Recently I bought a small wire of NiTinol to have fun with and use in demonstrations to kids. This video gives a spectacular demonstration and attempts to explain how it works. I did not know about their use in stents for heart surgery.
I am still struggling to understand exactly how shape-memory alloys work. According to Wikipedia
The shape memory effect occurs because a temperature-induced phase transformation reverses deformation...Typically the martensitic (low-temperature) phase is monoclinic or orthorhombic . Since these crystal structures do not have enough slip systems for easy dislocation motion, they deform by twinning—or rather, detwinning.
Martensite is thermodynamically favored at lower temperatures, while austenite (B2 cubic) is thermodynamically favored at higher temperatures. Since these structures have different lattice sizes and symmetry, cooling austenite into martensite introduces internal strain energy in the martensitic phase. To reduce this energy, the martensitic phase forms many twins—this is called "self-accommodating twinning" and is the twinning version of geometrically necessary dislocations.
In different words, I think the essential idea may be the following. In most metals large strains are accomodated by topological defects such as dislocations. These become entangled leading to work hardening and irreversible changes is macroscopic shapes. Shape memory alloys are different because of the low symmetry unit cell. The most natural defects are twinning domain walls and they are not topological and so their formation is reversible.
I am looking forward to reading the book chapter Shape memory alloys by Vladimir Buljak, Gianluca Ranzi
Another fascinating phenomena that is related to shape-memory is "superelasticity", which I discussed in an earlier post on organic molecular crystals, and has recently been reviewed.
I welcome clarification of the essential physics.
Think about a two dimensional example where the high symmetry structure is a square, and the low temperature structures are two equivalent rectangles with different orientations . If you just lower the temperature, you will get equal amounts of each rectangle domain that on average add up to a square. But if you strain the material along one axis, the system will prefer one rectangle over the other and switch to that orientation until the material elongates enough to relax the strain (without any plastic defects, just reversible domain wall motion). If you release the strain and raise the temperature again, the material returns to a single square domain with its original shape.
ReplyDeleteOne way to think about it is you are reversibly melting and refreezing a symmetry-breaking microscopic strain degree of freedom while the overall structure remains a solid. It is analogous to magnetic degrees of freedom in a ferromagnet, which can be reversibly poled by an external field and then returned to the paramagnetic state at high temperature.
Thanks. Your explanation is very helpful.
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