Text: A new class of metal alloys has a remarkable combination of unusual and useful properties: all its members are strong, heat-stable, supple and elastic1. The materials are compounds of titanium, zirconium, vanadium, niobium and tantalum - elements clustered together in the middle of the periodic table, in a larger group known as the transition metals. A small amount of oxygen provides an essential seasoning in the mix. Most metals would be permanently deformed if stretched to up to 2.5 times their original length. But the new alloys spring back again - earning them the title 'super-elastic'. When pulled harder, they extend by a further 20% before they snap. This degree of stretchiness is most unusual for a metal, and is dubbed superplasticity. The mixtures' super-elasticity means that they don't dent easily; their superplasticity means that they can be moulded without the need for heat. But it doesn't stop there, say developers Takashi Saito, of Toyota Central Research and Development Laboratories in Nagakute, Japan, and his colleagues. When warmed, the alloys barely expand. This rare, 'invar' behaviour is characteristic of some nickel-steel mixes that were discovered in the 1890s and are used in parts of delicate mechanisms such as wristwatches and scientific measuring instruments. This refusal to expand when warmed means that the devices are accurate across a range of temperatures. The new compounds also show 'elinvar' behaviour - their stiffness remains constant when they are heated. This effect holds over an amazingly wide temperature range - from as low as -194 °C to over 200 °C. To cap it all, the alloys are very strong. Their tensile strength - the amount of pulling that they can stand - is about twice that of steel. And they can be bent and straightened repeatedly without becoming brittle; they don't suffer from 'work hardening', in other words. Three magic numbers The alloys share three magic numbers. One, called their average valence, can be described as the number of bonds that each metal atom forms with others - it is 4.24 in this case. The bonds also have the same average strength or 'bond order'. And the average ability of each metal atom to gain an extra electron - a quantity called electronegativity - is also the same for each alloy. The metals are crystalline - their atoms are all lined up in rows. Occasionally these orderly rows don't line up properly, creating defects called dislocations. When most metals are bent, their dislocations shift around. After a lot of bending, the dislocations become tangled up and cannot move so easily, and the metal becomes hard and brittle. When the new materials are pushed and pulled, their dislocations don't move. Instead, cracks rather like geological faults form in the crystals, and these slide over one another. This explains why the alloys can stretch so much without snapping. References # Saito, T. et al. Multifunctional alloys obtained via a dislocation-free plastic deformation mechanism. Science, 300, 464 - 467, (2003). |Homepage| © Nature News Service / Macmillan Magazines Ltd 2003 http://www.nature.com/nsu/030414/030414-12.html

See Also:

Source: