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What is Shape memory

Author: Date:7/17/2012 8:59:15 PM

Materials science is all about choosing the best possible material to do a particular job. If you're designing a jet engine, for example, you choose strong materials that are extremely light and can cope with high temperatures. You might pickaluminum, titanium, or a metal alloy. But what if you want to make an airplanecomponent that behaves in one way at low temperatures and in a different way when it heats up? That's the kind of situation where you might use a shape-memory alloy, which can automatically reshape itself as the temperature changes.

Ordinary metal objects have no memory of their shape. If you sit on a pair of aluminum eyeglass frames and bend them permanently (in scientific words, "subject them to a plastic deformation"), it's tricky to get them back exactly how they were. You have to use your own memory of what the frames were originally like and laboriously twist and bend; even then, there's no guarantee the frames will look like they used to and they may even break entirely—throughfatigue—if you wiggle them back and forth too much.

Shape-memory materials behave differently. They're strong, lightweight alloys (generally, mixtures of two or metals) with a very special property. They can be "programmed" to remember their original shape, so if you bend or squeeze them you can get that original shape back again just by heating them. This is called the shape memory effect(or thermal shape memory effect, since heat energy makes it happen). Some shape-memory alloys remember one shape when they're hot and a different one when they're cold, so if you cool them they spring into one shape and if you heat them they "forget" that shape and flex into a different one. This is known as the two-way shape memory effect. Now what if you could make a shape-memory object that would bend and twist by a huge amount but still return perfectly to its original shape, even without heating? That's an aspect of shape memory called pseudo-elasticityor superelasticity and it's used in those super-bendy, virtually indestructible eyeglass frames, which manufacturers claim are at least 10 times more flexible than steel!

Although nitinol (also called nickel-titanium, Ni-Ti) is perhaps the best-known shape memory alloy, there are lots more, including alloys made from copper,zinc, and aluminum (Cu-Zn-Al); copper, aluminum, and nickel (Cu-Al-Ni); iron, manganese, and silicon (Fe-Mn-Si); and quite a few others.