In this blog post, we’ll explore the principles and characteristics of shape-memory alloys and introduce examples of their applications, ranging from everyday life to cutting-edge technology.
An antenna that folds into a compact size for easy transport but unfolds fully in space; a shirt whose sleeves automatically roll up when it gets hot; glasses that return to their original shape even after being bent—these scenarios, which once seemed like something straight out of a science fiction movie, are now a reality. What makes this possible is the “shape-memory alloy.” As the name suggests, “shape-memory alloys” are alloys that remember their shape. Even after an alloy is formed into a specific shape and then deformed into a different shape by applying force, it returns to its original shape when heated. The creation of this alloy—which seems like something that would exist only in the imagination—came about quite by accident. In 1960, during an experiment at the U.S. Naval Weapons Laboratory, a researcher noticed that a nickel (Ni)-titanium (Ti) specimen began to wriggle when a cigarette lighter was held against it. Further research into this phenomenon led to the development of the nickel (Ni)-titanium (Ti) shape memory alloys that are primarily used today.
The ability of shape memory alloys to return to their original shape despite external deformation is due to the metal’s crystal structure—that is, the arrangement of its atoms. All metals consist of atoms arranged in a regular pattern to form crystals, and these crystals have a repeating internal structure. In most metals, when bent, stretched, or heated from the outside, deformation occurs without changing the arrangement of the atoms. In contrast, shape-memory alloys possess two stable crystal structures that change with temperature; consequently, when the temperature changes, the atomic arrangement itself changes. For example, at high temperatures, steel has a face-centered cubic (fcc) atomic arrangement—known as austenite—among its various phases, but when cooled, it changes to a body-centered cubic (bcc) atomic arrangement known as martensite. Since martensite can be deformed externally, it is possible to shape it as desired; when heated, the alloy “remembers” that shape in its austenite state. Afterward, even if the shape is altered, it will return to its original form simply by raising the temperature.
To better understand the operating principle of shape-memory alloys, we can liken them to a living organism. Shape-memory alloys are like living organisms that “remember their form” under specific conditions and “recover” to their original state when circumstances change. In other words, they maintain their original shape at high temperatures, and when subjected to external impacts or deformation, they temporarily assume a new shape but eventually return to their original state. As such, shape-memory alloys are considered “smart materials” that transcend the limitations of simple metals by repeatedly deforming and recovering on their own, and research is underway to make them respond not only to temperature but also to various stimuli such as electrical impulses, magnetic fields, and pressure.
Based on this principle, dozens of alloy systems—including nickel-based (Ni), copper-based (Cu), and iron-based (Fe)—have been discovered through ongoing research. However, they all share two common characteristics. The first is “recovery force.” Recovery force refers to the force exerted on the alloy as it returns to its original shape in response to temperature changes; because this force is significant, it can be mechanically harnessed. The second characteristic is “repeatability.” Even after the alloy has undergone one cycle of deformation and recovery, applying deformation to it again causes it to return to its original shape. This process exhibits repeatability, allowing the alloy to return to its original form even after hundreds of cycles. Based on these properties of recovery force and repeatability, shape memory alloys have established themselves as essential materials in various fields.
Shape memory alloys, which possess the unique properties of “recovery” and “repeatability” unlike ordinary metals, were initially used only in space exploration, military, and industrial applications. However, they now play a significant role in many aspects of daily life, and their applications are virtually limitless. For example, in the field of space technology, shape memory alloys are used in components such as wings and solar panels; they can be designed to remain folded into a compact state during spacecraft launch and then unfold automatically upon entering space. This allows for a larger surface area while reducing bulk, thereby improving transport efficiency and lowering launch costs.
By applying the alloy’s property—which requires temperature changes for recovery and deformation—to body heat, there are numerous applications related to the human body. Examples include “memory bra wires,” where bent bra wires straighten back to their original shape upon contact with body heat during washing, and wrinkle-free shirts with shape-memory alloy fibers that adjust sleeve length based on weather and temperature, all of which make daily life more convenient. In addition, orthodontic appliances that use body heat to evenly align teeth are widely used, and shape-memory alloys are inserted into narrow blood vessels to expand at the desired location, serving medical purposes such as connecting or supporting damaged body parts. If the properties of shape-memory alloys were integrated with the biotechnology field, the synergistic effects would be immense. In addition to these applications, their sensitivity to temperature changes makes them useful as automatic temperature-control sensors in sprinklers and heaters, and they are also used in fields requiring high stability, such as pipe joints in submarines and aircraft.
Although shape-memory alloys possess excellent properties and have a wide range of applications, they do, of course, have drawbacks. They are difficult to process, challenging to shape, and expensive, which poses obstacles to their practical application. To overcome these drawbacks, extensive research is currently underway. As a result, shape-memory alloys made from copper—which is relatively less expensive than titanium—and shape-memory plastics, which possess the same properties as shape-memory alloys but are more cost-competitive, are on the verge of commercialization. Currently, research is in full swing in the field of materials engineering to develop more practical shape-memory materials. In particular, shape-memory properties are being expanded beyond alloys to include polymer materials, combining them with the advantages of polymers—such as lightness, adhesion, and ease of molding—to enable diverse applications in medical materials and textiles. Furthermore, efforts are ongoing to expand shape memory properties—which respond to temperature changes—to include various stimuli such as magnetic forces and acid-base reactions.
Looking back at human history, the development of civilization has been driven by the materials and substances predominantly used in each era, such as stone, bronze, iron, plastic, and silicon. The development and advancement of materials made computers possible, enabled spacecraft to travel into space, and, in a broader sense, made our current way of life possible. Viewed in this context, shape-memory alloys—which can be considered new materials—are also materials that will make a world once unimaginable a reality. We look forward to a future that takes another step forward, brought about by the development and application of shape-memory alloys, which possess sufficient potential to create a better world.
