The thermodynamic properties of solid Al−Mg alloys

By measurements of the electromotive force of reversible galvanic cells of the form Mg (solid)/MgCl₂ in fused LiCl/KCl eutectic/Mg _x Al _y (solid) between 370° and 550°C, values have been determined for the free energies, entropies, and heats of formation of all the solid phases of the Al?Mg binary alloy system. The aluminum-rich solid solution appears to be essentially regular with a small positive heat of formation, while the magnesium-rich terminal solution shows negative heats and entropies of formation. The intermediate phases β, γ, and ε all exhibit moderate negative heats and large negative excess entropies of formation; in the case of β and γ these properties pass through pronounced minima, at compositions within their homogeneity ranges, indicative of highly ordered stoichiometric conditions. The results are discussed in relation to known phase structures.