Muscle Wire

This muscle wire is made of a special alloy of equal parts nickel and titanium, commonly termed a shape memory alloy. It can be bent out of shape and, when heated, it springs back into its original “remembered” shape. 

The shape memory effect happens because of a solid-state phase transformation between two different crystal structures inside the metal wire. A material phase is a bit like its state of matter; either solid, liquid or gas. But some materials can have multiple solid phases, differentiated by how their atoms pack together in structures. The atoms in the shape memory alloy are in a certain structure, called martensite, at room temperature. Now it’s in its original shape, the one that’s about to be “remembered”. 

Most metals bend because the atoms inside can move; they can slide past one another, which involves breaking the bonds between them. The martensite in this special wire accommodates bending in an unusual way; the geometry of its structure changes, but the atoms don’t move and bonds don’t break. When heated, the metal wants to transform into a different solid phase, called austenite, which has a different atomic structure to martensite. But it can’t do this with the object in its deformed shape. The only way for it to transform to a different phase is for the whole shape of the object to return to its original “remembered” shape first. The transformation is fast, which is why shape memory alloys return to their “remembered” shape with a characteristic snap. When the shape cools, the austenite phase becomes martensite again, and the process can be repeated.

Shape memory alloys are used in a wide variety of applications, from actuators and dampeners in the aerospace and automotive sectors, to robotics, prosthetics, photography, surgery and medical devices like vascular stents.