The diffusional formation of ferrite from austenite in Fe-C-Ni alloys
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3/4/04
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The Diffusional Formation of Ferrite from Austenite in Fe-C-Ni Alloys C.R. HUTCHINSON, A. FUCHSMANN, and Y. BRECHET The kinetics of the → 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, 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 growth kinetics in all of the alloys over the timeframe investigated. We have proposed a new treatment for 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 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.
I. INTRODUCTION
THE proeutectoid ferrite () transformation is usually the first phase transformation to occur during cooling from the austenitic temperature range in most steels of industrial importance. The ability to form martensite (hardenability) upon cooling such steels therefore depends on suppressing the diffusional decomposition of austenite () and in particular the → proeutectoid transformation. In this regard, it is desirable to have a physically based quantitative understanding of the kinetics of formation and research toward this goal has been ongoing for over 50 years. The transformation involves both a structural rearrangement ((fcc) → (bcc)) and the long-range redistribution of C from the to the . Such phase transformations are often classified as either interface controlled or diffusion controlled depending on whether the majority of the free energy attending the transformation is dissipated by interfacial processes (e.g., the fcc → bcc structural rearrangement) or by solute diffusion in the bulk phases. Since all spontaneous processes in nature are irreversible, all transformations occurring under conditions of constant temperature and pressure that involve both a structural rearrangement and solute redistribution must be of a mixed nature with some free energy dissipated by interfacial processes and some by long-range solute diffusion. The growth aspect of the overall transformation kinetics can be cast either in terms of the interfacial processes and the free energy dissipated by them or in terms of the long-range solute diffusion and the free C.R. HUTCHINSON, Postdoctoral Research Fellow, A. FUCHSMANN, Postgraduate Student, and Y. BRECHET, Professor, are with the Laboratoire de Thermodynamique et Physico-Chimie Metallurgiques, Institut National Polytechnique de Grenoble, 3840
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