Structural changes associated with solid-state reactions in Al–Ag–Mg alloys

The structural changes associated with the precipitation of metastable and equilibrium phases during decomposition of three supersaturated Al-rich Al–Ag–Mg alloys at 393, 456 and 508 K have been established by analysing the diffraction of monochromatic X-radiation by single-crystal alloy plates. Precipitated phases observed were: (a) in the Al–0.54 at.% Ag–4.49 at.% Mg alloy, type 1 Guinier–Preston zones, T′ phase, T phase; (b) in the Al–0.87 at.% Ag–1.80 at.% Mg alloy, type 1 Guinier–Preston zones, type 2 Guinier–Preston zones (a newly observed body-centred tetragonal compound, a = 4.13, c = 8.60 Å, space group I 4₁/amd), T′ phase, η phase; (c) in the Al–1.30 at.% Ag–1.05 at.% Mg alloy, type 1 Guinier–Preston zones, type 2 Guinier–Preston zones, γ phase. Volume fractions of the precipitated phases, sizes of the precipitates and orientations of cell axes relative to the axes of the Al lattice are given for various stages of each decomposition sequence.