Thermodynamic properties of liquid Al-Mg alloys measured by the Emf method

Abstract

The thermodynamic properties of liquid Al-Mg alloys containing 0.027 to 95.50 at. Pct Mg were determined by measuring the emf, between 973 and 1073 K, of a magnesium concentration cell of the type Mg(l)|MgCl₂-CaCl₂ (eutectic melt, l)|Mg (in Al, l). Special attention has been given to low-magnesium aluminum alloys which are most commonly used in industry and for which definitive thermodynamic data are not reported in the literature. Alloys containing up to 12 at. Pct Mg follow Henry’s law, and the magnesium activity is given by the relation a_MG = 0.88 X_MG at 1073 K. Above 12 at. Pct Mg, the magnesium activity shows a small negative deviation from the ideal solution behavior. The activity of aluminum, however, closely follows the ideal solution behavior up to 75 at. Pct Mg and thereafter it shows a small negative deviation. The emf data have also been used to determine the free energy, entropy, enthalpy, and excess free energy for liquid Al-Mg alloys.