Three distinct stages are observed on the uniaxial stress-strain curve representing the superelastic behavior of an SMA, schematically shown in Figure 5.
For stresses below , the material behaves in a purely elastic way.
If the material is in the martensitic state and detwinning and twinning of the martensitic variants occur upon loading and unloading, respectively, by reversible movement of twin boundaries, this phenomenon is called effect .
The martensitic transformation possesses well-defined characteristics that distinguish it among other solid state transformations: is an important factor in characterizing shape memory behavior.
When the SMA is heated from the martensitic phase in the absence of stress, the reverse transformation (martensite-to-austenite) begins at the temperature ) is associated with the energy dissipated during the transformation.
In polycrystals, the differences in crystallographical orientation among grains produce different transformation conditions in each grain.
The polycrystalline structure also requires the satisfaction of geometric compatibility conditions at grain boundaries, in addition to compatibility between austenite and the different martensitic variants.
Due to the displacive character of the martensitic transformation, applied stress plays a very important role.
During cooling of the SMA material below temperature in absence of applied stresses, the variants of the martensitic phase arrange themselves in a self-accommodating manner through twinning, resulting in no observable macroscopic shape change (see the stress-temperature diagram shown in Figure 1).
Also, note in Figure 2 that, in going from A to B many variants will start nucleating from the parent phase, while in going from D to E there is only one variant of the parent phase that nucleates from the single remaining martensitic variant indicated by D. Schematic representation of the thermomechanical loading path demonstrating the shape memory effect in an SMA.
The stress-free cooling of austenite produces a complex arrangement of several variants of martensite.
Only during the last step the reverse transformation induced by heating recovers the inelastic strain.
Since martensite variants have been reoriented by stress, the reversion to austenite produces a large transformation strain having the same amplitude but the opposite direction with the inelastic strain, and the SMA returns to its original shape of the austenitic phase. Schematic of a stress-strain-temperature curve showing the shape memory effect. Such loading path in the stress-temperature space is schematically shown in Figure&4;.
At the end of the deformation (indicated by C) and after unloading it is possible that only one martensitic variant remains (indicated by D). The highly symmetric parent austenitic phase (usually with a cubic symmetry) forms only one variant, and thus the original shape (before deformation) is regained (indicated by E).