An outline has been given of how displacive (shear) transformations can effect a macroscopic shape change. These transformations occur by the cooperative, systematic motion of all the atoms in the region concerned by small distances with respect to their neighbours. Unlike a similar shape change generated by conventional plasticity (dislocation glide), such transformations, and hence the shape change, can be reversed. This can occur by simply removing the applied stress, giving rise to so-called Superelasticity. Shape changes can also be stimulated by changing the temperature, thus altering the relative stability of the two phases. This behaviour is exploited in the Shape Memory Effect, in which defined shape changes are induced by changing the temperature, either just recovering a prescribed shape after mechanical deformation or cycling between two defined shapes by thermal cycling.
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