An assessment of the Fe-C-Si system
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INTRODUCTION
PRECISE knowledge of the Fe-C-Si system, in terms of phase diagram or of thermodynamic properties of the phases involved, has been the subject of numerous previous studies because of the importance of this system for steelmaking processes and cast iron foundry. However, the most recent literature [~,z,3j is still plagued by uncertainties or missing information. The purpose of this work was to evaluate the Fe-C-Si system and to obtain a precise description of the system that would be simple enough for practical applications. For this purpose, an assessment of the Fe-Si system has first been achieved and then used together with the previous assessments of the Fe-C r4j and Si-C tS] systems for the description of the ternary Fe-C-Si system. The assessment of the Fe-Si system performed by Chart t61 gave a good fit to the phase diagram and to activities of the constituents in the liquid phase but led to a less precise description of the activities in the bodycentered cubic (bcc) phase. Attempts to improve this aspect turned out to be impossible without introducing at least a very simple treatment of the ordering phenomenon in the bcc phase. In the present assessment, the B2 ordering is to be described by means of a Bragg-WilliamsGorski (BWG) model, as previously proposed by Inden and Pitsch. tTj Modeling the D03 ordering was considered outside the scope of the present assessment, as the aim was to have a treatment simple enough to be useful for multicomponent calculations. In the present study of the binary Fe-Si and ternary Fe-C-Si systems, thermodynamic data have been combined with phase diagram knowledge for the optimization of the parameters describing the thermodynamic properties of the phases. The iron-rich corners of the
JACQUES LACAZE is Charg6 de Recherche with the Laboratoire de Science et Grnie des Matrriaux Mrtalliques, Ecole des Mines, 54042 Nancy, France. BO SUNDMAN is Lecturer with the Division of Physical Metallurgy, Royal Institute of Technology, 100-44 Stockholm, Sweden. Manuscript submitted October 31, 1990. METALLURGICAL TRANSACTIONS A
stable and metastable systems are described. This assessment has been achieved by means of Thermocalc software.t8] II.
T H E Fe-Si A N D Fe-C-Si D I A G R A M S
A. The Binary Fe-Si Phase Diagram Figure 1 presents the experimental phase diagram proposed by Kubaschewski. t91 The general shape of the diagram is similar to that described in previous reviews, t1~ except for the nature of the reactions occuring in the bcc-Fe-rich solution. It is now well established that the bcc phase exists in disordered (A2) and ordered (B2 and D03) modifications. The hatched lines in Figure 1 represent continuous changes associated with ordering reactions, which are second-order ones at high temperature. At lower temperature, the magnetic and chemical ordering interact, and the D03 ordering becomes a firstorder reaction, t12] The recent experimental work by Schiirmann and Hensgen [lq in the Fe-FeSi domain confirms the liquid/ solid equilibria determined previously and giv
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