Thermodynamics and thermophysical properties of Pb-Bi and Fe-Cr alloys

In: 2. Thermochemical and thermophysical properties of materials Thermodynamics and thermophysical properties of Pb-Bi and Fe-Cr alloys Rada Novakovic National Research Council (CNR) - Institute for Energetics and Interphases (IENI), Via De Marini 6, 16149-Genoa (Italy) Abstract Fe-Cr alloys are model materials for ferritic / martensitic steels used for applications to core and primary components of advanced reactors, while the Pb-Bi eutectic and Pb metallic melts are used as coolant in the same type of reactors (Gen. IV). Due to higher energy of Pb-atomic nucleus, Pb could be concurrent to the Pb-Bi eutectic alloy. Therefore the knowledge of their properties can indicate the parameters which are governing the behaviour of the aforementioned materials under real operative conditions. According to this, the investigation of general physico-chemical properties of these alloys are important for specific solutions of a complex mass transport circulation problem in the loops. In the present work an overview on the thermodynamic and thermophysical properties of the Pb-Bi and Fe-Cr systems in the liquid and/or solid state have been reported, giving the state of the art on these properties. The Fe-Cr and Pb-Bi systems and their properties have been studied by many authors. The Bi-Pb phase diagram shows an intermetallic hcp-A3 phase (called ?-Pb3Bi) formed by peritectic reaction at 460.15 K, while the Fe-Cr phase diagram exhibits the presence of an intermediate ? - phase with a close-packed tetragonal crystallographic structure. Although there are numerous studies on the Fe-Cr system, some discrepancies regarding its phase diagram still remain. In particular, for an intermediate ? - phase, its temperature and composition ranges are not yet well defined. Indeed, several authors gave 1093 K as the lowest value of the upper-limit of the existence of ? - phase as a single phase, but the literature data on this temperature for the bulk alloys differ by up to 100 K, indicating 1203 K as the largest value. A compositional range in which ? - phase can be formed has been determined by many authors and the corresponding experimental values show a large scatter. The last re-assessment of the Fe-Cr phase diagram has been done by postulating low-temperature and high temperature limits for the existence of ? - phase at 793 and 1093 K, respectively, and the composition range of 42-51 at.% Cr. The harmful effects of the ? - phase on the mechanical properties (e.g., toughness and creep ductility) and corrosion resistance of the commercial alloys and steels AISI 316 or AISI 310 stainless steels are well documented. Binary systems characterised by the presence of one or more intermetallic compounds in the solid state, exhibit in the liquid phase the associative tendency among the unlike constituent elements. The mixing properties of such molten alloys deviate from the regular solution behaviour and show a well-defined peak at one or more concentrations, which lie in the vicinity of the stoichiometric compositions of the intermetallic compounds. The asymmetry around equiatomic composition is more evident for strongly interacting binary alloys. Since the structure of a liquid alloy is, at least close to the melting point, similar to that of a crystal as it has been already demonstrated by diffraction experiments, the existence of ? and ?-Pb3Bi intermediate phases in the Fe-Cr and Pb-Bi systems, respectively, suggests short range ordering in their liquid phases. Accordingly, by postulating the formation of FeCr and Pb3Bi chemical complexes in the corresponding liquid phases, the mixing behaviour in Fe-Cr and Pb-Bi alloy melts was studied in by the compound formation model (CFM) in a weak interaction approximation. The energetics of mixing in liquid Fe-Cr and Pb-Bi systems has been analysed through the study of the concentration dependence of various thermodynamic, surface (surface tension and surface composition), transport (diffusivity) and structural properties (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter). The CFM, developed by Bhatia and Singh, in the framework of statistical mechanical theory in conjunction with the Quasi-Lattice Theory (QLT) provides the information on the energetics and structure at atomistic level that can be deduced from observable indicators, such as thermodynamic and thermophysical quantities, giving qualitative insight into the mixing processes that occur in an alloy melt. The energetics and the structure of the Fe-Cr solid alloys have also been analysed in terms of their thermodynamic and surface properties and compared with literature data.