The exciting field of smart materials is expanding rapidly, with one of the most interesting areas being that of shape memory alloys. A shape memory alloy (SMA) can undergo substantial plastic deformation, and then be triggered into returning to its original shape by the application of heat. These properties have led to a proliferation of diverse applications in a variety of industries, see table 2.

From early applications such as greenhouse window openers in which an SMA actuator provided temperature-dependent ventilation, through to plastic-coated mobile phone antennas made from a super-elastic SMA capable of recovering its shape even after an extreme deformation such as dropping the phone, the list of applications has increased enormously throughout the 1990s. Medical applications of SMAs, using their superelastic and shape recovery properties, are particularly interesting and beneficial, and are growing rapidly.
History

Surprisingly for materials with so many applications, shape memory alloys have not been around a long time. A key discovery occurred in 1962, when a binary alloy composed of equi-atomic nickel and titanium was found to exhibit a shape recovery effect when heated after being mechanically deformed. Although other reversible phase change materials were known at the time, the Ni-Ti alloys showed a large recoverable strain value when compared to other binary, ternary or quaternary shape memory alloy systems.

The physical performance of the Ni-Ti alloy made it a landmark discovery, and the range of commercially viable applications that have been found for the materials is proof of the importance of the nickel-titanium shape memory alloys. But the discovery may have been a happy accident. Rumour has it that William Buehler, who was working with high nickel-bearing alloys for gas turbine components, left a small ingot of Ni-Ti alloy made in a vacuum melt furnace on a desk in direct sunlight. When Buehler and his colleagues came back from lunch, they noticed the ingot’s shape had changed. Now known as Nitinol (derived from Ni-Ti Naval Ordinance Laboratories, part of the US Department of Defence), the name has become one of the commonly used titles for the SMAs emanating from Buehler's laboratory.