The structure of primary strengthening precipitates in an Al-1.5wt% Au-4.0wt% Mg-0.5wt% Ag alloy

Convergent-beam electron diffraction (CBED) has been used to study the primary strengthening precipitate phase (designated Z) in an alloy Al- 1.5wt°/oCu-4.0wt%Mg-0.5wt%Ag, aged at 473 and 513K. The CBED patterns obtained from individual precipitates indicated that they have a cubic structure, with unit-cell parameter ao = l'999nm The relative displacement of the first-order Laue zone net with respect to the zero-layer plane established a fcc Bravais lattice for the Z-phase; the whole pattern symmetry defined a point group m3m. Two orientation relationships were, identified with the matrix α-phase: (100)z‖(100)α, [010]z‖[010]α and (011)z‖(011)α, [011]z‖[011]α, the former being more dominant at shorter ageing times. Although earlier work on similar alloys suggested that these precipitates had the bcc T-phase structure of Bergman, Waugh and Pauling, the present results indicate that the Z-phase is not an isomorph of the T-phase that has been observed in the ternary Al-Zn-Mg and Al-Cu-Mg alloy systems. The Z-phase is also distinct from the metastable hexagonal X-phase that has been observed in the Al-Cu-Mg-Ag alloys which lie in the (α+S)-phase field, and which was also originally considered to be an isomorph of the T-phase of Bergman et al.