by Chris Woodford. Last updated: October 14, 2016.
You take it for granted that you can remember all kinds of useful things. You know what you look like, what your name is, and where you live; you can remember famous faces, family birthdays, and maybe you can even speak a foreign language or two. It's much more surprising to discover that inanimate objects can also have a kind of memory. The atoms in a metal teaspoon stay in the same place more or less indefinitely: once a spoon, always a spoon. Bend that spoon in a vise and you can deform it so it turns into an unrecognizable slice of metal. But if it's made of a special kind of material called a shape-memory alloy, it doesn't actually forget that it's spoon shaped, and if you heat it up again it will magically spring back into its original shape! Shape-memory alloys are probably best known as the magic materials behind "indestructible" eyeglasses and underwired bras—but they have all kinds of amazing uses, particularly in medicine and aerospace. How exactly do they work? Let's take a closer look!
Photo: A shape-memory wire "remembers" and returns to its original shape even if you bend and deform it. But it's not simply an elastic piece of metal: there's much more complex science at work. Photo by courtesy of NASA Glenn Research Center (NASA-GRC).
Elastic versus plastic
Before we can understand shape memory, it helps if we know a little about materials and how they behave when we stretch or squeeze them.
A solid object, such as a teaspoon, tends to stay spoon-shaped unless you apply a force to it—and that's no big surprise because it's essentially what "solid" means. Use a big enough force and an object will always change shape or deform; exactly how it changes depends on the material it's made from. Some materials are brittle and won't change shape at all. If your spoon happens to be made of glass or wood and you bend it too much, it'll just snap into pieces.
If it's made of thick rubber (the kind you find in a dog's chewy toy), and you stretch or squeeze it a little bit, it will return to its original shape as soon as you remove the force you're applying. We call behavior like this elasticity: if an object changes shape when a force is applied, but returns to shape when the force is removed, we say it's elastic—and the completely reversible stretching process is an elastic deformation. Often we talk about "elastic" as though it's a material in its own right ("Hey, the elastic has gone in my socks!"), but elastic actually means any artificially made, rubbery material that has elasticity. Even metals are elastic if you don't stretch them very much, especially if they're shaped like springs.
Maybe your spoon is made of plastic? If so, you might be able to bend it so it permanently changes into a new shape. When you stop bending, you'll find the spoon stays in that new shape without going back again. We call this kind of behavior plasticity—and it explains how plastics get their name. A plastic is a chemical made of long molecules (polymers) that's simple to shape during manufacture (and sometimes afterward), because the molecules can slide past one another very easily. Typically plastics are made in chemical plants as hot liquids, molded into shape, and then allowed to cool so they harden (set). But, just like "elastic," plastic really refers to how something behaves and not what it's made from. A lot of soft metals are actually plastic, in this sense, because you can bend them back and forth without them returning to shape. That kind of nonreversible change of shape is called a plastic deformation.
Photo: Top: Elasticity: If you stretch a rubber band and release it, providing the stretching force isn't too great, it returns exactly to its original shape and size, like the unstretched rubber band on my palm. Bottom: Plasticity: If you bend a paper clip with even a small force, it's permanently deformed and stays in its new shape unless you push it back again. Most soft metals are actually "plastic" like this.
What is shape memory?
Materials science is all about choosing the best possible material to do a particular job. If you're designing a jet engine, for example, you choose strong materials that are extremely light and can cope with high temperatures. You might pick aluminum, titanium, or a metal alloy. But what if you want to make an airplane component that behaves in one way at low temperatures and in a different way when it heats up? That's the kind of situation where you might use a shape-memory alloy, which can automatically reshape itself as the temperature changes.
Ordinary metal objects have no memory of their shape. If you sit on a pair of aluminum eyeglass frames and bend them permanently (in scientific words, "subject them to a plastic deformation"), it's tricky to get them back exactly how they were. You have to use your own memory of what the frames were originally like and laboriously twist and bend; even then, there's no guarantee the frames will look like they used to and they may even break entirely—through fatigue—if you wiggle them back and forth too much.
Shape-memory materials behave differently. They're strong, lightweight alloys (generally, mixtures of two or metals) with a very special property. They can be "programmed" to remember their original shape, so if you bend or squeeze them you can get that original shape back again just by heating them. This is called the shape-memory effect (or thermal shape-memory effect, since heat energy makes it happen). Some shape-memory alloys remember one shape when they're hot and a different one when they're cold, so if you cool them they spring into one shape and if you heat them they "forget" that shape and flex into a different one. This is known as the two-way shape-memory effect. Now what if you could make a shape-memory object that would bend and twist by a huge amount but still return perfectly to its original shape, even without heating? That's an aspect of shape memory called pseudo-elasticity or superelasticity and it's used in those super-bendy, virtually indestructible eyeglass frames, which manufacturers claim are at least 10 times more flexible than steel!
Although nitinol (also called nickel-titanium, Ni-Ti) is perhaps the best-known shape-memory alloy, there are lots more, including alloys made from copper, zinc, and aluminum (Cu-Zn-Al); copper, aluminum, and nickel (Cu-Al-Ni); iron, manganese, and silicon (Fe-Mn-Si); and quite a few others. You'll also find shape-memory alloys referred to under brand names such as FLEXINOL® and Muscle Wires®, which are proprietary nickel-titanium alloys made by Dynalloy.
Artwork: Comparing shape-memory effects. Top: The one-way shape-memory effect. 1) We "program" a wire by bending it into a specific shape at a high temperature. 2) Once it's cooled down, we can bend it into a different form. 3) We can bend it into any number of other shapes. 4) If we heat it above a critical temperature, it automatically springs back to its originally remembered shape. 5) If we cool it down, it stays in that shape. Bottom: The two-way shape-memory effect is simpler to understand: the material exists in one shape above a critical temperature and in a different shape below it. It flips back and forth between these two shapes as the temperature changes.
How does shape memory work?
The easiest way to understand shape memory is to remember that what's happening inside a material (at the nanoscale of atoms and molecules) may be quite different from what seems to be happening on the outside.
Stretch an elastic band and, inside it, the long, knotted rubber molecules untangle and tease apart. Remove the stretching force and the molecules pull back together again. That's essentially how elasticity works. Shape memory is different. Bend an object made from shape-memory alloy and you deform its internal crystalline structure. Let it go and it stays as it is, permanently bent out of shape. Now apply some heat and the crystalline structure inside changes into an entirely different form, prompting the object to revert back to its original shape. Pseudo-elasticity is similar, but no temperature change is needed to make the object return to shape after you deform it. If you bend a pair of shape-memory eyeglasses, the stress you apply makes the titanium alloy from which they're made flip into an entirely different crystalline structure; let go and the crystalline structure reverts back again, so the glasses spring back to their original shape.
What's happening with shape-memory and pseudo-elasticity is that the internal structure of a solid material is changing back and forth between two very different crystalline forms: in other words, its molecules are rearranging themselves in a completely reversible way. This is called a solid-state phase change—and it sounds more complex than it really is. We're all used to phase changes: every time you put an ice cube in a drink and watch it melt, you're watching a phase change. As the frozen water heats up, its molecules change from being in a tightly packed rigid structure into an arrangement that's much looser and more fluid, so the water transforms from its solid phase (ice) to its liquid phase (ordinary liquid water). A broadly similar thing happens in a solid-state phase change, it's just that the material is a solid both before and after the transformation because the molecules remain very close together throughout.
Shape-memory alloys flip back and forth between two solid crystalline states called austenite and martensite. At lower temperatures, they take the form of martensite, which is relatively soft, plastic, and easy to shape; at a (very specific) higher temperature, they transform into austenite, which is a harder material and much more difficult to deform. Let's say you have a shape-memory wire and you can bend it into new shapes relatively easily. Inside, it's martensite and that's why it's easy to deform. No matter how you bend the wire, it stays in its new shape; much like any ordinary wire, it seems to be undergoing a very ordinary plastic deformation. But now for the magic part! Heat it up a little, above its transformation temperature, and it will change into austenite, with the heat energy you supply rearranging the atoms inside and turning the wire back into its original shape. Now cool it down and it will revert back to martensite, still in its original shape. If the material is above its transition temperature the whole time, you can deform it but it will spring back to shape as soon as you release the force you're applying.
The surprising (and, for some people, confusing) thing about shape memory is that the change between austenite and martensite isn't a "symmetrical" one. You can take a "programmed" piece of shape-memory wire (one that has a definite shape that it will remember) and bend it in any number of different ways. But, having done that, if you then heat up your randomly bent piece of wire, it will always flip back to a single, very definite shape. We can understand this by realizing that our material can happily take any number of crystalline forms when it's in the martensite state. But when it's in the form of austenite, there is only crystalline form it can take. This is the most stable form—the one with the lowest energy state.
Photo: A shape-memory alloy wire in its original form (above) has the internal, crystalline structure of martensite: it's relatively soft and you can easily pull it into a different shape—to make something like the word NASA (below). Heat it up and it changes into austenite, springing back to its original shape in a matter of seconds. When it cools, it remains in the same shape (so externally it's unchanged), but internally the crystalline structure reverts back to martensite. Photos courtesy of NASA Glenn Research Center (NASA-GRC).
Pseudo-elasticity happens in a similar way when you apply force (stress) instead of heat energy. Let's say you have your shape-memory eyeglasses. Normally, the alloy from which they're made is in the tough form of austenite. Apply a force to your glasses (bend them, in other words) and the austenite changes to martensite, which deforms easily. Let go of the frames and the martensite changes back to austenite, so your glasses flex back to their original shape.
What are shape-memory alloys used for?
The shape-memory effect was discovered in a gold-cadmium alloy by Arne Olander in the 1930s, but practical shape-memory alloys (also called SMAs, muscle wires, memory metals, and smart metals) only started to become popular in the early 1960s after the development of nitinol at the US Naval Ordnance Laboratory (nitinol actually stands for Nickel Titanium Naval Ordnance Laboratory). Several decades later, shape-memory alloys are a popular choice for all kinds of medical and health-related equipment, including everything from dental implants to surgical tools and underwired bras to eyeglass frames (sold under brand names such as Flexon). Unlike plastics, metals, and traditional alloys, shape-memory alloys are both strong and flexible, easy to sterilize, and corrosion-resistant too. Being lightweight, tough, and capable of operating at high temperatures, they're also widely used in aerospace components in such things as space rockets and space probes.
Robotics is another rapidly growing application. Sometimes we need to design unusual robots to reach places that ordinary robots can't get to: they might need to blast into space in a super-compact space rocket, or perhaps they need to sneak under doorways to spy on criminals. With that in mind, engineers are now designing self-unfolding robots made from shape-memory materials. They start off folded flat; when they need to be activated, an electric current shoots through their shape-memory parts, heating them just enough to make them pop out into their "preprogrammed," permanent shape.
Photo: The Hubble Space Telescope (HST) used arms made of a shape-memory alloy to release its solar panels automatically once it reached space. During its launch, the telescope was packed inside the Space Shuttle at a relatively low temperature, so the shape-memory arms were bent inward and the solar panels remained safely folded up. Once in space, and clear of the Shuttle, the Sun's rays rapidly warmed up the craft, heating the shape-memory arms above a critical temperature (known as the transformation temperature) so they sprung back to their original shape, causing the solar panels to fold out automatically. Photo by courtesy of NASA on the Commons.
Photo: A piece of shape-memory polymer (SMP) foam developed for medical applications at Lawrence Livermore National Laboratory. Photo by courtesy of US Department of Energy (DOE).
Shape-memory alloys sound brilliant, but they do have some drawbacks: they fatigue (break after repeated deformations) much more readily than ordinary stainless steel and they're notably more expensive to manufacture than traditional steel or aluminum alloys. In the 1990s, materials scientists started developing inexpensive shape-memory polymers (SMPs) (plastics) with a shape-memory effect similar to shape-memory alloys. Just as ordinary plastics revolutionized the world of materials, so shape-memory polymers are likely to expand hugely the list of applications for shape memory in coming years, because they're lighter, cheaper, and more flexible than metal-based alloys. Closely related to SMPs are shape-changing polymers (SCPs), which change shape more gradually when they're heated (or energetically stimulated in some other way) and return to shape when cooled. Although self-healing materials (ones that repair themselves after damage) can work in various different ways, some of them are very similar to shape-memory polymers. For example, it's easy to imagine a plastic airplane fuselage that absorbs the kinetic energy from an incoming bullet, converts it into heat, and uses that to activate a shape-memory effect that makes the polymer revert to its original shape, promptly healing and sealing the damage (NASA scientists have been developing materials like this for well over a decade).