IDENTIFICATION OF ASSOCIATES OF BINARY METAL SOLUTIONS IN ANALYTICAL FORM

Binary liquid-metal thermodynamic systems with strong negative deviations from Raoult’s law can be satisfactorily described within the model of ideal associated solutions when three types of associates are participating: Ар В, АВ and АВq , where А and В are the solution components; р and q are stoichiometric coefficients numerically equal to 2, 3 or 4. More complex associates, Ар Вq in particular, do not form in the solution. In these conditions, according to the rule of phases of associated solutions, the number of associate types in each point of a binary solution must not be greater than two. Since Ар В and АВq associates interact chemically producing the АВ associate, a subsystem containing only Ар В and АВ associates and a subsystem containing only АВ and АВq associates can be conventionally formed out of the complete thermodynamic system.This method allows determining and expressing concentrations and thermodynamic properties of the assumed associates in an explicit analytical form in each subsystem. The obtained results, however, are normally checked by the numerical solution of a system of initial equations with set calculated thermodynamic parameters. Two separate calculated solutions at the junction of subsystems are “jointed” using the previously proposed special functions imitating diffusive leveling of the associate concentration difference in this nonequilibrium zone. Identification of real solution associates with this method can be considered as valid and not requiring any adjustable parameters if calculated free energies of the associates are close to the reference values of free energies of the respective intermetallides. For example, when experimental data of component activities at 1600 °C were analyzed in the Ni – Al system for the identified associates Ni₃Al, NiAl and, the free formation energies were found to be 129.6, 93.1 and 124.2 kJ/mole, respectively. These values are close to the average formation energy values of the respective intermetallides specified in the modern data bases, i.e. 130.1, 92.5 and 126.0 kJ/mole, respectively. Similar analyses were also performed for seven other binary systems with aluminum, including systems containing one or two types of as sociates. The absolute error of approximation of activity isotherms of solution components for this method was 0.001 – 0.035.

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