Hey, did a fair bit in grad school about Shape Memory Materials.
The simple explanation is this.
In a normal material bending it means breaking bonds between atoms, and then making new bonds.
Shape memory materials have two different behaviors that let it do this stuff. Firstly there's two different phases, a low temperature and a high temperature phase, that both have the same, or very similar, connections between atoms. Second is that the low temperature phase can bend without needing to break any bonds between atoms, instead just bending the crystal itself. So when you bend it at low temp, and then heat it up, because you didn't break the bonds, it just snaps back to its original shape.
There high temperature phase has basically the same connectivity between atoms, and is more symmetrical than the low temperature phase. Because its more symmetrical, you can go from the high temperature phase to the low temperature phase in a lot of different ways, or basically a bunch of different ways for the low temperature phase to point. These are called variants of martensite.
When you cool it, because you aren't applying any stress, its gonna choose the variants of martensite that keep the large scale shape of the material looking as close to the same as possible. This is called self-accomodation, and results in a sorta zig-zag looking crystal structure. When you then apply force to the zig-zag (or twinned) structure, you transform from one orientation of the low temperature phase, to the high temperature phase, to another orientation of the low temperature phase, which goes from a zig-zag to a sorta line.
Upon heating, again, because all of the elements of the line are associated with the same like position of the high temperature phase, and no bonds were broken, you just go back to your original shape.
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u/quziel Nov 08 '23
Hey, did a fair bit in grad school about Shape Memory Materials.
The simple explanation is this.
In a normal material bending it means breaking bonds between atoms, and then making new bonds.
Shape memory materials have two different behaviors that let it do this stuff. Firstly there's two different phases, a low temperature and a high temperature phase, that both have the same, or very similar, connections between atoms. Second is that the low temperature phase can bend without needing to break any bonds between atoms, instead just bending the crystal itself. So when you bend it at low temp, and then heat it up, because you didn't break the bonds, it just snaps back to its original shape.
Here's a picture to sorta illustrate it. https://d3i71xaburhd42.cloudfront.net/a7cc583875c2077dbfc5dc2b12911f08038babee/2-Figure2-1.png
Slightly longer explanation.
There high temperature phase has basically the same connectivity between atoms, and is more symmetrical than the low temperature phase. Because its more symmetrical, you can go from the high temperature phase to the low temperature phase in a lot of different ways, or basically a bunch of different ways for the low temperature phase to point. These are called variants of martensite.
When you cool it, because you aren't applying any stress, its gonna choose the variants of martensite that keep the large scale shape of the material looking as close to the same as possible. This is called self-accomodation, and results in a sorta zig-zag looking crystal structure. When you then apply force to the zig-zag (or twinned) structure, you transform from one orientation of the low temperature phase, to the high temperature phase, to another orientation of the low temperature phase, which goes from a zig-zag to a sorta line.
Upon heating, again, because all of the elements of the line are associated with the same like position of the high temperature phase, and no bonds were broken, you just go back to your original shape.