Volume 98, Issue 10

Various aspects of the modeling of short-range order in binary and ternary solutions using the Bragg – Williams (random mixing), associate, and quasichemical models are examined. The models are compared with respect to their ability to predict properties of ternary solutions from optimized binary model parameters in the presence of short-range order. It is shown how introducing short-range order through the quasichemical model can explain the observed flattened shape of most miscibility gaps. The combination of the quasichemical model for nearest-neighbor short-range order with the Bragg – Williams model for the remaining lattice interactions is described.

This paper shows the potential of three-dimensional diagrams, using up-to-date computer graphics techniques. In contrast to the past perspective diagrams can now be generated from quantitative calculations of phase equilibria for systems with any number of components. These diagrams can even be made quantitatively readable. Furthermore new visualisation techniques from the field of virtual reality are used to make the complex data easier to understand.

Computation of chemical equilibria and phase diagrams for multi-phase and non-ideal systems by the material-balance constrained Gibbs free energy minimisation is a modern application of the thermodynamic theory with increasing practical uses. However, in many prospective applications, the behaviour of matter is influenced by additional constraining factors, such as kinetic inhibitions, or electrochemical, charge transport and surface phenomena. For those situations, an extended Gibbs energy method can be applied. In this technique, the supplementary work-coefficient or affinity related condition is introduced to the Gibbs energy calculation as an additional undetermined Lagrange multiplier, which represents the constraint potential. A number of new phenomena can thus be included in Gibbsian calculations. The extended Gibbs energy method is illustrated with some very basic examples.

ChemApp is a thermochemical software library which enables the user to perform thermochemical calculations across a wide spectrum of applications by providing an easily programmable interface to complex equilibrium calculation techniques for multicomponent, multiphase chemical systems. ChemApp is described, and an overview of selected application examples from areas such as metallurgy, gas phase and aqueous chemistry, combustion technology, corrosion, geochemistry, and more is given.

SimuSage is an innovative software tool for process simulation and flowsheeting tasks. Based on ChemApp and its rigorous Gibbs energy minimizing technique, it provides a library of components for the development of highly customized process simulation models. The SimuSage concept is described, and a number of examples from typical application areas such as metallurgy, combustion technology, and other industrial high-temperature processes involving inorganic chemistry are introduced.

A thermochemical optimisation of the intermetallic phases in the Mn – Ni system is presented using the CALPHAD approach. The ordered phases of the fcc family are described with a 4-sublattice model and the ordered B2 phase is described with a 2-sublattice model. The L1 and L1 branches are transformed from the 4-sublattice representation into equivalent 2-sublattice descriptions in order to combine the data with existing databases which are usually based on 2-sublattice models only for the description of ordering phases.

In the present work a Calphad type thermodynamic assessment of the Al – Li system is presented. The system is dominated by the central AlLi phase, which has an ordered bcc (B32) structure. fcc Al dissolves considerable amounts of Li. If the fcc solid solution is quenched from high temperature, coherent metastable AlLi precipitates with an ordered fcc (L1) structure can form. There are two nearly stoichiometric phases; AlLi and AlLi. The disordered fcc phase and the ordered AlLi phase as well as the disordered bcc phase (Li) and the ordered AlLi phase are described by four-sublattice models. There is a fairly complete set of experimental data available for this system, and most of them are satisfactorily described by the currently optimised parameters.

The liquidus lines of the binary system Mn – C were studied, mainly by solubility determinations (at high carbon) and thermal analysis (at low carbon). The low carbon part of the liquidus was mapped in unprecedented detail. The poorly defined phase Mn 4 C 1 – X is probably the highest melting manganese carbide (i. e. not Mn 7 C 3 ). No (stable) peritectics exist below 1326 °C, but three eutectics were identified: one meta-stable and two stable. Also, transistion temperatures were identified for some Mn carbides, as well as γ-Mn (austenitic manganese), and δ-Mn (ferritic). Literature data were co-opted to quantify the enthalpy of melting for δ-Mn (8.3 ± 1.5 kJ · mol −1 ) which is 3 to 5 kJ lower than given by current tabulations and databases. Several transition temperatures within Mn – C (including the melting of pure δ-Mn) are markedly affected by even minor amounts of oxygen and nitrogen.

A thermodynamic dataset for the Al – C – O – Si – Y system was used for calculations of multicomponent, multiphase reactions. Some aspects of the liquid phase sintering of silicon carbide using alumina and yttria sintering additives were analyzed. The phase relations in the SiC – Al 2 O 3 , SiC – Al 2 O 3 – SiO 2 and SiC – Y 2 O 3 – SiO 2 systems were calculated. Phase fraction diagrams, isopleths, isothermal sections, and potential phase diagrams are presented to illustrate the reactions between silicon carbide and sintering additives. The effect of Ar inert gas as an additional component and the related volume change of the gas phase were considered. In addition, the influence of surface silica on silicon carbide powder is taken into account.

A complete literature review, critical evaluation, and thermodynamic modeling of the phase diagrams and thermodynamic properties of all oxide phases in the binary systems B 2 O 3 – SiO 2 and B 2 O 3 – Al 2 O 3 have been performed. The molten oxide is described by the Modified Quasichemical Model. Discrepancies in the available experimental data have been resolved and an optimized set of self-consistent thermodynamic functions of all phases has been obtained. This thermodynamic database can be used with the FactSage TM thermodynamic computing system to calculate all thermodynamic properties and phase equilibria from 25 °C to above the liquidus temperatures at 1 atm of total pressure and over the entire composition range.

A thermodynamic model of the copper P-S converter has been developed to predict the behavior of magnetite in the converter slag, amounts of slag and matte, compositions of slag and matte, volume of the bath, bath temperature, slag blow endpoint determination and SEMTECH OPC signals. The predictions of bath temperature, slag and matte compositions, and magnetite content in the slag have been validated by industrial data. The effects of fluxing strategy, returns and skim charges, oxygen enrichment, and temperature on the magnetite formation in the slag have been predicted and discussed. The effect of oxygen potential, SiO 2 /Fe in slag, detection of slag blow endpoint, and temperature on slag viscosity, liquidus temperature of converter slag and skimming operation has been modeled and discussed as well. Some applications to industrial operations have been presented.

Many projects involving nuclear fuel rest on a quantitative understanding of the co-existing phases at various stages of burnup. Since the fission products have considerably different abilities to chemically associate with oxygen, and the metal-to-oxygen molar ratio is necessarily increasing, the chemical potential of oxygen is a function of burnup. Concurrently, well-recognized small fractions of new phases such as inert gas, noble metals, zirconates, etc. also develop. To further complicate matters, the dominant UO 2 fuel phase may be non-stoichiometric and most of the minor phases themselves have a variable composition dependent on temperature and possible contact with the coolant in the event of a sheathing breach. A thermodynamic database has been in development to predict the phases in partially burned CANDU (CANada Deuterium Uranium) nuclear fuel containing the major fission products. The building blocks are the standard Gibbs energies of formation of the many possible compounds expressed as a function of temperature. To these data are added mixing terms associated with the appearance of the component species in particular phases. In operational terms, the treatment rests on the ability to minimize the Gibbs energy in a multicomponent system using the algorithms developed by Eriksson. The treatment, considered applicable in the range 300 to 2000 °C, is capable of handling non-stoichiometry in the UO 2 fluorite phase, dilute solution behaviour of significant solute oxides, noble metal inclusions, a second metal solid solution U(Pd – Rh – Ru) 3 , zirconate, molybdate, and uranate solutions as well as other minor solid phases, and volatile gaseous species. The paper highlights the current capability of an ongoing project.

The use of borates as an autocausticizing agent in kraft recovery boilers of pulp mills is an interesting concept for decreasing the lime consumption at pulp mills. The main autocausticizing reaction is between NaBO 2 and Na 2 CO 3 forming Na 3 BO 3 and CO 2 at high temperatures in a liquid phase in the recovery boiler. The thermodynamic and phase equilibrium data of the Na 2 CO 3 – NaBO 2 – Na 3 BO 3 system were evaluated and optimized. The liquid phase was modeled with the modified quasichemical model. The thermodynamic database obtained can be used to calculate the phase equilibrium and thermodynamic properties of multicomponent sodium salt mixtures of importance for the borate autocausticizing concept. The thermodynamic database is a part of an extensive thermodynamic database for alkali salt mixtures of importance for black liquor combustion in the pulp and paper industry.

Low superheat casting with and without a cooling slope was employed to produce AA6061 thixoforming feedstock. The dendritic features were more frequent in the latter, suggesting that the degeneration of the dendritic structure was promoted by the cooling slope casting process. Minor differences in the primary phase morphology in the as-cast state were no longer evident after partial melting, with nearly equiaxed polygonal grains in all ingots cast with and without a cooling slope. 5 min soaking at 640 °C was identified as the best reheating practice as it provided the smallest grains and the most complete grain boundary wetting. The slugs machined from AA6061 ingots cast from a pouring temperature of 680 °C with and without a cooling slope were both successfully thixoformed into sound parts with no signs of porosity and segregation. The T6 heat treatment produced hardness values as high as 107 HB which compare very favourably with those of the extruded products and are even better than that obtained in the thermomechanically processed feedstock.

The results of a comparative thermodynamic investigation of the Bi – Ga – Sn system, giving experimentally obtained results and calculated thermodynamic data, are presented in this paper. Oelsen calorimetry was performed for the experimental determination of thermodynamic properties in the section with a molar ratio of Bi: Sn = 1: 3, while the Redlich – Kister – Muggianu method was applied for thermodynamic predicting in the sections with molar ratio Bi: Sn = 1: 3, 1: 1, 3: 1. Thermodynamic parameters – partial and integral molar quantities, as well as gallium activities, were determined at the temperatures of 873 and 973 K. Phase equilibria in the Bi – Ga – Sn ternary system have been studied experimentally – using differential thermal analysis and scanning electron microscopy with energy dispersive X-rays, while the calculation of phase diagram was done according to the CALPHAD method. The phase diagram of the section with molar ratio of Bi: Sn = 1: 3 is given, as well as the phase diagram of the ternary Bi – Ga – Sn system at 373 K.

The grain growth process controlled by particles able to move together with grain boundaries is investigated by means of numerical simulation. The particles either located on grain boundaries or randomly distributed over the material volume are shown to retard grain growth. In the first case, the growth kinetics is described by a power law D n – D 0 n = kt with the exponent n = 3, at particle volume fractions as low as 0.001. In the case of polycrystals with randomly distributed mobile particles the growth exponent n is larger than 3 and increases with an increase in the particle size or volume fraction.