The Shape Memory Effect
“Dad? You know how I’ve been taking driving lessons and you said I could use your car to practice?”
“Yeah?”
“And you know how you said to be extra careful and not dent it or anything?”
“… What happened?
“I… might have dented it. But it was an accident, and it won’t happen again and I’ll buff it out or I’ll pay you back for it! … Eventually.”
“… … Put the kettle on.”
“You want a cup of tea? Sure, milk and no sugar, right?”
“It’s for the car – just carefully pour some hot water over the dent and it’ll be good as new. But I’ll still have that cuppa!”
This is the shape memory effect – where a material can “remember” its original shape by being heated up. This might sound like the kind of magic trick that only Harry Potter could pull off, but it is far from science fiction: the shape memory effect is an engineering reality that is already used in a wide range of fields from medicine and dentistry to planes and even space exploration!
The shape memory effect is a unique kind of phase transformation. Phase transformations are common in everyday life – freezing, melting, boiling, and condensing are all examples of phase transformations. However, in something like melting, the phase transformation is from a solid to a liquid. By contrast, the shape memory effect involves going between two solid phases — a high-temperature solid phase, and a low-temperature solid phase. The difference between the two solid phases is that the crystal structure (the pattern in which the atoms in the solid arrange themselves) is different at high temperatures compared to low temperatures.
But how does the shape memory effect work?
We start with the blue structure at the top. At low temperature, the atoms prefer to be arranged as diamonds. These diamonds zig-zag. In the first layer, the diamonds point one way. In the next layer, the diamonds point the other way, and so on. The overall shape is a rectangle.
When we stretch this material, we can force all these diamonds to point in the same direction. We haven’t changed the crystal structure, because technically all of the atoms are still arranged as diamonds. However, it is clear to see that the material’s dimensions have changed and it has become longer.
At high temperature, the atoms prefer to be arranged as squares. So when we heat up our stretched material, the atoms move into this square arrangement. This gives us our original rectangular shape back.
When we let our material cool back down, the atoms arrange themselves as diamonds again. But, because we aren’t forcing the diamonds to align, they zig-zag to keep the same overall rectangle shape. This can keep on going in a cycle where we can repeatedly stretch, heat, and cool the material.
We know some materials display the shape memory effect. Most of them are metallic alloys, but the shape memory effect is a relatively rare and exotic phenomenon that requires careful alloy design. The amount of each element in the alloy and how much of each element to include must be precisely controlled.
As exciting and interesting as shape memory alloys are, there is still some work we can do to improve them. One area of my project is looking at how we can make shape memory alloys better suited for biomedical applications. One current use for shape memory alloys is in bone implants, such as hip replacements. If the implant is too flexible, it won’t be able to support the weight on it. But if the implant is too stiff, the remaining bone becomes weaker. At the moment, most implants are too stiff, so one area of research is in finding a way to make the implant more closely match the stiffness of human bones.
Another challenge for implants is preventing infection and making sure the implant doesn’t dissolve away in the body over time. Luckily, it turns out that adding small amount of precious metals like silver or gold to existing shape memory alloys might be the answer to these challenges!
Ayush Prasad
NanoDTC Associate, a2021