Shape Memory

When a metal like Nitinol undergoes a martensitic to austenitic phase transition, its crystal structure shifts from a monoclinic (martensite) to a cubic B2 structure (austenite) without diffusion.

This occurs over a temperature range defined by the austenite start (As) and finish (Af) temperatures—typically between 50–100 °C depending on alloy composition.

In the martensitic state, the material can be easily deformed due to twin boundaries in the lattice.

When heated above Af, the structure reverts to the original austenitic form, erasing the deformation.

This solid-state phase shift is highly strain-reversible and occurs in milliseconds, driven by thermal energy overcoming internal stress barriers, making it ideal for cyclic use in actuators and thermal triggers.