The structure of metastable 111, precipitates in an Al-2.5 wt0025 Cu-1.5 wt0025 Mg-0.5 wt0025 Ag alloy

Electron microdiffraction experiments have been used to study a fine metastable precipitate (designated X) that forms as platelets on the [111]_aplanes of the alloy Al-2.5 wt0025 Cu-1.5 wt0025 Mg-0.5 wt0025Ag, aged at 513 K. Microdiffraction patterns observed from individual platelets indicated that they have a hexagonal structure with the unit-cell parameters a = 0.496-nm and c = 1.375 nm, and an orientation relationship of (0001)a- (111)_aand [1010]a- [110]a. Although earlier work by Bergman, Waugh and Pauling on similar alloys suggested that the platelets had a b.c.c. T-phase structure, the present results indicate that X phase is not in fact an isomorph of the T-phase that has been observed in the ternary Al-Zn-Mg and Al-Cu-Mg alloy systems.