How Does Memory Metal Work?

A metal coil

Memory metal was invented in 1965 by The Naval Ordinance Laboratory. It is an alloy of nickel and titanium in almost equal proportions. The amount of each metal is varied depending on what the desired properties of the final product are. Memory metal gets its name from its ability to be mechanically deformed, then return to its original shape by applying heat. The temperature at which this change occurs is determined by the ratio of nickel to titanium.

Phases

Solid metal

Most materials on Earth can exist in three phases: a solid phase, liquid phase and gas phase. However, memory metal has two different solid phases. The austenite phase exists above the temperature of phase change and has a very rigid, cube-like atomic structure while the martensite phase exists below the temperature of phase change and is much more malleable.

Changing Phases

Forming metal with heat

While in the martensite phase, memory metal can be manipulated into various shapes. With the application of a relatively low amount of heat such as that from hot tap water or a hair dryer, it will change back to its original austenite phase. To change the shape of the metal in the austenite phase, it must be formed to the intended shape and held there mechanically while it is heated to temperatures higher than 500 degrees Celsius, then cooled very slowly.

Medicinal Applications

Medical supplies

The properties of memory metal are very applicable in medicine and dentistry. Memory metals are used in orthodontic braces because the heat of the body causes the memory metal wire to apply pressure to the teeth and push them into position. These properties also cause memory metal stents to expand to their original shape when placed inside an artery to hold the desired blood vessel open.

Mechanical Applications

Memory metal is used in helicopters.

Memory metals are also useful in household products and heavy industry. Common appliance thermometers such as those in coffee pots cause the device to switch off when heated to a certain temperature. This property can also allow eyeglasses to better retain their shape or cause a memory metal clamp to tighten. More large-scale applications include using memory metals to regulate the balance on helicopter blades.

Other Alloys

Most coins are metal alloys.

Although the nickel-titanium alloy is the most common memory metal, other alloys do exist. Examples include iron-manganese-silicon, copper-aluminum-nickel, copper-zinc-aluminum and gold-cadmium alloys.

References

• University of Wisconsin-Madison: Memory Metal
• Chemistry Comes Alive: Memory Metal