The diffusional formation of allotriomorphic ferrite from austenite in Fe-C-X systems. The simplest case: Fe-C-Ni

The diffusional formation of α from γ has been investigated using two complementary approaches. In the first approach, the kinetic transitions in α growth were examined as a function of Ni content using an Fe-1Ni/Fe-5Ni diffusion couple carburized to ∼0.1 (wt.%) C. In agreement with previous investigators, significant α formation was observed in compositions lying above the Local Equilibrium-Partition (LE-P)/Local Equilibrium-Negligible Partition (LE-NP) boundary. On the basis of these observations, a new series of Fe-C-Ni alloys containing ∼0.1C (wt.%) and Ni contents ranging from 2.0 to 3.4 (wt.%) were made and the isothermal transformation kinetics were measured at 700°C. Neither the Paraequilibrium (PE) nor the LENP treatments were able to consistently describe the a growth kinetics in all of the alloys over the time frame investigated. We have proposed a new treatment for α growth, based on a local energy balance at the interface, that allows for the local transport of Ni atoms across the reaction interface and predicts accurately both the fast initial α growth rate and the transition to much more sluggish kinetics that were observed in the alloys with increasing Ni contents. PE interfacial conditions are the assumed starting point for the model and a continuous transition to local equilibrium conditions is observed with time. The model can quantitatively explain the growth of α both above and below the LE-P/LE-NP boundary. It was not found necessary to include any free energy dissipation due to Ni diffusion within the interface.