The rotation reorients the lattice but has no effect on its crystal structure. The rotation required to generate convert B to an ILS precisely predicts the observed orientation from the Bain orientation. The strain transforms it to an ellipsoid of revolution. Thus, it is impossible to convert austenite into martensite by a strain which is an invariant-plane strain. A corrolarly to this statement is that the two crystals cannot ever be joined at an interface which is fully coherent and stress-free.
Martensitic transformation is diffusionless.
It may then precipitate as cementite, giving an upper bainite microstructure. Widmanstatten ferrite grows by a displacive paraequilibrium mechanism involving the diffusion of carbon. It thus grows at a rate which is controlled by the diffusion of carbon in the austenite ahead of the interface. Furthermore, there is little driving force available at the temperatures at which grows so two plates have to grow together in a self-accommodating manner in order to reduce the overall strain energy. This is why it optically appears to be in the form of a thin wedge, since the component plates have slightly different habit planes.