A nitinol alloy hardens when heated

Do shape memory alloys (SMA) harden?

We had a piece of this thin shape memory alloy in class (I think it was called Nitinol, but I think that's a brand name). The teacher showed how flexible and bouncy it is and will return to its original shape when put in warm water. Normal metals, when bent back and forth, become hard and then break. Will an SMA break like this? Would the work hardening be reversed by returning it to its original shape?

WetSavannaAnimal aka Rod Vance

These guys use nitinol as an electrical "muscle". A small current flowing through Nitinol causes it to heat up slightly and contract a remarkable amount for the small temperature difference. The shrinkage is used to allow the user to move the focal plane of the instrument axially.

Prathyush Poduval

Go here.

When we change the shape of an SMA, its internal crystal structure rearranges into a different shape. However, when we heat the SMA, it uses the thermal energy to convert its crystalline structure to its original shape, thus regaining its original shape. In the end, you can say that hardening work will be undone if it returns to its original state.


According to your reasoning, you automatically anneal the metal (unhardened if you wish) by changing its shape. Just to put in the keyword that may help relate it to how one normally undoes work hardening.