An assessment of the Fe-S system using a two-sublattice model for the liquid phase
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THE liquid phase in binary systems between elements with a large difference in electronegativity generally shows a very rapid variation of the activities at some stoichiometric composition. This feature is often taken as a sign of the existence of molecular-like aggregates of atoms in the liquid phase, t In recent times there have even been many efforts to formulate thermodynamic models of the liquid phase by postulating the existence of such aggregates? They are usually referred to as "associated solution" models and have recently been applied to many systems with considerable success. 2 When considering the structure of a liquid phase without having access to direct experimental information, it seems advisable to look at the structures of the crystalline phases in the same alloy system. Already Wagner 3 emphasized that the existence of individual molecules corresponding to definite groups of atoms is improbable if they do not exist in the crystalline state. When examining the crystalline phases in the binary systems where the associated solution models have been applied, one might usually find that there are no molecules or molecular-like aggregates of atoms. Instead there is often an ordered solid phase at the same composition as the rapid variation in activities. The deviation from the stoichiometric composition is due to defects in the ordered atomic arrangement. It should be interesting to attempt to describe the thermodynamic properties of the liquid phase in such systems by postulating that the liquid phase has an atomic arrangement similar to the ordered solid phase. Of course, in order to describe the much wider variation in composition of the liquid phase, it is necessary to consider much higher concentrations of defects but, at least in the first instance, one should consider the same type of defects. Many types of defects are known from the ordered solid phases and they would all be considA. FERNANDEZ GUILLERMET is Researcher at the Centro de Investigaci6n de Materiales, C.C.884 Cordoba, Argentina. At present he is on leave for studies at the Dept. of Metallurgy, Royal Inst. of Technology, S-10044 Stockholm. M. HILLERT is Professor of Physical Metallurgy, Royal Inst. of Technology, S-10044 Stockholm. B. JANSSON is Research Assistant, Department of Metallurgy, Royal Inst. of Technology, S-10044 Stockholm. B. SUNDMAN is Research Associate, Dept. of Metallurgy, Royal Inst. of Technology, S- 10044 Stockholm, Sweden. Manuscript submitted December 29, 1980.
METALLURGICAL TRANSACTIONS B
ered in a general treatment of the liquid phase. However, before one has gained more insight in the structure of liquid phases, it may be justified to simplify the treatment by considering only a few types of defects. The present work is an attempt to describe the liquid Fe-S phase by postulating one sublattice for Fe and another for S. The deviation from stoichiometry will be described by introducing vacancies on both sublattices. In fact, this method has already been used in an analysis of the properties of the half-system
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