Evolution of plate-like precipitates of the Omega phase in an Al-1.9Cu-0.3M
g-0.2Ag (at.%) alloy has been investigated by three-dimensional atom probe
(3DAP) and transmission electron microscope (TEM) techniques. In an as-quen
ched alloy, all solute atoms are uniformly dissolved. After 5s aging at 180
degrees C, co-clusters of Ag and Mg atoms are present. Initially, these co
-clusters do not have well-defined shape, but after aging for 120 s, they a
re aligned on the {111} planes incorporating Cu atoms. Initially, they do n
ot have the distinct structure consistent with the Omega phase, thus these
are designated as {111}GP zones. The SL platelet which appears after the {1
11}GP zones contain all solute elements within the precipitate having a com
position of approximately 25 at.% Cu, 20 at.% Mg and 10 at.% Ag. In the wel
l-defined Omega phase which appears after 2 and 10 h aging at 180 degrees C
, Ag and Mg atoms are strongly segregated to the broad face monolayer inter
faces of alpha/Omega. No Mg and Ag are incorporated within the platelet, an
d its chemical composition is Al-33 at.% Cu. Based on these observations, i
t is concluded that the Omega phase evolves from co-clusters to plate-like
precipitates continuously, rather than precipitating discretely by heteroge
neous nucleation to their precursors. In a well-grown Omega plate, elementa
l mapping near a ledge was observed by 3DAP for the first time. (C) 1998 Ac
ta Metallurgica Inc. Published by Elsevier Science Ltd. All lights reserved
.