A shape memory alloy (SMA, also known as a smart alloy, memory metal, or muscle wire) is an alloy that "remembers" its shape, and
can be returned to that shape after being deformed, by applying heat to the alloy.
When the shape memory effect is correctly harnessed, this material becomes a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems.


Overview
The three main types of SMA are the
copper-zinc-aluminum-nickel,
copper-aluminium-nickel,
nickel-titanium (NiTi) alloys.


NiTi alloys are generally more expensive and change from austenite to martensite upon cooling;
Mf is the temperature at which the transition to Martensite is finished during cooling.
Accordingly, during heating As and Af are the temperatures at which the transformation from Martensite to Austenite starts and finishes.
Repeated use of the shape memory effect may lead to a shift of the characteristic transformation temperatures (this effect is known as functional fatigue, as it is closely related with a change of microstructural and functional properties of the material).

SMA's can also be used by alloying zinc, copper, gold, and iron.

The transition from the martensite phase to the austenite phase is only dependent on temperature and stress, not time, as most phase changes are, as there is no diffusion involved.

Similarly, the austenite structure gets its name from steel alloys of a similar structure. It is the reversible diffusionless transition between these two phases that allow the special properties to arise.

While martensite can be formed from austenite by rapidly cooling carbon-steel, this process is not reversible, so steel does not have shape memory properties.

In this figure, ξ (T) represents the martensite fraction. The difference between the heating transition and the cooling transition give rise to the shape of the curve depends on the material properties of the shape memory alloy, such as the alloying and work hardening.