An assessment of thermodynamic equilibria in the Ag-Al-Cu-Mg quaternary system in relation to precipitation reactions

Thermochemical assessments of phase equilibria in the binary and ternary alloy subsystems which describe the addition of Ag to Al-Cu-Mg ternary alloys have been undertaken. The assessed Gibbs free energies show that the ternary intermetallic AlMgAg, silver-rich T-Al(Ag,Cu)Mg and AgMg3 phases are introduced into Al-rich alloys of the Al-Cu-Mg ternary system for atomic ratios Ag/Mg 1 the B2 phase AgMg and the Mg-based hcp solid solution become stable. Phase diagrams computed from the assessed Gibbs free energies for Al-Cu-Mg alloys with 0.1 at.% Ag agree with observations of the enhanced stability of the phase T-A132(Ag,Cu,Mg)49 relative to the θ-Al 2 Cu phase compared with the ternary Al-Cu-Mg system, and the enrichment of Ag in the T phase in the Al-Cu-Mg-(Ag) quaternary system compared with its Cu content in the Al-Cu-Mg ternary system. The assessment accepts the Ω precipitation as a growth morphology of θ-A1 2 Cu and strongly suggests that the ternary intermetallic AlMgAg rather than AgMg3 is the precursor on which it nucleates.