Although it sounds like science fiction, metals that can shape-shift and “remember” their previous forms are used today throughout the automotive, aerospace, biomedical and robotic industries.
Since their uses are so diverse, these metals go by many names, including shape memory alloy (SMA), smart metal, muscle wire and smart alloy. Simply speaking, SMAs are specially formulated mixtures of metal that can re-work themselves back into their original shapes after being deformed. The trick here is to heat up the deformed SMA to its “transition temperature”, which activates the SMAs memory property so that it returns to its prior shape. Of course, the alloy does not actually remember its original shape as a person remembers what they ate for breakfast. Instead, when the SMA is manufactured, the metals are processed in such a way that establishes a "parent shape."
In their 1993 paper “Memory Metal,” George Kauffman and Issac Mayo describe the process for manufacturing one of the most versatile and common SMAs, a combination of nickel and titanium called nitinol. “The metal must be held in position and heated to about 500 ° C. The high temperature causes the atoms to arrange themselves into the most compact and regular pattern possible, resulting in a rigid cubic arrangement known as the austenite phase.” After this phase is established—a process also called training—it is ready to be used.
SMAs can be broken down into two types; either they have one-way memory or a two-way memory. One-way memory indicates that the metal can be shaped and reshaped to hold different positions, but once exposed to its transition temperature, it returns to its parent shape. The two-way memory effect is seen in SMAs that train the metal to remember one shape at a lower temperature and a different shape at a high temperature.
The way they are used varies by industry; SMAs have been included in aerospace designs for quieter engines, shape-memory coupling for oil line and water pipes, the seats of cars, the “muscles” of robots and more. Medical applications of SMAs have been very successful. SMA stents are used in peripheral artery angioplasty procedures to keep arteries open, SMA allows for flexible eyeglass frames and dental braces that exert a constant force on the wearer’s teeth.
If you haven’t seen shape memory alloys in action yet, you probably will. New uses for them are being envisioned every day. From smart phones to robotics to engines, SMAs are a futuristic material that is being used right now.