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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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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