Influence of the Chemistry and Structure of Austenite:Ferrite Boundaries upon Growth Kinetics and Composition of Grain Boundary Ferrite Allotriomorphs in Fe-C-X Alloys

Abstract

A review is presented of the growth kinetics of grain boundary ferrite allotriomorphs in Fe-C and Fe-C-X alloys, of the models which have been developed to explain these kinetics and of the deficiences which repeated comparisons with experiment have brought to light in these models. These models, principally that of “local equilibrium” and of paraequilibrium, also encounter problems in explaining experimental data on the composition of ferrite allotriomorphs as determined earlier by electron probe analysis and more recently by STEM. Addition of the rejector plate mechanism to the “local equilibrium” model is useful in explaining growth kinetics and composition of partitioned ferrite in Fe-C-Mn and Fe-C-Ni alloys, but the rejector plate component seems on firmer ground than that of “local equilibrium”. Effects of carbide precipitation at austenite: ferrite boundaries and, especially, the partially coherent structure which allotriomorphs now appear to have instead of the incoherent structure originally predicted can explain some of the discrepancies. However, the paraequilibrium model, as modified by a solute drag-like effect, seems presently to be the most promising avenue of approach to the growth kinetics and composition of allotriomorphs formed at lower temperatures. At higher temperature, though, the comparisons with experiment so far reported suggest the need for further theoretical developments.