When an external force is applied to a metal, deformation occurs. In ordinary metals, once plastic deformation takes place, the material remains permanently deformed. However, in shape memory alloys, heating causes the material to recover its original shape that was manufactured through prior treatment. This phenomenon is known as the shape memory effect. A shape memory alloy is a general term for alloys that exhibit this effect.
The development and practical application of shape memory alloys as engineering materials progressed rapidly beginning in the 1980s. In the latter half of that decade, more than 1,000 patents and utility model applications related to their applications were filed annually. These applications spanned a wide range of fields, including electrical engineering, mechanical engineering, transportation, chemistry, medicine, energy, and consumer products. Their presence across nearly all industrial sectors indicates the strong expectations for practical applications.
Historically, the shape memory effect in alloys was first discovered in 1951 in an Au–Cd alloy, and subsequently reported in 1953 in an In–Tl alloy. At that time, however, the effect was regarded merely as a subject of academic interest. It was believed to occur only in limited materials, and there was no movement toward practical application.
Interest in the practical use of the shape memory effect increased significantly after the report of the Ti–Ni shape memory alloy in 1963. Thereafter, the shape memory effect was successively discovered in many non-ferrous and ferrous alloys. Fundamental properties, such as crystal structure, the mechanism of the shape memory effect, phase transformations, and deformation behavior, have since been thoroughly investigated and are now well understood. Nevertheless, from a practical standpoint, no material has yet surpassed Ti–Ni alloys, and most applications continue to rely on them.
Although extensive research has been conducted on Ti–Ni alloys since their discovery, the material remained enigmatic for nearly two decades, and much of the early research was later found to be incorrect. Beginning in the 1980s, research difficulties associated with Ti–Ni alloys gradually began to be resolved. At the same time, as material patents expired, the fundamental materials science properties were largely clarified. As mentioned above, this period also saw a significant increase in patent applications, continuing to the present day.
Familiar practical applications include temperature control components in coffee makers and rice cookers, as well as components requiring precise and rapid control, such as retractable automobile headlights and thermostats. Currently, research on shape memory alloys continues with the aim of achieving higher response speeds and improved safety, and their range of practical applications is expected to expand further.