Shape memory alloys (SMAs) are a unique class of alloys that have ability to 'remember' their shape and are able to return to that shape even after being bent. The shape memory effect (SME) refers to the thermally triggered shape recovery of SMA when the alloy is in the martensitic phase (low temperature phase); however, the superelasticity effect (SE) observed in the austenitic phase (high temperature phase) refers to the ability of mechanically stressed alloys to recover their original shape even when unloaded beyond their linear range.
Typically, SMA is present in 100% martensitic phase, 100% austenitic phase, or a mixture of both phases. A key factor in determining the alloy phase is temperature. Figure 1 [1] shows the stress-strain behavior of SMA in the austenitic (top) and martensitic (bottom) phases. The figure also shows four horizontal lines representing the four transition temperatures for the unique characteristics of each alloy. As shown, at temperatures above the austenitic surface temperature (Af), the alloy is 100% austenitic and therefore exhibits super-elastically (i.e., it recovers its pre-deformed shape upon unloading), while at temperatures below the martensitic surface temperature (Mf), the alloy is 100% martensitic and therefore exhibits inelasticity (i.e., a large residual strain persists upon unloading). The shape memory effect (SME) is a unique phenomenon in which a martensitic alloy can recover its pre-deformed shape after excessive strain by simply heating the alloy to a temperature above Af. If constrained, heating the deformed martensitic alloy will result in the induction of large recovery stresses.
Fig. 1 Stress-strain behavior of SMAs in the austenite and martensite phases
Fig. 2 SMA shape memory effects
Basically, the SMA shape memory effects can be divided into the following three types [2]:
When the SMA is in the martensite phase, it can be deformed and will remain this shape until its temperature is raised above the transition temperature. Upon heating, the SMA transforms into the austenite phase and restores its original shape. The SMA will remain in this shape after cooling until deformed again.
When the SMA is deformed with/without an external force and heated past the transition temperature, the SMA will enter the austenite phase and return to the original shape. When the temperature goes below the transition temperature, the SMA will automatically return to the memorized shape in the martensite phase. This is called the intrinsic two-way effect.
The SMA will automatically shrink when its temperature goes below the transition temperature. When the temperature is lower, it will change to the reverse direction of the deformation. The SMA will restore the memorized shape if the temperature goes above the transition temperature of the austenite phase again.
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In general, according to the composition, there are mainly 3 types of shape memory alloys, thus nitinol, Cu-based and Fe-based SMAs. We mainly focus on the nitinol SMAs.
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