The Magic of Shape-Memory Alloys

Metals are typically thought of as solid, rigid materials—and this is usually true. However, many metals are soft, malleable, and can be bent easily. Some are even liquid at room temperature, such as gold and mercury. However, while many metals can warp easily, most cannot return to their original shape. For example, it is nearly impossible to reshape a paper clip after stretching it out. What if there was a metal that could reshape itself back to its original form?

Nitinol is known as a shape-memory alloy. It is made up of 50% nickel and 50% titanium atomic units, which translates to about 55% nickel and 45% titanium by mass. Its property of superelasticity allows it to stretch, bend, and warp into various shapes without breaking. It is also corrosion-resistant and biocompatible.

Nitinol is shape-memorable because of its atomic structure. When it is at a cooler temperature, it can be deformed, and when it is heated past its “transformation temperature,” it can return to its original shape without deformities. Nitinol’s atomic structure changes between “martensitic” and “austensitic.” It is martensitic when it is deformed, and it is austensitic when it is heated and returned to its original shape. Nitinol is superelastic above the transformation temperature, and this elasticity causes the martensitic structure to undergo stress and rebound to an austensitic structure.

Nitinol requires a meticulous manufacturing process to have shape-memory. To shape it, a piece of nitinol is held in a mold, heated to 500 degrees Celsius, and fast-cooled. This process sets the nitinol’s shape, and if it is warped under its transformation temperature, then it can return to its original shape when heated.

Nitinol’s biocompatibility gives it numerous medical uses. It is commonly used in orthodontic wires, filters, and bone anchors. Its elasticity allows nitinol stents to fit inside catheters and hold the blood vessel open after the catheter is removed. Outside of medicine, nitinol has aerospace applications: it is used in robotically controlled instruments, landing gear, control panels, and thermal-actuated controls. It is even used in household items, such as eyeglasses and appliances. Further applications are being researched, and nitinol’s uses could be endless.

Sources:

https://www.asme.org/topics-resources/content/metal-that-remembers-its-shape

https://www.proplate.com/nitinol-uses-and-applications-in-the-medical-industry