Shewmon recognized that the conditions for nucleation are different if there is partial or full coherency between preexisting
alpha-ferrite and the parent phase, and equilibrium solute distribution will not exist at the interface.
The immobility of the ledges of a coherent ferrite/austenite interface leads to solute diffusing away from the interface, resulting in supersaturation within the austenite and, hence, to an increase in the chemical thermodynamic driving force for
sympathetic nucleation.