text: The two most prevalent shape-memory alloys are copper-aluminium-nickel and nickel-titanium (NiTi), but SMAs can also be created by alloying zinc, copper, gold and iron.
  Although iron-based and copper-based SMAs, such as Fe-Mn-Si, Cu-Zn-Al and Cu-Al-Ni, are commercially available and cheaper than NiTi, NiTi-based SMAs are preferable for most applications due to their stability and practicability as well as their superior thermo-mechanical performance. SMAs can exist in two different phases, with three different crystal structures (i.e. twinned martensite, detwinned martensite, and austenite) and six possible transformations. The thermo-mechanic behavior of the SMAs is governed by a phase transformation between the austenite and the martensite.
  ; M is the temperature at which the transition to martensite completes upon cooling. Accordingly, during heating A and A are the temperatures at which the transformation from martensite to austenite starts and finishes. 
  Applying a mechanical load to the martensite leads to a re-orientation of the crystals, referred to as “de-twinning”, which results in a deformation which is not recovered (remembered) after releasing the mechanical load. De-twinning starts at a certain stress σ and ends at σ above which martensite continue exhibiting only elastic behavior (as long as the load is below the yield stress). The memorized deformation from detwinning is recovered after heating to austenite.
  The phase transformation from austenite to martensite can also occur at constant temperature by applying a mechanical load above a certain level. The transformation is reversed when the load is released.
  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 receives its name from steel alloys of a similar structure. It is the reversible diffusionless transition between these two phases that results in special properties. 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 the vertical axis represents the martensite fraction. The difference between the heating transition and the cooling transition gives rise to hysteresis where some of the mechanical energy is lost in the process. The shape of the curve depends on the material properties of the shape-memory alloy, such as the alloy's composition and work hardening.