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Cellular segregation reactions, such as discontinuous precipitation and pearlite formation, often accomplish the segregation solely by diffusion along the cell boundary as it sweeps through the original unsegregated phase. Previous theories, which have assumed that the reaction is diffusion controlled, have been inadequate in describing many of its quantitative aspects.

If the diffusion is limited to the advancing cell boundary, the lamellae of the cells or nodules cannot reach equilibrium composition at any non-zero growth rate and for this reason the growth rate of the cells can no longer be determined by the diffusion rates alone. It is concluded that at least two controlling kinetic processes are required to describe such a complex reaction.

A simple model, which, in addition to diffusion control, assumes that the cell boundary moves with a velocity proportional to the net free energy decrease (taking into account the incompleteness of the segregation as well as the creation of lamellae surfaces), reproduces many of the observed features of these processes. Among these are the incompleteness of the segregation and the interlamellar spacing.

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