Sequence of microstructural changes during the shape memory effect
This figure shows schematically what happens inside the material during the demonstration. The material is initially in the martensitic form and the applied "plastic" deformation induces straining in the form of (shear) reorientation of small regions (to form different "variants"). When the specimen is heated, the austenitic structure becomes the one that is thermodynamically more stable. The transformation to austenite takes place in the "trained" shape - the coiled spring in this case. Final cooling leads to the crystal structure reverting to martensite, while the shape remains the same. "Training" of this sample involved heating and then fairly rapid cooling while it was constrained into the shape of the coil. This created a set of residual stresses that ensure that the phase transformation takes place most readily when the specimen has this shape.