On theories of growth during discontinuous precipitation

Growth theories for the lamellar eutectoid transformation are reviewed from a free energy point of view in order to form a background for an examination of the various treatments of growth during discontinuous precipitation. The shortcomings of Cahn’s theory of discontinuous precipitation are pointed out and the treatment based upon the individual consideration of growth conditions for the two phases is discussed, with particular emphasis on how the total free energy available to drive the reaction is divided into many parts which are used for various purposes. New sets of calculations are presented in order to demonstrate the effect of free energy losses due to boundary friction and volume diffusion. The effect of coherency stresses in the parent grain is shown to decrease the free energy loss due to volume diffusion and may thus provide a driving force for grain boundary movement. This paper is based on a presentation made at a symposium on “The Cellular and the Pearlite Reactions,” held at the Detroit Meeting of The Metallurgical Society of AIME, October 20, 1971, under the sponsorship of the IMD Heat Treatment Committee.