on an unnotched Pd79Ag3.5P6Si9.5Ge2 glassy specimen.One way of making the metal stronger and harder is to make a glass with no large scale structure. When metals are molten their atoms are arranged randomly so if you can cool them quickly enough they won't crystallise especially if you are using a complex alloy with lots of different sizes of atoms. These are very hard and wear resistant, and very efficiently elastic, but the are not tough because unlike a normal metal they can't absorb energy in an impact by deforming, so cracks can pass through them very easily like normal glass.

Marios D. Demetriou and collegues from the California Institute of Technology have managed to greatly increase their toughness by making a glass involving a palladium alloy, this made the glass much better at deforming slightly under large shear forces. This means that at the front of a crack the glass distorts slightly blunting the crack instead of making the crack longer. This has produced a material that is both stronger and tougher than any other metal.

It does however have the problem that Palladium is much rarer than Gold and costs about $16000 per kilogram, so it is unlikely to be used for anything very large. But the group is now working on alloys with similar properties which are a bit cheaper.

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