Thermodynamics and Kinetics of Austenite Decomposition in the Iron-rich Iron-Manganese System

Abstract

<p>A thermodynamically self consistent phase diagram together with T₀ line for iron-rich Fe-Mn system has been evaluated using the experimental tie-line information from this study and other sources and thermodynamic data from the literature.</p> <p>The competing morphologies resulting from the decomposition of Fe-Mn austenite due to variation in composition and cooling rate are equiaxed, ragged ferrite and lath morphology. The former two may result via a massive mode, whereas the last one may be due to a martensitic reaction. The massive mode of decomposition has been observed in the two phase field of this system.</p> <p>The solute drag theory for massive transformation due to Hillert and Sundman has been further developed for the low velocity branch. Our theory predicts a C-shaped TTT curve and an optimum in the rate of entropy production with respect to a structure parameter related to defect density.</p> <p>Isothermal transformation tests have yielded a C-shaped TTT curve for Fe-3.1%Mn alloy. The transformation behaviour has been explained as controlled by trace interstitial impurities (C and N) diffusion above the nose and by the massive mode at lower temperatures.</p>