A study of the influence of Mn and Ni on the kinetics of the proeutectoid ferrite reaction in steels

The kinetic transition between partitioned and unpartitioned growth of proe utectoid ferrite has been studied for high-purity Fe-C-Mn and Fe-C-Ni alloy s, and for temperatures just above the eutectoid. These results land certai n results of previous investigations) are compared with computed paraequili brium and equilibrium ternary phase diagrams, and it is shown that the tran sition occurs well within the paraequilibrium two-phase regions, but signif icantly outside the limit predicted by the local equilibrium analysis of th e ternary precipitation reaction. These observations are interpreted in ter ms of solute drag theory. It is inferred that both Mn and Ni exert a drag o n the moving ferrite/austenite interfaces, and that this drag force is due to substitutional solute diffusion within the moving interface. The equilib rium binding energies of each of the substitutional solutes to the boundary are expected to be of order RT. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.