Materials that remember their shape What are shape memory alloys?

Learn about shape memory alloys, which are materials that remember their shape.

 

Antennas that unfurl in space, shirts that automatically turn up their sleeves when it’s hot, glasses that bend back into their original shape after being bent – these are all science fiction stories that are happening in the real world. Shape memory alloys make this possible. A shape memory alloy is exactly what it sounds like: an alloy that remembers its 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 can be reversed by simply increasing the temperature.
Research on shape memory alloys has led to the discovery of dozens of different alloys based on this principle, 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.
Shape memory alloys, which have the characteristics of ‘resilience’ and ‘repeatability’ that are different from ordinary metals, were used only in space exploration, military, and industrial applications in the early days of research, but now they are showing their properties in the depths of everyday life and have endless applications. There are many applications related to the human body by incorporating the characteristics that require temperature changes to recover and deform the alloy to the body temperature. For example, ‘memory bra wire’, which allows bent bra wires to be stretched back to their original shape by body heat when they come into contact with the human body during washing, and shirts that adjust their sleeves according to the weather and temperature by adding shape memory alloy fibers and do not wrinkle, are making life easier. 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 has 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.