Preprint of a manuscript submitted to Acta Materialia EVOLUTION OF W PHASE IN AN Al-Cu-Mg-Ag ALLOY Ð A THREE DIMENSIONAL ATOM PROBE STUDY

Evolution of plate-like precipitate of the W phase in an Al-1.9Cu-0.3Mg-0.2Ag (at.%) alloy has been investigated by three dimensional atom probe (3DAP) and transmission electron microscope (TEM) techniques. In an as-quenched alloy, all solute atoms are uniformly dissolved. After 5 s aging at 180¡C, coclusters of Ag and Mg atoms are present. Initially, these co-clusters do not have well defined shape, but after aging for 120 s, they are aligned on the {111} planes incorporating Cu atoms. Initially, they do not have the distinct structure consistent with the W phase, thus these are designated as {111}GP zones. The W platelet which appear after the {111}GP zones contain all solute elements within the precipitate having a composition of approximately 25at.%Cu, 20at.%Mg and 10at.%Ag. In the well-defined W phase which appear after 2 and 10 h aging at 180¡C, Ag and Mg atoms are strongly segregated to the broad face monolayer interfaces of a/W. No Mg and Ag are incorporated within the platelet, and its chemical composition is Al-33at.%Cu. Based on these observations, it is concluded that the W phase evolves from co-clusters to plate-like precipitates continuously, rather than precipitating discretely by heterogeneous nucleation to their precursors. In a wellgrown W plate, elemental mapping near a ledge was observed by 3DAP for the first time. On leave from Department of General Physics, Eštvšs University, Budapest, H-1145, P.O.B. 323, Hungary