Activities, concentration fluctuations and complexing in liquid Ca–Al alloys

Abstract

Activity of calcium in Ca–Al alloys (X_Ca<0.38) was determined by the Knudsen effusion technique. Activity of aluminium in Ca-rich alloys (X_Ca>0.44) was calculated from the measured distribution coefficient for aluminium between the alloy and a thin foil of pure iron, using data for the Fe–Al system reported in the literature. By combining the results of Knudsen effusion and distribution studies with the Gibbs-Duhem relation, activities of both components and integral Gibbs energy of mixing for the complete composition range have been obtained at 1373 K. The concentration-concentration structure factor of Bhatia and Thornton at zero wave vector has been computed from the thermodynamic data. The mean square thermal fluctuation in composition exhibits a minimum for the Ca–Al system at X_Ca=0.26, suggesting the presence of a strong complex. The thermodynamic properties of the liquid alloy can be described by the associated solution model, with Al₂Ca as the predominant complex. The thermodynamic properties computed on the mole fraction basis are more compatible with experimental data than those based on volume fraction.