Nickel-Titanium Shape Memory Effect

Nickel-titanium alloys have this special trick that they remember their original shape when you heat them up. That's due to the transformation between two crystal lattices: when cooling down, the material becomes soft and deformable (martensite), when heating up it snaps back into the hard, original structure (austenite). When welding, you really have to be careful. If you overheat the alloy too much or cool it down too slowly, you can alter the microstructure so that the memory effect is gone. The temperatures at which the transformation happens depend strongly on the exact composition. In practice, this means you have to keep the heat input during welding well under control, otherwise the shape-memory function is ruined. And if you want to bring the parts back into shape after welding, you have to heat them to the right temperature and hold them there for a while so the lattice completely transforms back.