HOWEVER much a shape-memory alloy is pulled and beaten about, a temperature change can push it back into its original form. But how does it remember what shape it should be?
One theory says the memory is stored by individual atoms, with each keeping tabs on its nearest neighbours. Another says that stresses in the structure store the information. Both explain the behaviour of nitinol, an alloy of nickel and titanium used in surgical instruments and spectacle frames. Nitinol has a 鈥渉ot鈥� form thought to be governed by structural memory, and a 鈥渃old鈥� crystal form controlled by atomic memory.
But previous calculations of the cold structure of nitinol got it wrong, reports Graeme Ackland from the University of Edinburgh in Scotland, in Nature Materials (DOI: 10.1038/nmat884).
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The cold crystal form of the alloy has too many atoms for each to properly track its neighbours, so both memory mechanisms must operate at once. 鈥淚n some sense,鈥� says Ackland, 鈥渢his unites the two theories.鈥�