AN EXTRA bendy mixture of metals is astonishing the normally conservative world of materials science. It does not conform to any known theory of metal deformation and is already sparking talk of a new generation of 鈥渟uperalloys鈥�.
The complicated alloy of titanium, niobium, tantalum, zirconium and oxygen was developed by Toyota researchers based in Nagakute, Japan. It has been nicknamed 鈥済um metal鈥� and is being used to manufacture unbreakable spectacle frames that can be twisted or bent almost to right angles and still spring back to their original shape. A wire of the alloy can be stretched to twice its original length without breaking. And the alloy鈥檚 super-powers hold good at any temperature between 鈭�200 掳C and 200 掳C, which should catch the eye of spacecraft designers.
The material has been on the market for a year, but until now its discoverers have said nothing about what it is or how it works. They were not keeping company secrets, explains Takashi Saito, who led the research, but simply double-checking their results. He says the way the material behaves on an atomic scale is unique and surprising: 鈥渢oo abnormal to be accepted by the scientific society of metallurgy鈥�. The team wanted to be sure they were right before facing the scrutiny of others.
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Proposing a new theory of metal deformation is a brave move. The standard model has been entrenched since the 1950s, when electron microscopes first revealed the patterns of atoms inside metal crystals. The atoms were found neatly organised into layers, occasionally interrupted by a slight defect or wrinkle. When the metal was deformed or stretched, the wrinkles moved through the structure. It was assumed these dislocations could explain how all metals change shape under stress.
But Saito and his team now report in Science (vol 300, p 464) that their microscope images of the new alloy show almost no such rearrangements in its structure as it deforms. Instead, entire layers of atoms seem to bend and slip along 鈥済iant faults鈥�.
鈥淚t鈥檚 tremendously interesting,鈥� says Graeme Ackland of the University of Edinburgh, Scotland (who is urging another metallurgy rethink in this month鈥檚 Nature Materials, see page 26 of this issue). He says he has seen similar effects in simulations of metal deformation, but the results were so unusual he assumed they were artefacts of the computer program. 鈥淚t鈥檚 astonishing these deformations are seen, but disappointing that we predicted it and binned our results.鈥�
Saito and his team still don鈥檛 understand why this particular mix of metals behaves in this way. But they have now discovered a whole family of alloys that share gum metal鈥檚 striking properties. What鈥檚 more, the family is united by three 鈥渕agic numbers鈥�.
The first number describes the average number of electrons in the atoms鈥� outer shells. Saito was looking for an alloy that was soft enough to mould, but strong enough to be used in Toyota鈥檚 cars. His computer model indicated that materials with an average of 4.24 outer-shell electrons per atom should be easy to deform.
The team then mixed up hundreds of different alloys with this property in the hope of finding one that was also strong, and in the course of this investigation they discovered the super-alloys. They found that these mixtures shared other 鈥渕agic numbers鈥�, which describe the bond strengths and electron energies in the materials used.
Gary Shiflet, a materials scientist at the University of Virginia, Charlottesville, says that calculating the first magic number was key to the team鈥檚 success. 鈥淲hy else would you pick these elements from the periodic table, in very precise amounts?鈥� he says. 鈥淵ou need some computational tool to help you.鈥�
The magic numbers herald a new approach to metallurgy, in which researchers will be able to pick out the most promising combinations of metals without having to test millions of mixtures, Shiflet predicts. 鈥淚n 10 to 20 years, people will design alloys from their desks.鈥�