Diffusional Transformations: A Theory for the Formation of Superledges

It is well known that the growth of proeutectoid ferrite in steels often occurs by the superledge mechanism, in which the piecewise displacement of the interface is accomplished by the motion of steps, whose heights can amount to several hundreds of lattice spacings. The present work is an attempt to understand the factors controlling the height (h) of experimentally observed superledges. A theory based on step nucleation is developed, which predicts a lower limit for h, for a specified set of transformation conditions and steel composition; published experimental data seems to be in good agreement with the theory, particularly with respect to the variation of h with transformation temperature. It is also shown that the critical height below which the nucleation of a ledge is not possible, is directly related to the free energy of the singular interface on which ledge motion is supposed to occur.