Experimental Investigation and Thermodynamic Calculation of Mg-Al-Sn Phase Equilibria and Solidification Microstructures

Experimental work using differential scanning calorimetry, differential thermal analysis, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis was conducted on key samples with more than 33 at.% Mg in the Mg-Al-Sn system. In addition to the thermal analysis and study of the solidification microstructure, an equilibration at 400 °C was performed. The results were used together with all available phase equilibria and thermodynamic data from earlier experimentation to create a consistent thermodynamic description of the Mg-Al-Sn alloy system. No ternary stable phase was discovered, and no ternary solubilities of the binary intermetallic phases were found. With the addition of a single Gibbs energy parameter describing the interaction between Al and the associate Mg2Sn in the liquid phase, the thermodynamic model of the system could be made to describe all pertinent experimental data throughout the ternary system. The phase precipitation sequence during slow solidification of all present samples is reflected by the thermodynamic calculations as well. Intricate details of the liquid miscibility gap, occurring in ternary alloys only, are also highlighted.