ACS Chemistry for Life

One of the saddest ways of finding a pair of lost eyeglasses is by sitting on them. Inexpensive frames get bent and are impossible to straighten out again.  If the frames are made of the shape memory alloys Flexon® or Titan Flex® however, they can be heated up and the frames will return to their original shape. 

The general name for the alloys that display a memory effect is Nitinol, which first referred to the nickel-titanium alloy made at the Naval Ordinance Laboratory in White Oak, Md. The property was demonstrated at a laboratory meeting during which David S. Muzzey, a scientific researcher, used his lighter to heat up a sample of Nitinol that had been bent repeatedly. The heat caused the alloy to return to its initial unbent form. 

Shape memory alloys work because they have two different solid state phases. The high temperature phase, known as the austenite phase, is the parent phase and is characterized by the most compact and regular arrangement of the metal atoms.  At lower temperatures, the alloy is in the martensite phase. In this phase, the alloy can be bent and deformed into other shapes, but when heated, the sample will shift back to the austenite phase, and it will readopt whatever shape it possessed in the austenite phase as well.

There are numerous applications of this behavior in the aeronautics industry to reduce engine noise and in the automobile industry in the seat adjustment mechanism, but one of the most interesting uses is for dental braces. The wire that spans the teeth can be made from a shape memory alloy, and the force exerted by the metal trying to return to its parent form can be exploited to exert a constant pressure to move the teeth into the desired positions without requiring constant tightening and adjustments.

More information on shape memory alloys may be found at http://www.stanford.edu/~richlin1/sma/sma.html