Dec 21 2018
A shape memory alloy, sometimes known as a smart alloy, is a type of metal that remembers a predetermined shape at room temperature. When deformed, a shape memory alloy is able to return to its original shape when heat is applied. Shape memory alloys were first developed by Arne Olander in the 1930s but did not start to become popular until the early 1960s.
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Shape memory alloys are very versatile and therefore can be applied in multiple industries, most notably the automotive, aerospace, biomedical, and robotics sectors. This is because the material is lightweight and can be used as an alternative to traditional-style actuators. In bioengineering, shape memory alloys have long been used to mend broken bones and have also been used for hip replacements and dental wires as a result of the shape memory effect.
The most industrially used metallic shape memory material is the Ni-Ti alloy, which is a mixture of nickel and titanium, and was first developed by the US Naval Ordnance Laboratory. However, research has started to move towards Cu-based alloys as well as high-temperature shape memory alloys. Most copper-based alloys are also found to be corrosion-resistant.
The topic of shape memory alloys has been well researched in recent years with a growing interest in the various properties and structures of each metal. Research has shown that smart alloys can be structurally transformed using thermal, mechanical, and magnetic inductions. Due to this, many industries have been finding uses for shape memory and superelasticity.
Shape memory alloys work by transferring between two crystal structure states, a martensite phase and an austenite phase. These phases are determined by the temperature and internal stresses of the material. In conventional shape memory alloys, martensite changes to austenite at higher temperatures and changes back to martensite when temperatures decrease. The metal can be ‘programmed’ to transfer through these states over different temperature ranges depending on the metals used in the alloy. For example, the Ni-Ti alloy system transforms between the temperature range -50°C to 110°C, while the In-Ti alloy works at the much smaller temperature range of 60°C and 100°C. Therefore, it can be said that the choice of the shape memory metal will dictate the composition. It should be noted that shape memory alloys are used with the idea that systems always go back to their preferred lower energy states rather than remaining permanently deformed. In addition to this, it is known that all shape memory alloys have high rates of superelasticity.
Traditionally, the material is trained using a process called thermal cycling. However, according to NASA’s Glenn Research Center, researchers program their shape memory alloys by “reconceptualizing the entire stabilization process”. This means that the smart material undergoes mechanical cycling under a fixed temperature in order to increase the efficiency of the phase transformation process. This fabrication process uses an isobaric response to create a stabilization point that is specific to the space agency’s needs. It also means that the training can be completed in minutes, rather than the conventional method which can sometimes take weeks. NASA believes that this new method, known as the Glenn Method, will result in smart memory alloys being a much more practical solution to a variety of industrial applications, from aerospace engineering to everyday uses such as eyeglass frames.
References and Further Reading
J. Van Humbeeck, R. S. (2002). Shape Memory Alloys, Types and Functionalities. Encyclopedia of Smart Materials.
M Follador, M. C. (2012). A general method for the design and fabrication of shape memory alloy active spring actuators. Smart Materials and Structures, Volume 21, Number 11.
NASA. (n.d.). How to Train Shape Memory Alloys. Retrieved from technology.nasa.gov
University of Wasington. (n.d.). Shape Memory Alloys. Retrieved from depts.washington.edu: https://depts.washington.edu/matseed/mse_resources/Webpage/Memory%20metals/shape_memory_alloys.htm