Dilute solutions in molten metals and alloys

Abstract

Measurements have been made of the activity coefficients of sulphur in molten copper and in binary alloys of copper with gold, silicon, platinum, cobalt, iron and nickel at temperatures between 1100° and 1500°C. The results indicate that the thermodynamic properties of the dissolved sulphur either in elementary metals or in alloys, cannot be explained solely on the basis of the electron chemical potential of the metals or the corresponding electron: atom ratio. A chemical approach has therefore been developed and this leads to a qualitative relation between the activity coefficients of sulphur in elementary metals and the stabilities of the metal sulphides. It leads to a semi-quantitative relation between the effect of an alloying element at high dilution on the activity coefficient of sulphur at low concentrations. The effect is a function of the activity coefficients of sulphur in the two elementary metals and the activity coefficient of the alloying element. This relationship has been further developed to apply at high concentrations of an alloying element and tested with success on a variety of ternary systems. It follows, as previously suggested by Darken on purely thermodynamic grounds, that the integral free energy of mixing of a binary system can be determined within about 1 kcal by experiments of the type reported here.