How some metals remember and snap back to their shape

HostMost of us have messed around with a paperclip at some point. You bend it out of shape to pick a lock or just to have something to do with your hands, and it stays bent. It stays in that new, messy shape until it eventually snaps. But there are these odd metals out there that act like they have a mind of their own. You can twist them into a knot, and then, with just a little bit of heat, they snap right back to how they looked at the start. It looks like a magic trick, but it's actually a bit of very clever physics. How's it that a solid piece of wire can remember where it started?

GuestIt does look like magic when you see it for the first time. You take a wire that's as straight as a needle, crumple it up into a ball, and then drop it into a bowl of hot water. In a split second, it jumps and uncoils itself back into that perfect straight line. People call these memory metals, and the most common one is a mix of nickel and titanium. The secret to the memory isn't in a brain, but in how the tiny bits that make up the metal, the atoms, are stacked together. In a normal piece of steel, when you bend it, you're sliding those atoms past each other and forcing them into new spots. Once they move, they're stuck there. They have no way to find their way home.

HostSo when I bend a normal paperclip, I'm basically breaking the internal map of the metal. The atoms are lost in a new neighborhood and they don't know how to get back.

GuestThat's a good way to put it. You're permanently moving the furniture. But in these memory metals, the atoms are arranged in a very specific grid. This grid can exist in two different ways. Think of a big crowd of people standing in neat rows and columns. When the metal is cold, the atoms are in a state where they can lean over. Imagine that whole crowd of people all leaning to the right at the same time. They're still in their rows, and they're still holding hands with the people next to them, but they're all tilted. Because they can tilt without breaking their grip on each other, you can bend the wire without ruining the overall pattern.

HostBut if they're all leaning over, they're still in a new shape. If I let go of the wire while it's cold, it stays bent. It doesn't feel like it wants to move. Where does the memory part come in?

GuestIt stays bent because the atoms are comfortable leaning over. They're stable that way. But the metal has a second state, a hot state. When you add heat, you're giving those atoms a burst of energy. At a certain point, that energy is enough to make every single atom stand up straight again. Since they never let go of their neighbors when they were leaning, they all pull each other back into those perfect, upright rows. The whole crowd stands up at once, and the wire snaps back to its first shape. The memory is just the fact that the atoms never lost track of who they were standing next to.

HostWait, if they're just leaning, why does the metal feel so solid? If I bend a piece of wire, I have to use some force. It feels like I'm changing something real, not just tilting a few tiny dots.

GuestIt's real force. You're shifting billions and billions of these atoms. But here is the thing that trips people up. In most metals, when you push that hard, the atoms slide and roll over one another like grains of sand. Once they slide, the connection is gone. In memory metal, they don't slide. They just shift their angle. Scientists call this twinning. It's like a mirror image. The atoms move into a new position that's a perfect reflection of the old one. Because it's so organized, the path back to the start is always open. It's like a folding chair. You can fold it up and it looks different, but all the parts stay bolted together so it can only ever unfold back into a chair.

HostI can see how that would be useful for a toy or a trick, but it seems like a lot of work to have to heat something up every time you want it to work. Does this have a use beyond just being a weird thing to show off at a party?

GuestIt's actually inside a lot of people right now. Think about a tiny tube used to keep an artery open in someone's heart. They call these stents. Doctors need to get that tube through a very small space to reach the heart. So they take a memory metal tube, chill it down until it's in that easy to bend state, and crush it down until it's tiny. They slide it into the body, and as soon as it hits the warmth of the person's blood, the metal reaches that trigger heat. It remembers its big, open shape and pops open, pushing the artery walls out and holding them there. It does the work all by itself just by being warm.

HostThat's a much higher stake than a paperclip. But it makes me wonder about the limits. If I keep bending and heating it, does it ever get tired? I mean, even a folding chair eventually gets a wobbly leg or a rusty bolt.

GuestIt does get tired. Even though the atoms stay connected, the grid can get little flaws over time. If you do it thousands of times, some atoms might finally slide out of place or get stuck. The memory gets a little fuzzy. The wire might not go back to being perfectly straight, or it might need more heat to get there. There's also the issue of the heat itself. If you get it too hot, you can actually reset the memory. You can force the atoms into a new home shape. That's how they make the things in the first place. They hold the metal in a shape, blast it with high heat, and tell the atoms, this is your new home. From then on, that's the shape it'll always try to find.

HostIt sounds like we're still finding ways to use this. Is it just for small things like heart tubes and glasses frames that don't break, or can we go bigger?

GuestWe're going much bigger. Some engineers are looking at using these metals in airplane wings. Right now, wings have flaps with lots of heavy motors and gears to move them. But imagine a wing made with memory metal. Instead of a motor, you just send a bit of heat through the metal and the whole wing tip curls or flattens out to help the plane fly better. It would be lighter and have fewer parts to break. We're even using it on Mars. The wheels on some of the rovers use a mesh made of this metal. If a wheel hits a sharp rock and gets a big dent, the metal just flexes and pops back out as the rover moves. It's a wheel that can never go flat and never stays dented.

GuestThe most amazing part is that we're now making metals that can remember two shapes, one for when it's hot and a different one for when it's cold, so it can flip back and forth forever just by changing the air around it.

HostThe paperclip on my desk seems pretty boring now that I know there are wires out there that can act as their own motors and tools just by feeling a bit of warmth.