Shape-Memory Alloy

Bend a wire or a coat hanger hard with your hands, and it stays bent. That's because when ordinary metal takes a strong force, its atoms (the basic particles that make up matter) slide into new positions for good.

But eyeglass frames made from a shape-memory alloy are hard to break, even if you accidentally sit on them and squash them flat. The moment you let go, they pop back to their first shape, as if time had been rewound. Most frames like this use a property called “superelasticity,” which is built to snap back at room temperature. It's the same underlying principle as the shape-memory effect, which needs heat, just in a different form.

Cool a shape-memory wire and bend it, and it stays bent. Drop it in warm water, and it straightens out instantly. The metal wasn't broken. It was just waiting for the temperature to change. This ability to return to its original shape under the right conditions, even after being deformed, is called the shape-memory effect.

The secret behind this odd behavior is a phase transformation of the crystal structure, in which the whole arrangement of atoms inside the metal changes with temperature.

When it's cold, a shape-memory alloy settles into a soft state called “martensite,” where the atoms are linked in a slanted, loose pattern. In this state, pushing on the metal doesn't snap the bonds between atoms. Instead, the layers tilt and slide flexibly, so the shape bends out of place easily.

Now add heat and raise the temperature. All at once, the atoms switch into a firm, neat, grid-like state called “austenite.” The tilted atoms spring back, without a hair's breadth of error, to the regular, high-temperature positions they remembered from the start. And the whole piece of metal returns forcefully to its first shape.

The unusual properties of shape-memory alloys are put to work in everyday life, and also in space exploration and advanced medicine. The best-known material is an alloy called “Nitinol,” made from roughly equal parts nickel and titanium.

In space, researchers have studied letting antennas or solar panels ride folded up small inside a rocket, then unfold on their own when they warm up in orbit, and some are already used this way. Shape-memory alloys are also used in release latches that free folded parts. A big plus is that they can cut down on heavy motors.

They're just as vital in hospitals, where they help save lives. A prime example is the vascular stent, a mesh device that holds open a narrowed blood vessel. They're especially common in stents for vessels that are easily squeezed from outside, such as those in the neck or legs. A stent folded down very thin is tied under a sheath inside a slim tube and carried to the vessel. When the sheath is pulled back, the alloy, which becomes more springy at body temperature, opens wide to its original size and clears the blocked passage.