Shape memory alloys (SMAs) are types of smart materials. By definition, smart materials are materials whose properties can react to the changes within their environment. This implies that an external condition, such as pressure, temperature, electricity or light, can change one of the properties of these materials (Roy, 2016). However, the change is reversible. SMA is an alloy that can be folded, twisted or bent into a shape (i.e. get deformed) and return to or regain its original shape when heated above a certain temperature level (Ivanic, Gojic and Kozuh, 2014), as shown in Figure 1 below (GCSEScience.com, 2015). In other words, SMA can “remember” its original shape.
Figure 1: Changes of an SMA
Some characteristics of SMAs are as follows: their properties change with changing conditions such as temperature, pressure, light or electricity; change memory effect; they are highly super elastic; they are lightweight; they have high wear and tear resistance; they have high ductile and fatigue properties; their yield strength is relatively low than that of steel but higher than that of aluminium or steel; electrical resistance that changes with temperature; high resistance to vibration; resistance to acoustic damping.
The composition of SMAs depend on the specific type of metals that make up the alloy. The most common SMAs is Nitinol, which comprises of nickel and titanium. Other compositions of SMAs include: copper-aluminium-nickel (Cu-Al-Ni), silver cadmium (Ag-Cd), Au-Cd, Cu-Sn, copper zinc (Cu-Zn), In-Ti, nickel and aluminium (Ni-Al), Mn-Cu, Fe-Pt, among others. These metals are the ones that determine the properties of a particular SMA.
There are two main categories of SMAs: one way SMA and two way SMA. One way SMAs are those that when they are in their cold state i.e. below temperature at the start of Martensite-to-Austenite transformation (As), they can be stretched or bent while still holding their shape until when they are heated to temperatures exceeding the transition temperature. When they get heated, their shape changes to the original shape. On cooling, they retain their hot temperature shape until they deform again. Therefore the macroscopic shape of these SMAs does not change when they are cooled from the high temperatures. Two way SMA are those that remember two shapes. One of these shapes is when the SMA is at high temperature and the other one is when it is at low temperature. Two way SMAs exhibit shape memory effect when they are being cooled and heated. They can also be manipulated so as to leave some properties they acquire when they get deformed at low temperature during the high temperature phase. But when they get heated beyond a certain temperature level, two way SMAs lose the two way memory effect. When this happens, it is referred to as amnesia.
According to Shuai, Yen-Yu and Xi (2009), the history of SMAs goes back to 1930s. It was in 1932 when Au-Cd alloy’s pseudoelastic behavior was discovered by Olander. Greninger and Mooradian then made observations on how martensitic phase formed and disappeared when temperature of Cu-Zn alloy was decreased and increased. A decade later, there was extensive discussion about the fundamental concept of memory effect that was governed by martensite phase’s thermoelastic behavior. During early 1960s, shape memory effect was discovered in equiatomic nickel and titanium alloy by people working at the U.S. Naval Ordnance Laboratory. This was a major breakthrough in discovery of shape memory materials. The nickel and titanium alloy was given the name Nitinol to mean Nickel-Titanium Naval Ordnance Laboratory. After that, there followed numerous investigations towards understanding the basic behavior of Nitinol and its mechanics. Nitinol, also referred to as NiTi, started being used widely due to its shape memory effect and superelasticity. These two are very new properties in comparison with traditional metal alloys.
There are several production methods used to produce SMAs. These include: vacuum melting, induction melting, vacuum arc melting, plasma arc melting, hot and cold working (which comprises of rolling, forging and wire drawing), electron beam melting, rapid solidification methods such as continuous casting and melt spinning (Ivanic, Gojic and Kozuh, 2014). In general, the key processes of manufacturing SMAs are: casting process, heat treatment process, forming process, and machining process (Markopoulos, Pressas and Manolakos, 2016). After manufacturing, the SMAs go through fabrication where they are welded, soldered, joined, machined and coated/plated.
SMAs have a wide range of applications including industrial (automotive, spacecraft and aircraft, and robotics), civil structures (piping and telecommunication), medicine (optometry, dentistry and essential tremor), crafts and engines.
Limitations on use of smart materials
There are also several factors that limit use of SMAs. Some of these are: response symmetry, response time, functional fatigue, structural fatigue, unintended actuation, high cost, low energy efficiency and limited availability.
The future of SMAs is very promising because these materials are expected to be improved further so as to make them better and increase their applications (Weber, 2010). SMAs have great potential of transforming several industries including manufacturing, robotics, healthcare, etc. These materials are expected to find more applications in production of different products used in industries, homes and offices. It is also expected that researchers of these materials will continue developing strategies of overcoming the limitations of SMAs. Additionally, new types of SMAs are expected to be discovered in the near future (Brown, 2015). For instance, there are several ongoing research and development projects exploring the uses of SMAs. Some of these include use of SMAs to make the following products: amplitie, puddlejumper coat, cooling jacket, adaptable airplane wings, automatic rolling shirt sleeves, opaque glass, disappearing ink, etc. (Cooper, 2013). All these products are made by applying the fact that SMAs are able to learn and change their properties because of surrounding conditions. If properly used, SMAs can improve the performance of almost all present products including robots, automobiles, airplanes, electrical appliances, etc. (Rossiter, 2017). Therefore as the global population continues to increase and natural resources become scarcer, SMAs are expected to play a major role in coping up with scarcity of resources (Busscher, 2015). In general, SMAs are anticipated to improve the future and those who want to build the future must understand them.
Brown, J. (2015). Shape Memory alloys Continue to Improve the Future. Retrieved May 8, 2017,
Busscher, P. (2015). Smart materials: why the future face of manufacturing matters to investors.
Retrieved May 9, 2017, from https://www.cityam.com/209559/smart-materials-why-future-face-manufacturing-matters-investors
Cooper, B.B. (2013). If you want to build the future, you need to understand smart materials.
Retrieved May 8, 2017, from https://www.attendly.com/if-you-want-to-build-the-future-you-need-to-understand-smart-materials/
GCSEScience.com. (2015). Extraction of Metals. Retrieved May 8, 2017, from https://www.gcsescience.com/ex38.htm
Ivanic, K., Gojic, M. and Kozuh, S. (2014). Shape Memory alloys (part II: Classification,
Production and application). Journal of Chemists and Chemical Engineers, Vol. 63, No. 9.
Markopoulos, A.P., Pressas, I. and Manolakos, D. (2016). Materials Forming and Machining Research and Development, pp. 155-180. Cambridge: Woodhead Publishing.
Rossiter, J. (2017). Robotics, Smart Materials, and their Future Impact for Humans. Retrieved May 9, 2017, from https://www.technologyreview.com/s/604097/robotics-smart-materials-and-their-future-impact-for-humans/
Roy, B.N. (2016). Future of Shape Memory alloy and Its Utilization. International Journal of Current Research, Vol. 8, Issue 5, pp. 31646-31651.
Shuai, S., Yen.Yu, L. and Xi, L. (2009). Fundamental Characteristics of Shape Memory Alloys.
Retrieved May 8, 2017, from https://smagroup.blogspot.co.ke/2009/02/fundamental-characteristics-of-shape.html
Weber, A. (2010). Smart Materials Have a Bright Future. Retrieved May 9, 2017, from https://www.assemblymag.com/articles/87695-smart-materials-have-a-bright-future
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