Evolution of Omega phase in an Al-Cu-Mg-Ag alloy - A three-dimensional atom probe study

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 .