A shape-memory alloy is exactly what it sounds like: an alloy of two (or more) metals that somehow can “remember” the original shape it was folded into. One of the more famous examples of this is nickel-titanium, or nitinol, will spontaneously fold from a crumpled state back to the ordered, cold forged state when heated. A video of this process can be seen here. This works because of a small phase change in the metal itself, when shaped the atoms arrange themselves into organized crystal structures. Distorting the metal then causes these crystal structures to become disorganized and energetically unfavourable, application of heat then allows the original crystal structure to be formed again by overcoming the energy barrier. The special thing about SMA’s is that the crystal structures can be reversed while in most alloys the structures naturally decay due to diffusion of atoms within the metal.
Shape-memory alloys have many applications, ranging from uses in medicine and robotics right through to the more novel, as seen in this lamp designed by Japanese design group Nendo. In this case the heat from the bulb causes the lamp to “bloom” as the strips of alloy move back to their preformed shape.