Hands-on Science Carnival 2010 Activity Stations: Material Properties

Wire with a Memory

[Shopping List: Nitinol shape memory wire, cut into 2” lengths; ice water bath; gloves; tweezers or pliers; candles; aluminum tubing; “HOT” wire; bending teaspoon; hot plate with water bath and/or blow dryer]

1. Warm the thin wire with your fingers, then gently try to flex it. Notice that it resists bending when it’s warm.
2. Dip the wire into an ice water bath. Now it is very easy to bend, so go ahead and bend it a little.
3. Remove the bent wire and lay it on the table. It should keep it’s bent shape.
4. Pick up the wire with your fingers, and hold it for a few seconds. It may begin to straighten all by itself. Breath on it to warm it even more, and it should completely straighten out.

You can also demonstrate two higher temperature samples: a spoon that will bend 90° when placed in hot water; and a long wire that will bend to form the word “HOT” when placed in warm water. The critical temperature for these samples is about 120°F, so you will need a hot plate to heat the water bath, or try using a blow dryer. Cool the samples in the ice bath a few seconds before straightening them out to repeat, and be particularly careful with them, as they are expensive.

What’s Happening: This wire is made from a special alloy of two common metals, nickel and titanium called Nitinol. As is often the case when metals are combined, the alloy has very different properties than either of its component metals alone. Steel is a common alloy containing mostly iron and carbon, but adding other elements gives it special properties different than iron alone, such as strength, corrosion resistance, etc. In this case, Nitinol has a very strange property, it can exist in two different kinds of solid phases depending on its temperature. At lower temperatures it is in its martensite phase, but as it is warmed above a critical temperature it almost instantly changes into its austenite phase. The critical temperature depends on the exact mixture of metals in the alloy, so it can be controlled. For our samples, the critical temperature is about room temperature.

What makes Nitinol alloys really interesting, is that they can “remember” the shape they had in their austenite phase, even if they have been cooled to the martensite phase and deformed or bent. When they are warmed back into the austenite phase they will spring back into the shape they remember, so they’re called shape memory alloys (SMA). They are also much more difficult to deform while they are in the austenite phase. To make them even more useful (and fun), they can be “trained” to remember just about any shape desired. This is done by firmly clamping the material into the desired shape, then heating above a second much higher critical temperature (for our sample it’s about 400°F). We can do this by placing our wire into a small aluminum or copper tube, bending the tube to the shape we want, then heating the tube (with the wire inside) in a candle flame. After a few seconds, cool the tube, remove the wire, bend it in an ice bath, warm it with your hands and watch it sprint into the new trained shape.

Nitinol and other SMA alloys are used in many applications, not only because they return to a trained shape, but because in doing so they can generate very large and precisely repeatable forces for their size and weight. For this reason they can often replace costly and complex motors. The most common place you will find them is in eyeglass frames, where they make the frames resist bending at normal temperatures (but they can be bent easily when cold, then return to their original shape when warmed again.