Shape memory alloys are new materials that have the ability to remember and revert to their original shape at certain temperatures. They are used in a variety of fields, including aerospace, medical, and apparel, especially for parts that expand in tight spaces or products that respond to body temperature.
Antennas that can be packed into a small volume for easy transportation, shirts that unfurl in space, sleeves that automatically roll up when the weather gets hot, and glasses that can be bent back into their original shape even when they’re bent – these are all science fiction stories that are becoming reality. Shape memory alloys make this possible. Shape memory alloys are exactly what they sound like: alloys that remember their shape. It is an alloy that can be shaped into a certain shape and then deformed into a different shape by applying force, but if the temperature is increased, it will return to its original shape. The birth of this seemingly imaginary alloy was quite accidental. In 1960, a researcher at the U.S. Naval Ordnance Laboratory in the U.S. noticed that when a cigarette was lit on a nickel-titanium (Ni) specimen, the specimen began to warp, and further research led to the development of the nickel-titanium (Ni) shape memory alloys that are used today.
The fact that shape memory alloys can return to their original shape despite external deformation is due to the metal’s crystal structure, or atomic arrangement. All metals have an internal structure in which the atoms are arranged in a certain way to form crystals, and these crystals are repeated. Most metals deform when bent, stretched, or heated externally without changing the arrangement of the atoms. Shape memory alloys, on the other hand, have two stable crystal structures that change with temperature, meaning that the arrangement of the atoms changes as the temperature changes. For example, at high temperatures, steel has one of several phases, a face-centered cubic arrangement of atoms called austenite, which changes to a body-centered cubic arrangement called martensite when cooled. Martensite is externally deformable, so you can shape it into a desired shape at this time, and when heated, the shape is memorized in Austenite. From then on, any deformation of the shape only requires an increase in temperature to return it to its original shape.
To better understand how shape memory alloys like this work, you can think of them as living organisms. A shape memory alloy is like an organism that “remembers” its shape under certain conditions, and then “reverts” to its original shape when the conditions change. In other words, it maintains its original shape at high temperatures and temporarily assumes a new shape when subjected to external shock or deformation, before reverting back to its original shape. Shape memory alloys are considered to be “smart materials” that can deform and recover themselves beyond the limits of simple metals, and are being studied to respond not only to temperature but also to various stimuli such as electrical impulses, magnetic fields, and pressure.
Shape memory alloys are based on this principle, and research has led to the discovery of dozens of different alloys, including nickel (Ni), copper (Cu), and iron (Fe). However, they all share two common properties. The first is ‘resilience’. This is the force exerted on the alloy as it returns to its initial shape in response to a change in temperature, and it is so large that the force generated during recovery can be applied mechanically. The second characteristic is ‘repetitive behavior’. After the alloy has been deformed and recovered once, it can be deformed again and recovered to its original shape again. This process can be repeated hundreds of times and still return to its original shape. These properties of resilience and repeatability have made shape memory alloys an essential material in many fields.
In the early days of research, shape memory alloys were used only for space exploration, military, and industrial purposes, but now they are being used in everyday life, and their applications are endless. In space technology, for example, shape memory alloys are used in components such as wings and solar panels, which can be designed to fold up small when a spacecraft is launched and unfold on their own once it enters space. This allows for a large surface area while reducing volume, making transportation more efficient and reducing launch costs.
The fact that the alloy requires temperature changes to recover and deform has many applications in the human body. For example, “memory bra wire” that stretches back to its original shape when it comes into contact with the body during washing, shirts that do not wrinkle and sleeves that adjust to the weather and temperature thanks to the addition of shape memory alloy fibers, etc. In addition, there are many braces that use body temperature to evenly align teeth, and shape memory alloys are also used for medical purposes such as connecting and supporting damaged body parts by placing shape memory alloys in narrow blood vessels and expanding them in the desired area. If the properties of shape memory alloys can be applied to the biological field, the synergistic effects will be enormous. In addition to these applications, it is used as a sensor for automatic temperature control, such as a sensor in sprinklers and heaters, and is also used in areas that require great stability, such as piping seams in submarines and airplanes.
Despite their excellent properties and wide range of applications, shape-molded alloys have their drawbacks. They are difficult to process, difficult to shape, and expensive, which hinders their practical use. To overcome these disadvantages, many researchers are currently working on shape memory alloys using copper, which is relatively cheaper than titanium, and shape memory plastics, which have the same properties as shape memory alloys but are more competitive in terms of price, are on the verge of commercialization. Currently, research is underway in the field of materials engineering to develop more practical shape memory materials. In particular, shape memory properties are not limited to alloys but are being expanded to polymeric materials to combine the advantages of polymers such as lightness, adhesion, and ease of molding, and are being used as medical materials and fibers. In addition, attempts are being made to extend the shape memory property that responds to temperature and change to various stimuli such as magnetic force and acid-base.
Throughout human history, the development of civilization has been driven by the materials and materials used at the time, such as stone, bronze, iron, plastic, and silicon. The development and advancement of materials made computers possible, sent spacecraft into space, and in a broader sense, made our lives possible. In this context, shape memory alloys, a new material, are also a material that will enable a world that was once unimaginable. We are looking forward to the future of shape memory alloys, which have the capacity to create a better world.
