Shape Memory Alloy (SMA) based Actuators

Shape memory effect (SME) is the property of Shape Memory Alloys (SMA) by virtue of which these can recover apparent permanent strains to revert back to its original shape when they are heated above a certain temperature.

SMA's have two stable phases:

• The high temperature phase called austenite (parent phase). Austenite has cubic crystal structure.
• The low temperature martensite phase. Martensite has monoclinic crystal structure

SME occurs because of phase transformation of Martensite to Austenite beyond re-crystallization temperature.

In the absence of any applied load, upon heating an SMA, the Martensite phase simply gets transformed to Austenite. During this phase transformation, though there is change in the microstructure of SMA, macroscopic shape of the SMA does not change. Again, on cooling the SMA, a reverse Martensite transformation takes place; however, the Austenite now gets transformed to twinned Martensite. This transformation is called self-accommodating transformation. Figure 34.1 shows the phase transformations in SMA.

Figure 34.1: Phase tranformation of SMA

Now, upon loading SMA in the Martensite phase, de-twinning of Martensite takes place resulting in large straining of SMA. The strain produced in SMA during complete transformation of twinned Martensite to de-twinned Martensite is known as transformation strain [Λ_t], and is the maximum strain that SMA can recover upon heating. Further loading of SMA causes permanent strain which cannot be recovered. Recovered transformation strain of SMA is directly proportional to the degree of transformation of twinned Martensite to de-twinned Martensite.