NUCLEATION KINETICS OF GRAIN-BOUNDARY ALLOTRIOMORPHS OF PROEUTECTOID FERRITE IN FE-C-MN-X(2) ALLOYS

The steady-state nucleation rates of ferrite allotriomorphs at the ''faces'' of austenite grain boundaries were measured in Fe-C-X(1)-X(2) alloys, where X(1) was Mn and X(2) was successively Si, Ni, and Co, using techniques previously developed for counterpart studies on Fe-C and Fe-C-X alloys. The results were compared with the predictions of the classical nucleation theory, using the pillbox-shaped critical nucleus model. The volume free energy changes associated with nucleation in Fe-C-X(1)-X(2) quaternary systems were evaluated from the central atoms model (CAM) for both para- and orthoequilibrium modes of transformation. The nucleation process was assumed to be controlled by volume and/or grain boundary diffusion of alloying elements. The so-called synergistic effects of alloying elements were considered in terms of the volume free energy change and interfacial energies on the basis of the results of the nucleation rate measurements.