Acta Materialia 50 (2002) 3597–3608 www.actamat-journals.com Enhanced creep performance in an Al–Cu–Mg–Ag alloy through underageing R.N. Lumley a,* , A.J. Morton a , I.J. Polmear b a CSIRO Manufacturing Science and Technology, Private Bag 33, Clayton South MDC, Victoria 3169, Australia b School of Physics and Materials Engineering, PO Box 69M, Monash University, Melbourne, Victoria 3800, Australia Received 7 December 2001; received in revised form 5 April 2002; accepted 5 April 2002 Abstract Tests at 130 °C and 150 °C have shown that the creep resistance of an Al–Cu–Mg–Ag alloy is significantly increased if it is heat-treated at an elevated temperature to an underaged condition rather than the fully hardened, T6 temper. This beneficial effect of underageing is manifest in reduced rates of secondary creep. Similar results have been obtained for the commercial alloy 2024. Delays at ambient temperature after underageing and before testing lead to secondary precipitation and a progressive decrease in creep performance that eventually reverts to close to that for the T6 condition. This detrimental effect may be overcome by slow cooling from the underageing temperature, which arrests or impedes subsequent secondary precipitation. Microstructural observations suggest that the enhanced creep resistance in the unde- raged condition is a consequence of the presence of “free” solute in solid solution that is not yet involved in precipi- tation.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Creep; Aluminium alloys; Age hardening; Dynamic precipitation 1. Introduction Micro-alloying additions of Ag (0.1 at%) are known to modify precipitation processes in a wide range of age-hardenable Al–Cu–Mg alloys, so that their response to age hardening is enhanced [1–3]. Special interest has been shown in Al–Cu–Mg–Ag alloys with high Cu:Mg ratios in which Ag pro- motes the formation of the finely dispersed, meta- stable precipitate Omega () that nucleates and * Corresponding author. E-mail address: roger.lumley@cmst.csiro.au (R.N. Lumley). 1359-6454/02/$22.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(02)00164-7 grows as thin hexagonal plates on the {111} α planes [4–8]. The  phase is now considered to be an orthorhombic form of the incoherent, equilib- rium phase θ (Al 2 Cu) that has a tetragonal structure and forms on the {100} α planes in binary Al–Cu alloys aged at high temperatures [7,8]. The inco- herent θ phase is also the equilibrium phase that forms when the metastable body-centred tetragonal θ' phase in the form of thin octagonal plates loses coherency with the matrix in binary Al–Cu alloys aged at relatively high temperatures (e.g. 250 °C). Omega precipitates appear to maintain coher- ency along the {111} α planes at temperatures up to 200 °C, which is facilitated by segregation of the Mg and Ag atoms to the precipitate–matrix