How Does Shape-Memory Work

The easiest way to understand shape-memory is to understand that what’s going on inside a material (at the nanoscale of atoms and molecules) can be quite different from what seems to be going on outside.

Stretch the elastic band and, inside, untangle the long, knotted rubber molecules and cut them apart. Remove the stretching force and the molecules will pull back together again. That’s exactly how elasticity functions. The memory of the form is different. Bend an object made of a shape-memory alloy and deform its internal crystalline structure. Let it go, and it stays as it is, bent out of shape permanently. Now apply some heat and the crystalline structure inside shifts to a completely different form, causing the object to return to its original form. Pseudo-elasticity is similar, but you do not need to change the temperature to make the object return to shape after you deform it. If you bend a pair of shape-memory eyeglasses, the stress that you apply makes the titanium alloy from which they are made flip into a completely different crystalline structure; let go, and the crystalline structure returns, so that the glasses return to their original shape.

What happens to shape-memory and pseudo-elasticity is that the internal structure of a solid material shifts back and forth between two very different crystalline forms: In other words, the molecules rearrange them in a totally reversible way. It’s called a solid-state phase change—and it sounds more complicated than it actually is. We are all used to phase changes: Every time you place an ice cube in a drink and watch it melt, you are seeing a change of phase. As the frozen water heats up, its molecules change from being in a tightly packed rigid structure to an arrangement that is much looser and more fluid, so that the water changes 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 solid both before and after transformation, because the molecules remain very close together.

Shape-memory alloys rotate 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 tough material and much more difficult to deform. (Note that this is different from steel, where martensite is the hardest of the two.) Let’s say you have a shape-memory wire and you can bend it into new shapes relatively easily. It is martensite inside, and that’s why it’s easy to deform. No matter how you bend the wire, it’s still in its new' shape; like any ordinary wire, it seems to be undergoing a very ordinary plastic deformation. But now it’s the magic part! Heat it up a little, above its transformation temperature, and it will turn into austenite, with the heat energy you supply to rearrange the atoms inside and turn the wire back into its original shape. Now cool it down and it’s going to return to the martensite, still in its original form. If the material is above its transition temperature all the time, you can deform it, but it will spring back to shape as soon as you release the force you apply.

The surprising (and, for some people, confusing) thing about shape-memory is that the change between austenite and martensite is not a “symmetrical” one. You can take a “programmed” piece of shape-memory wire (one that has a definite shape that you can remember) and bend it in any number of different ways. But, having done that, if you heat up your randomly bent piece of wire, it will always turn back into a single, very definite shape. We can appreciate this by understanding that our material can happily take any number of crystalline forms while it is in a martensite state. But when it’s in the form of austenite, there’s only a crystalline form it can take. It is the most stable form—the one with the lowest state of energy.