The precipitation of a new phase sometimes results in the simultaneous growth of one of the matrix grains at the expense of another one, so called discontinuous precipitation. The two growing crystals may arrange themselves in the lamellar fashion, known from eutectoid transformations. The mathematical analysis of this diffusion-controlled reaction would be incomplete if the nature of the driving force for the matrix grain is not identified.
There have been many suggestions regarding the nature of the driving force. The recent discovery that grain boundaries in a single-phase material can migrate in connection with grain boundary diffusion supports the hypothesis of a chemical driving force, acting in discontinuous precipitation.
Another reaction where the chemical driving force may be important is discontinuous coarsening. The theory for this reaction is developed in sufficient detail to show the role of the chemical driving force.