Solute clustering in Al-Cu-Mg alloys during the early stages of elevated temperature ageing

The evolution of atomistic-level nanostructure during the early stages of elevated temperature ageing of rapid hardening (RH) Al-Cu Mg alloys has been characterised by a combination of atom probe tomography (APT), transmission electron microscopy (TEM) and positron annihilation spectroscopy (PAS). APT analysis confirms that significant dispersions of small solute clusters form during ageing for 60 s at 150 degrees C. No zone-like precipitate structures were observed by TEM and APT examinations. These small clusters are believed to be responsible for the RH effect. Careful quantitative APT analysis reveals that a high density of Cu-Mg clusters with high Mg:Cu ratio gives the most potent strengthening response. Positron annihilation measurements also show that Cu Mg clusters provide additional sites for vacancy stabilisation. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.