Ni-Ti shape memory alloys (SMA) are extremely interesting materials both for their ability of showing the shape memory effect (SME) and for their elevated strength and ductility. 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 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, the material can be shaped as desired and it retains this geometry as it cools. When cooled, the material can be bent and twisted, but on heating, the low-temperature atomic arrangement reverts to its previous form, miraculously returning the object to its previous shape.