Metals That Do Not Forget

A crystal is defined by the pattern in which the atoms within are arranged. This pattern can in the right circumstances transform into a new arrangement that has a different symmetry. One of the ways in which such a change can be achieved is by a homogeneous deformation which leaves all near-neighbour relationships intact. This represents a particular class of transformations that are known as martensitic transformations. There is no diffusion required and there is no composition change. As a result, the martensite retains a memory of the arrangement of atoms in the parent structure so a reversal of the transformation also reverses the deformation. We shall see in this chapter how this memory effect can be exploited to create devices that are life-changing and others that improve the ability to engineer improved mechanical devices.