If a paper clip is bent out of shape and then heated it will go back to the original shape. First, how does this even work? Second, if heated up and a new shape is created what happens if this process of cooling, bending, reheating is repeated? Does it retain memory of both or just the last heated shape?
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When a paper clip is bent out of shape, the atoms that make up the metal are forced out of their natural positions and into new ones. When the metal is heated, the atoms gain energy and begin to move around more quickly. As they move around, they can potentially return to their natural positions, which would cause the paper clip to return to its original shape.
If a paper clip is heated and then bent into a new shape, and this process is repeated multiple times, the paper clip will retain its most recent shape. It will not retain any memory of its previous shapes. This is because when the metal is heated and bent into a new shape, the atoms are rearranged into new positions, and there is no way for the metal to retain a “memory” of its previous arrangement.
You set the shape by heating it up to very high temperatures, usually glowing, and then bending it. This causes the crystal structure within the metal to change and set into this new shape. When you cool it down and then bend it cold the crystal structure does not change, it just bends. This is similar to a rubber band but it does not have the energy to snap back. By heating up the metal a bit you give it enough energy to bend back. The heat causes the atoms to jiggle back and forth enough to settle in the relaxed crystal structure again. I do not think you can actually bend it back and forth many times. The crystal structure does change a bit every time you bend it and heat it up. It will become more hardened and brittle over time. And this does limit the practical applications of memory alloys.
Shape memory alloys work because they do not deform like other metals in the first place.
Normally deformation is caused by dislocations (tiny errors in the crystal lattice) being created and moving around.
But shape memory alloys undego a specific transformation (martensitic transformation). This is unlike other phase changes in metals, because it doesn’t involve atoms hopping around, changing places and altering the chemical composition. It causes the whole crystal lattice to change shape almost at once.
So you create an SMA by heating it up, shaping it to what you want the “remembered” shape to be, then cooling it down to undergo the transformation. Then you can bend it out of shape. But what will happen is that rather than moving dislocations (like it would do normally), you deform the whole structure directly. This can happen because it’s already”out of shape” due to the transformation and doesn’t resist as much as it normally would. But this is important, the previous structure isn’t destroyed (like it would be with dislocations). It deforms it very specific and ordered ways.
Because of how specific and ordered all of this deformation is, when you heat it up again (reversing the original transformation), the crystal will literally snap back to its original shape.
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