Thermodynamic assessment of the Ag-Mg binary system

The heats of decomposition of γ-AgMg 4 and e'-Ag 17 Mg 54 have been measured as 1800 ± 200 Jmol -1 for the peritectoid reaction γ → (Mg) + e' at 469°C and 19 ± 1 kJmol -1 for the peritectic reaction e' → β' + Liq at 491°C. A thermodynamic assessment of phase data for the full range of the Ag-Mg binary alloy system reproduces the features of the phase diagram except for details of the ordered α' and β' phase fields. The single-parameter order-disorder model, chosen for its compatibility with models for higher-order systems, is insufficiently flexible to reproduce the variation in order in these phases. The sequence of four reactions observed on heating γ-AgMg 4 indicates that the liquid composition in the peritectic decomposition of e'-Ag 17 Mg 54 is approximately 81 at.% Mg.