The shape change

An atomic force microscope image of the beautiful surface relief caused on a pre-polished sample when bainite grows. Notice the sharp tips of the platelets which allow the bainite to be distinguished from the austenite, which is deformed plastically by the transformation strain. After Swallow and Bhadeshia, Materials Science and Technology, volume 12 (1996) 121-125. An atomic force microscope is needed because the plates are too fine to use the traditional light interference microscopy (e.g. Tolansky).

The invariant-plane strain (IPS) surface relief caused by the growth of bainitic ferrite has a large shear strain component of 0.24 in addition to the volume strain (0.03) on transformation Figure. There is therefore, a coordinated movement of atoms as the transformation occurs. Consistent with this, the iron and substitutional solutes such as Mn, Si, Ni, Mo and Cr, have been demonstrated using high resolution techniques to be frozen into position during transformation Figure. The change in crystal structure is therefore achieved by a deformation of the austenite crystal. If the strain is elastically accommodated, then the strain energy of bainitic ferrite amounts to about 400 J/mol. Some of the strain can be relaxed by plastic deformation in the adjacent austenite.

These pictures are taken at atomic resolution though the reproduction here does not do justice to the original work. (a) A field-ion micrograph of a vertical interface beween austenite and bainitic ferrite. (b) An image using just the iron atoms. (c) An image using just the silicon atoms. (d) An image using just the carbon atoms. The substitutional atoms clearly are frozen during transformation. After Bhadeshia and Waugh, Acta Metallurgica, volume 30 (1982) 993-998.

The movement of interstitial atoms during the change in crystal structure does not influence the development of surface relief. Conversely, the observation of relief cannot yield information about whether or not carbon diffuses during transformation.