The diffusional formation of ferrite from austenite in Fe-C-Ni alloys

The kinetics of the γ →γ + α phase transformation in a series of Fe-C-Ni alloys containing ∼0.1C (wt pct) and Ni contents ranging from 2.0 to 3.4 (wt pct) have been examined at 700 °C. In agreement with previous investigators, α formation in alloy compositions lying significantly above the calculated local equilibrium-partition/local equilibrium-negligible partition (LE-P/LE-NP) boundary was observed. Neither the paraequilibrium (PE) nor the LE-NP treatments were able to consistently describe the α growth kinetics in all of the alloys over the timeframe investigated. We have proposed a new treatment for α growth, based on the 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. The PE interfacial conditions are the assumed starting point for the model and a continuous transition to local equilibrium conditions is observed with time.