Usually, under external forces, a common metallic material deforms elastically first, then plastic deformation occurs after its yield point, and finally, even if the force is removed, the permanent deformation will be reserved. But for some other alloys, even when a plastic deformation occurs, they can still return to their original shapes after being heated up to a certain temperature. Such a shape recovery phenomenon is called the Shape Memory Effect (SME), which is due to the Martensitic transformation in these alloys. When an alloy with a given shape cools from the Austenite form to the Martensite form, it is easily deformed to a new shape (the restriction is that the deformations must not exceed a certain level), but if the same alloy is heated up to its transformation temperature, it will recover its previous shape due to the reversible reverse transformation.

After being deformed, Martensite can recover its parent shape via reverse transformation. This effect is called the one-way memory effect (Fig. 18.2a). After given proper training, some alloys can memorize to return to not only the parent shape during heating, but also the deformed Martensite shape during re-cooling. This effect is called the two-way memory effect (Fig. 18.2b). The latter can be obtained only after a specific thermomechanical treatment, which is usually called “training”. The amount of shape change that can be obtained by the two-way memory effect is always significantly less than that by the one-way memory effect.

Generally, the shape memory effect can be expressed by the shape recovery ratio (i.e., η). If the initial shape of the alloy in Austenite form is l₀ (expressed as length), the shape of deformed Martensite (e.g., tension) is l₁, and the shape after reverse transformation at high temperature is l₂, the η can be expressed as [8].

Superelasticity, as the name implies, refers to a phenomenon that the alloy can exhibit strain far beyond its elastic limit upon loading, whereas once the stress is removed, the original strain will be returned completely. According to the characteristics of the stress–strain curves, the superelasticity can be classified into two types: linear and non-linear superelasticity. The latter is caused by a stress that occurs during a loading and unloading process, which leads to Martensitic transformation and its reverse transformation at a temperature range above A_f. The former is probably related to the contribution of microtwins to the deformation. The two kinds of superelastic behavior are shown in Fig. 18.3. As for the non-linear superelasticity, when the stress reaches a critical level, the alloy will start to transform into Martensite, accompanied by an increasing strain at constant stress until the alloy is fully transformed into Martensite (see Segment A–B in Fig. 18.3). When the stress is removed, the reverse transformation will occur at a lower stress level (see Segment C–D in Fig. 18.3). The SME described above depends on temperature changes. In contrast, the superelastic effect of shape memory alloys is a kind of isothermal phenomenon and the temperature changes are not necessary. The critical stresses can be adjusted by alloy composition, treatment and temperature. In general, the stress levels increase linearly with increasing temperatures [6]. Reversible strain obtained by the superelastic effect is always up to 8 %, which is 10–20 times higher than the normal elastic strain of conventional metallic materials. As shown in Fig. 18.3, the stress upon loading and unloading does not overlap and shows a hysteresis as well.

The shape memory alloys have a high damping capacity in the Martensite state or two-phase state. The high damping capacity of SMA is related to the hysteretic movement on interfaces (Martensite variants interfaces, twin planes, parent-Martensite interfaces) whereas a contribution of dislocations is not excluded [5].