In this blog post, we’ll explore the principles behind shape-memory alloys—which return to their original shape in response to temperature changes—as well as the characteristics of Nitinol, a representative material, and its various applications and limitations.
Generally, metals have an elastic limit—that is, a limit beyond which they cannot return to their original shape through elastic deformation; if deformed by a force exceeding this limit, they will not return to their original form. However, there are metals that, when treated to “remember” a specific shape within a certain temperature range, exhibit the property of reverting to that remembered shape once they return to that temperature range—even if they undergo significant deformation at temperatures higher or lower than that range. These are shape-memory alloys. Shape-memory alloys do not simply “imprint” a shape onto the metal; rather, they recover their original shape by utilizing the property where their crystal structure reversibly changes in response to temperature or stress.
Let’s first examine the shape memory effect, which is the most important characteristic of shape memory alloys. For example, suppose we program an alloy to remember the shape of a fully bloomed flower. At a low temperature, we can bend this alloy by hand into the shape of a flower bud. Then, if we place this bud in water at a temperature higher than the one at which the memorized shape appears, the alloy’s temperature rises, causing it to gradually unfold and return to its original flower shape. This phenomenon can be repeated as long as the material conditions and deformation range are appropriate. This phenomenon is called the shape memory effect, and metals that exhibit this characteristic are called shape memory alloys. In South Korea, when people hear the term “shape memory alloy,” many probably think first of the women’s underwear that once appeared in advertisements. This is because the so-called “memory bra,” made with shape-memory alloy material, was heavily promoted through advertisements and garnered significant public attention. The memory bra utilized the principle of using shape-memory alloy in the underwire instead of plastic or ordinary metal, ensuring that even if the underwire bent during washing, it would return to its original shape when exposed to a temperature close to body temperature. So, how does shape-memory alloy—which has become widely known to the public as a material for women’s underwear—remember its original shape?
Research into the shape-memory phenomenon has a long history, with the early work attributed to Arne Ölander of Sweden, who observed temperature-dependent martensitic transformations in a gold-cadmium alloy in 1932. Subsequently, various researchers confirmed similar phenomena in different alloys, and Nitinol—an alloy composed of nickel and titanium—began to attract attention for its practical shape-memory properties. In particular, researchers at the U.S. Naval Ordnance Laboratory discovered unique shape memory properties in an alloy containing nearly equal proportions of nickel and titanium; this alloy subsequently became known as “Nitinol,” a name derived from the elements nickel and titanium and the name of the U.S. Naval Ordnance Laboratory. Nitinol possesses excellent shape recovery properties, high corrosion resistance, biocompatibility, and superelasticity, and remains the most widely used shape memory alloy to this day.
The principle behind these shape memory alloys is closely related to their crystalline structure. A crystalline structure refers to the arrangement of atoms—which make up a material—in space according to specific rules to form a crystal. Nitinol generally forms a B2-type austenite crystalline structure at high temperatures and transforms into a B19′-type martensite crystalline structure upon cooling. In some cases, an intermediate structure known as the R phase may appear during this process.
While the martensitic transformation in typical metals can result in permanent deformation as the crystal structure changes, in Nitinol, the transformation between austenite and martensite can occur reversibly in response to changes in temperature and stress. In particular, in the low-temperature martensitic state, the material can be deformed relatively easily by external forces; when heat is applied to Nitinol deformed in this state, the martensite transforms back into austenite, restoring the originally “memorized” shape. In other words, the shape memory effect of shape memory alloys is a phenomenon that occurs because reversible martensitic transformations take place between different stable crystal structures in response to temperature changes.
Although shape memory alloys were developed through research into military materials, they are now utilized in a wide variety of fields. One of the most representative applications of shape memory alloys is eyeglass frames. Eyeglass frames made from shape memory alloys can recover their original shape under certain conditions when bent by an external force, and have thus been used to create products that are resistant to impact. Second, they are used in structures and actuators in the aerospace sector, such as antennas. Shape-memory alloys can be designed to be folded or deformed into a compact form during launch and then unfold into a preset shape once they reach the desired temperature in space or are electrically heated; this makes them highly suitable for the aerospace industry, where reducing volume and weight is essential. Third are medical devices. In particular, the shape memory effect and superelastic properties of Nitinol are utilized in stents, guidewires, and orthodontic wires. For example, a stent can be designed to be compressed into a small size when inserted into a blood vessel and then expand to its original shape in response to body temperature and the surrounding environment. Orthodontic wires can also take advantage of nitinol’s ability to continuously apply a constant force. In addition, shape-memory alloys are used in mechanical components such as pipe fittings, valves, and actuators, where the force generated during shape recovery is utilized to tighten or move parts.
Currently, research and development is underway on a variety of alloy systems that exhibit the shape memory effect, including nitinol, copper-based alloys, and iron-based alloys. In particular, nickel-titanium shape memory alloys are widely used in industrial and medical fields due to their excellent shape memory effect, superelasticity, corrosion resistance, and biocompatibility. However, shape memory alloys also have drawbacks. First, their properties change in response to variations in temperature and stress; repeated deformation and shape recovery can lead to fatigue accumulation, which may degrade their shape recovery capability or cause material damage. Furthermore, since the transformation temperature and mechanical properties vary significantly depending on the alloy composition, heat treatment, and processing methods, precise manufacturing and processing are required to reliably achieve the desired performance. In the case of Nitinol, because it contains nickel, considerations regarding surface treatment, biocompatibility, and reliability during long-term use must be taken into account even when used for medical applications. Above all, nickel-titanium-based shape-memory alloys have the disadvantage of being more difficult to manufacture and process than general metal materials, and they tend to be relatively expensive. Therefore, research is ongoing to develop shape-memory alloys that are more affordable yet offer stable performance. In addition to shape memory alloys, active research is being conducted on new shape memory materials such as shape memory polymers. Research is also underway on composite materials that combine the properties of shape memory alloys and polymers, as well as manufacturing technologies for shape memory materials using 3D printing.
What will our daily lives look like in the future with the widespread use of shape-memory alloys? In the past, when a car was involved in a minor collision and the bumper or body was dented, parts had to be replaced or sheet metal work performed. However, in the future, components designed to utilize the properties of shape-memory alloys—such as those that can self-repair damaged areas or return to their original shape through simple heat treatment or an activation signal—are likely to emerge. Of course, the approach of constructing an entire automobile body from shape-memory alloys so that it returns to its original form when heat is applied has not yet been widely commercialized; several technical challenges remain for practical application, including cost, cyclic fatigue, required resilience, and safety. Nevertheless, by utilizing the deformability and phase-transition characteristics of shape-memory alloys in response to temperature and stress, new applications can be expected in robotic actuators, aerospace structures, precision machine parts, and various medical devices. In particular, recent research has been focusing on utilizing shape-memory alloys not merely as “metals that return to their original shape,” but as smart materials that move autonomously in response to electrical heating or external stress. As materials engineering continues to advance, shape memory alloys will likely be utilized in far more diverse ways than we can currently imagine—in everyday machinery, medical devices, robots, and space technology.
