Nitinol

This is a shape memory alloy, which has appoximately equivalent weights of titanium and Nickel. It is used in many engineering applications, for fasteners, actuators and couplings.

Nitinol couplings have been used in Submarine plumbings and fittings. These are expanded by ~4% in Liquid Nitrogen and placed around tubes. After warming to room temperature, these contract to their original shape, producing a tight seal.

Nitinol is an acronym for Nickel Titanium Naval Ordnance Laboratories. It was developed in 1965 at (surprise) Naval Ordnance Laboratories

Shape memory alloys are not the only thermally responsive substance; recently, shape memory polymers have also been developed.

Some SMAs are not thermally responsive; Ferromagnetic shape memory alloys are a kind of SMA that change shape in magnetic fields

Nitinol and other SMAs (such as copper-zinc-aluminum alloy) are used in an increasing number of applications. One major new area of use is in medicine. For example, dental braces made of SMAs can exert a constant force on the teeth.

Nitinol is fun to play with. Bend it anyway you like, and dipping it in hot water will make it spring back to its original shape. In order to change the original shape, bend the nitinol and then dip it in cold water to make it set. Upon subsequent twisting and emersion in hot water, the nitnol will return to its new "original" shape.

Sources include wikipedia article on shaped memory alloy and personal experience with nitinol wires