Shape Memory Alloy: A New Frontier in the World of Sensors and Actuators

Published in

IEEE SB KUET

5 min readJan 12, 2021

Have you ever given a closer look at the sunflowers? This fascinating little specimen from your garden faces the sun from the east while rising and follows it to the west till sunset. Then, without any external assistance, it returns to its starting position. Just like this special flower, shape memory alloy (SMA) is also sensitive to sunlight due to its inherent superior thermal-mechanical properties. So what is SMA? SMA is an intermetallic compound that can memorize its predefined shape, which is able to recover during a thermal cycle. Let’s prepare to dive into the realm of SMA that can remember its shape.

Probably you have seen the depiction of an uncoiled spring coiled back and returns to its original shape when it is placed in hot water. Well, this happens because the wire is made of shape memory alloy

This behavior of the SMA happens due to its crystallographic phase transformation. It undergoes a specific type of crystallographic phase transformation which is referred to as the martensitic phase transformation. In low temperatures, the material exhibits martensite to form. Martensite is a soft crystalline structure that can be easily deformed. When a load is applied the martensite transform to detwinned martensite resulting in large deformation. The detwinned martensite phase will remain deformed unless it is heated. While heating, the material from the detwinned martensite structure (α+ phase) transforms into austenite (β phase) which leads to strain recovery, and the material regains its original shape. The Austenite phase has a crystalline structure that cannot be easily deformed. It has a definite fixed long-range crystalline structure. When the heating process is released, the austenitic phase transforms into twinned martensite, without apparent shape change. This phenomenon is also called the thermo-elastic martensitic phase transformation as no additional lattice defect is created during these reversible processes.

Figure 1: Schematic Diagram of Conversion between Martensite and Austenite Molecular Structure | Image edited by Author

This solid-solid phase transformation process can be understood by taking a closer look into solid-liquid transformation. For example, the water turns into ice while cooled down and the ice turns into water while heating. The sole difference is in solid-liquid transformation where the molecules loosen up and their respective position changes. In this solid-solid phase transformation process there will be a change in the molecular arrangement but the position of atoms remains unchanged with respect to their neighboring atoms. Thus no change in chemical concentration occurs. From Figure 1(a) you can see, the transition simply looks like stretching a rhombus or parallelogram-shaped martensite Structure into a square or rectangle-shaped austenite structure and vice versa.

Figure 1(b) illustrates the transition stage between the martensite and austenite phases. At the temperature denoted as A_s, the martensite phase starts to transform into the austenite phase. The transition finishes at the temperature A_f as noted in the curve. Again the transition starts from austenite to martensite at M_s and finishes at M_f. This transformation exhibited by the SMA due to the applied temperature is known as the shape memory effect. As said, martensite can be easily deformed when a force is applied and stays in the deformed shape even after the removal of the force. Thus, it implies that martensite can exist in more than one form as depicted in Figure 2. When force is applied the twinned form of martensite form turns into detwinned form.

Figure 2: Schematic Diagram Twined and Detwinned Martensite Molecular Structure | Image Edited by Author

When heated, any form of martensite can transform into austenite, and upon cooling they return to the shape before deformation. Inventors have exploited this superior thermal-mechanical property of SMA to sense the difference in temperature. This shape-changing capability SMA enables it to act as a sensor and as an actuator concurrently. When the material senses low temperature it will turn into the martensite phase and while the temperature increases up to the start of the austenite phase temperature it will transform into the austenite phase. So without any doubt, the SMA is capable of responding to the solar radiation and will be able to actuate the solar collector towards the sunlight. Integration of these temperature-sensitive materials with photovoltaic or other solar collecting systems can remove the need for a conventional electrical tracker which consumes a significant amount of electricity. Therefore, we should turn our attention to this technology to build a more efficient and promising source of energy.

Moreover, by making temperature differences by employing electric heating or other thermal methods this thermal response of SMA can be exploited beneficially. If programmed cleverly, the sensing and actuation mechanism can operate thermal protection, compensation, and many more. The lightweight, fast response properties are attracting researchers more and more. The most commonly used SMA is Copper-Zinc-Aluminum and Copper-Zinc-Nickle. Besides, SMA also exhibits pseudo-elasticity. The term elasticity refers to the property of reversibility of material to the original shape after the removal of the applied force. But in the case of pseudo-elasticity, the change occurs only at the atomic level. Assume that we have a shape memory alloy that is at a high temperature at this time the material will be in an austenite state. When a force is applied the material deforms into martensite and returns to austenite when the force is removed. With this property shape memorization, the alloy is capable of responding to the fluctuation of mechanical force. This property has already been introduced in the intelligent force sensing system design. Investigating the variation in electrical resistance in an SMA wire opens a new possibility of using them as an actuated sensor. Thus, Integrating SMA enables to ensure more accuracy in static and dynamic force measurement.

Therefore, it is beyond saying that the shape-changing the property of SMA in different conditions has opened many possibilities for sensing and generating motion and force in actuators, fastenings, and couplings by exploiting the shape memory effect. Despite having this superior property SMA exhibits failure issues during continuous usage, which is known as functional fatigue. But the great thing is that functionally graded SMA has been introduced which can be fabricated by additive manufacturing to check this issue.

Shape Memory Alloy: A New Frontier in the World of Sensors and Actuators

Written by Sumiya Afrose