Institute of Metals Division - Kinetics of the Proeutectoid Ferrite Reaction at an Incoherent Interface, as Determined by a Diffusion Couple (TN)

THE formation of ferrite in supersaturated aus-tenite has been the subject of considerable speculation and investigation. Aaronson has recently reviewed the data pertinent to this reaction.' Since these data in all cases refer to ternary or high-order alloys it is not possible to put them to the test of simple binary diffusion formalism. It has not, therefore, been possible to make a certain statement as to the controlling mechanisms in the process. An assessment of the difficulties inherent in the conventional statistical metallographic approach suggested that a study of the problem by means of a macroscopic two-phase diffusion couple might contribute useful information. Accordingly, such a couple was synthesized, and the rate of motion of a ferrite-austenite phase boundary (the growth of an austenite layer at the expense of a ferrite layer) was determined. Wagner originally produced a diffusion solution applicable to the motion of a planar phase interface in a binary diffusion couple, assuming local equilibrium throughout the diffusion zone. This solution, which is stated free of typographical error* in Ref. where a is the position of the interface in A space (A = x/fi), C; and CL1 are the equilibrium inter-facial concentrations in phases I and 11, C: and C*1 are the initial bulk concentrations in phases I and 11, and & and are diffusion coefficients (assumed constant). If a diffusion couple consisting of homogeneous layers of ferrite and austenite in equilibrium is raised to a higher temperature, the austenite layer will grow at the expense of the ferrite. Provided that the thickness of the ferrite layer is much less than the diffusion length of carbon in ferrite, we can set , so that Wagner's solution reduces to