Shape Memory Effect - "Training" of the Transformation
The shape memory effect also involves martensitic transformations, but in this case they are stimulated, not by imposed mechanical strain, but by changes in temperature. It also involves the material being “trained” to have a preferred shape. This is done in the following way.
The component is given a thermo-mechanical treatment, which involves holding at a high temperature (usually well above Af), followed by cooling (to below Mf), while mechanically constrained to have a particular shape - eg a spring. Stress relaxation occurs during the holding period and then, during cooling (in the constrained shape), the austenite-martensite transformation takes place in such a way as to minimise the overall shape change. When a portion of the austenitic lattice shears so as to form the martensitic phase, there are usually several alternative directions in which it can do this – forming what are often termed different “variants” – so it’s possible for groups of variants to be formed which, taken together, have a very similar shape to the original parent material. This training predisposes the component to adopt the shape concerned when the phase transformation occurs, since this minimises the associated elastic strain energy.
Once a component has been “trained” in this way, then, after it has been deformed in an arbitrary way, it can recover its “trained” shape just by reheating it to above its Af temperature
It’s also possible, using slightly more complex thermo-mechanical treatments, to create components which exhibit a “Two-way shape memory effect”, such that they can be cycled between two pre-determined shapes by thermal cycling. Such components are used in devices such as those designed to automatically open greenhouse windows in hot weather, and close them when it gets cooler.