In this blog post, we will look at the principles and characteristics of shape memory alloys and introduce examples of their use in everyday life and cutting-edge technology.
They are compact and easy to carry. Antennas that spread out in space, shirts that automatically raise their sleeves when it’s hot, glasses that return to their original shape even if they are bent. Stories like these, which could only have been seen in science fiction movies, are now becoming a reality. This is made possible by shape memory alloy. As the name suggests, it is an alloy that remembers its shape. It is an alloy that is given a certain shape and then deformed into a different shape by applying force, but when the temperature is raised, it returns to its original shape. The birth of this alloy, which seems to exist only in the imagination, was made possible by a very accidental event. In 1960, a researcher at the U.S. Naval Weapons Laboratory noticed that a nickel-titanium specimen began to move when a cigarette was placed on it during an experiment. This discovery led to further research, which resulted in the development of the nickel-titanium shape memory alloy that is commonly used today.
The shape memory alloy can return to its original shape despite external deformation due to the crystal structure of the metal, that is, the atomic arrangement. All metals have an internal structure in which atoms are arranged in a regular pattern to form crystals, and these crystals are repeated. At this time, most metals undergo deformation without changing the arrangement of atoms when bent, stretched, or exposed to heat from the outside. On the other hand, shape memory alloys have two stable crystal structures that change with temperature, so the arrangement of atoms changes when the temperature changes. For example, at high temperatures, steel has an atomic arrangement of the face-centered cubic structure called austenite among various phases, but when cooled, it changes to an atomic arrangement of the body-centered cubic structure called martensite. Martensite can be deformed externally, so if you create the desired shape at this time and heat it, the shape will be remembered in austenite. After that, even if the shape is deformed, it will return to its original shape by simply increasing the temperature.
To make it easier to understand the working principle of shape memory alloy, it can be likened to a living organism. Shape memory alloy is like a living organism that “remembers its body” under certain conditions and “recovers” to its original state when the situation changes. In other words, it retains its original shape at high temperatures and temporarily takes on a new shape when subjected to external impact or deformation, but then returns to its original state. Shape memory alloys are considered “smart materials” that can repeat deformation and recovery beyond the limits of simple metals, and they are being studied to respond to various stimuli such as electrical stimulation, magnetic fields, and pressure, in addition to temperature.
Using this principle, shape memory alloys have been created, and through research, dozens of alloy systems have been discovered, including nickel-based (Ni), copper-based (Cu), and iron-based (Fe) alloys. However, all of them have two common characteristics. The first characteristic is “resilience.” Resilience refers to the force exerted on the alloy when it returns to its original shape due to changes in temperature. The magnitude of this force is so large that it can be mechanically applied to recover. The second characteristic is “repetitive motion.” Even after deformation and recovery have been applied to the alloy once, it will return to its original shape if it is deformed again. This process has the property of returning to its original shape even if it is repeated hundreds of times. Based on these properties of resilience and repeatability, shape memory alloy has become an essential material in various fields.
Shape memory alloy, which has characteristics of resilience and repeatability different from those of general metals, was initially used only for space exploration, military, and industrial purposes, but now its properties are being demonstrated in the depths of our daily lives, and its applications are endless. For example, in the field of space technology, shape memory alloys can be used in parts such as wings and solar panels, which can be designed to fold up into a compact shape when the spacecraft is launched and then unfold themselves once they enter space. This allows for a large surface area while reducing the volume, which increases transportation efficiency and reduces launch costs.
There are many applications related to the human body by incorporating the characteristics that require temperature changes in the recovery and deformation of alloys into body temperature. “Memory bra wires” that straighten out when they come into contact with the human body during washing, and shirts with sleeves that adjust to the weather and temperature and do not wrinkle thanks to shape memory alloy fibers are making our lives more convenient. In addition, orthodontic braces that use body temperature to evenly align teeth are widely used, and shape memory alloys that are clumped together in narrow blood vessels are used for medical purposes such as connecting and supporting damaged body parts by spreading them out in the desired areas. If the characteristics of shape memory alloys are combined with the bio field, the synergy effect will be enormous. In addition to these applications, shape memory alloys are also used as temperature-automatic-adjustment sensors, such as sprinklers and heating sensors, by taking advantage of their sensitivity to temperature changes, and are also used in areas that require a high degree of stability, such as the pipe joints of submarines and aircraft.
Shape memory alloys have excellent properties and a wide range of applications, but of course, they have their drawbacks. The processing of shape memory alloys is difficult, and the shape is difficult to make, and the high price is an obstacle to their practical application. A lot of research is still being conducted to overcome these shortcomings, and as a result, shape memory alloys using copper, which is relatively cheaper than titanium, and shape memory plastics with the same characteristics as shape memory alloys but more competitive in terms of price, are on the verge of commercialization. Currently, research is in full swing to develop more practical shape memory materials in the field of materials engineering. In particular, the shape memory characteristics are not limited to alloys but are also being expanded to polymer materials to combine with the advantages of polymers, such as lightness, adhesiveness, and ease of molding, and are being used in various ways as medical materials and fibers. In addition, there are ongoing attempts to expand the shape memory characteristics that respond to temperature and changes to various stimuli such as magnetic force and acid-base properties.
Looking at the history of mankind, the development of civilization was made possible by the materials and materials that were mainly used in those days, such as stoneware, bronze, ironware, plastic, and silicon. The development and advancement of materials has made computers possible, made it possible to launch spacecraft into space, and, broadly speaking, has made our lives possible today. In this context, shape memory alloy, which can be considered a new material, is also a material that will make a world that was unimaginable in the past possible. We look forward to the future that will be brought about by the development and application of shape memory alloy, which has the capacity to make a better world.
