Unidirectional transformation of Fe-0.8C-Co alloys: Part II. Kinetics of the eutectoid reaction

TLDR: In this article, unidirectional transformation was applied to a study of the kinetics of eutectoid growth in Fe-0.8C-Co alloys.

Abstract: Unidirectional transformation techniques have been applied to a study of the kinetics of eutectoid growth in Fe-0.8C-Co alloys. The technique readily yielded kinetic data which it is shown could be used to indicate the rate controlling process for pearlitic growth. Accurate measurements of interlamellar spacing (λ) could be made at controlled growth rates (V) and analyzed in terms of the expressionVλn, where the exponentn can indicate the rate controlling process. The results obtained by unidirectional transformation were compared with those achieved by conventional isothermal transformation, both to aid in the initial interpretation of the more unfamiliarV:λ data and also to show that the two different experimental routes lead to equivalent kinetic data. Analysis of the results obtained for Fe-0.8C-Co alloys suggested control by interfacial diffusion of carbon at high growth rates (n=3) changing towards volume diffusion of carbon at lower growth rates (n=2), but also revealed an unexpected region at very slow growth rates (n=4). This anomalous region could be explained in terms of the partitioning of cobalt as the growth rate decreased. It was also shown that cobalt additions decreased the pearlite interlamellar spacing at constant undercooling or growth rate.