Activities, Concentration Fluctuations and Complexing in Liquid Ca–Al Alloys

Abstract

Activity of calcium in Ca–Al alloys (XCa<0.38) was determined by the Knudsen effusion technique. Activity of aluminium in Ca-rich alloys (XCa>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 XCa=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.