One of the theories on the specific formation mechanism for bainite is that it occurs by a shear transformation, as in martensite.  The crystal structure change is achieved by a deformation rather than by diffusion. The shape change  associated with bainite is an invariant—plane strain with a large shear component. This kind of deformation implies a disciplined motion of atoms (rather than a chaotic transfer associated with diffusion), and is typical of all displacive transformations in steels, for example, martensite, bainite and Widmanstaetten ferrite. There is a strain energy associated with such relief, that leads to the plate shape of the transformation product Any diffusion is subsequent to the diffusionless transformation of austenite, for example the partitioning of carbon from supersaturated bainitic ferrite, or the precipitation of carbides; this is analogous to the tempering of martensite.
There are many features of bainite that are correctly predicted by this theory, including:
* the plate shape, which is a consequence of the minimisation of strain energy due to the shape deformation accompanying transformation.
* The fact that excess carbon is retained inside the even defect-free regions of bainitic ferrite.
* The fact that the unit cell of bainitic ferrite can be tetragonal rather than cubic.
* The fact that the bainite transformation can be dramatically retarded when the austenite is first plastically deformed, a phenomenon known as mechanical stabilisation, which is unique to displacive transformations.
* The obvious fact that displacements occur when bainite grows. The transformation is a combination of deformation and crystal structure change, just like martensite.