What’s so special about this spring? It’s made of nickel titanium, which is a shape memory alloy. Shape memory alloys are unique because they can remember their original shape after they are deformed. This is possible because shape memory materials have two crystalline structures. Martensite is the structure at lower temperatures and Austenite is the structure at higher temperatures. When a temperature change or mechanical stress occurs, the structure changes from Martensite to Austenite or the other way around. When we heat this wire by dropping it in hot water, it returns to a coil, as if it remembers its original shape..