THE STRUCTURE OF METASTABLE (111)(ALPHA) PRECIPITATES IN AN AL-2-CENTER-DOT-5 WT-PERCENT CU-1-CENTER-DOT-5 WT-PERCENT MG-0-CENTER-DOT-5 WT-PERCENT AG ALLOY

Electron microdiffraction experiments have been used to study a fine metastable precipitate (designated X) that forms as platelets on the {111}(alpha) planes of the alloy Al-2.5 wt% Cu-1.5 wt% Mg-0.5 wt% Ag, 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)(X)\\(111)(alpha) and [<10(1)over bar 0>](X)\\[1(1) over bar0$](alpha). 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.