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Many alloys exhibit the so-called shape-memory effect: they can be deformed and retain their original shape upon heating. Explore this effect yourself in this video (courtesy of Ingo Müller, TU Berlin). Furthermore with the help of special microscopes one can observe fascinating structures, the so-called microstructure. See the pictures of a Copper-Aluminum-Nickel alloy made by T. W. Shield:
Where do these effects come from?
Shape memory alloys are characterized by a single lattice structure at high temperatures and several symmetry related lattice structures at low temperatures. The high temperature phase is called the austenite, the low temperature phases are called martensites.
The interesting point now is that phase transitions may occur, that is one phase transforms to another one by a slight motion of the grid atoms. Each transformation goes along with a deformation of the outer shape. For example, when the austenite crystal above transforms to martensite M1, this corresponds to a shear deformation.
Phase transitions can be induced by loading. Consider this crystal:
In the left it completely consists of the martensite phase M1, indicated in green here. When a shear load is applied to it as indicated by the arrows, the M1 phase transforms partially to the M2 phase, indicated in red. Hereby the crystal anticipates the new outer shape forced by the loading. The typical pattern observed here is a laminate: layers of two alternating phases. The fine pattern of different phases is called microstructure. If we would continue loading the crystal, it would transform completely to the M2 phase.
Phase transitions can also be induced by temperature changes. At high temperatures the crystal will only consist of the single austenitic phase. But at low temperatures, several martensitic phases are possible, even a mixture of martensitic phases as mentioned above. Therefore cooling an austenitic crystal transforms it to a martensitic crystal, and vice-versa heating a martensitic crystal transforms it to an austenitic crystal.
A combination of transformations induced by loading and by temperature changes produces the so-called shape memory effect:
Let us start with an austenitic crystal like the first one. If we cool it, it transforms to martensite. Hereby a transition is preferred that resembles the previous shape as much as possible. For example the first yellow crystal in the picture above transforms to the second one, a laminate of martensitic phases. Then we deform the crystal, e.g. by a shear load. This leads to a phase transformation: all red M2 phases transform to green M1 phases. If the crystal is heated then, it must go back to the single austenitic phase, which is necessarily connected with a deformation back to the cube: the crystal has retained its original shape. The complete process can be repeated then.
For related information on the physical background see also What are Martensites?
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