Shape Memory Alloy

Shape memory alloys do exactly as their name would suggest; they’re metallic materials that are able to remember, and return to, their original shape (the shape they are given when made, say as a casting or an extrusion) under the right stimuli. The principle behind this unusual phenomenon comes from two diverse atom arrangements (crystal structures) present in this material – one at high temperatures and one at low temperatures – that are reversible.

At high temperatures, where one of the crystal structures is favoured, the material can be shaped as desired and it retains this geometry as it cools to lower temperatures. Cooled, the material can be bent and twisted beyond all recognition of its previous guise but, on heating, the low temperature atomic arrangement reverts to its previous form and, in so doing, miraculously returns to its previous shape.

Shape memory alloys are widely applied in actuators - mechanical mechanisms that control a system by responding to external stimuli. Moreover, half of all stents (artificial tubes used in the human body to allow fluid to flow through a constriction) are made of shape memory alloys, since they can respond to changes in body temperature, and increase or reduce the diameter of the stent as appropriate.