Solute clustering in Al–Cu–Mg alloys during the early stages of elevated temperature ageing R.K.W. Marceau a,b, * , G. Sha a,b , R. Ferragut c , A. Dupasquier c , S.P. Ringer a,b a Australian Centre for Microscopy & Microanalysis, The University of Sydney, Madsen Building F09, NSW 2006, Australia b ARC Centre of Excellence for Design in Light Metals, The University of Sydney, NSW 2006, Australia c Dipartimento di Fisica, LNESS and CNISM, Politecnico di Milano, Via Anzani 42, 22100 Como, Italy Received 3 August 2009; received in revised form 6 May 2010; accepted 6 May 2010 Available online 22 June 2010 Abstract 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 °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. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Aluminium alloys; Age hardening; Solute clustering; Atom probe tomography; Positron annihilation 1. Introduction Al–Cu–Mg alloys that lie in the a + S phase field and have a medium/low Cu:Mg ratio (0.5–2) exhibit a two- stage age hardening response, separated by a prolonged hardness plateau, upon isothermal T6 ageing (solution heat treated, quenched and artificially aged) between 110 and 240 °C [1–3]. The first stage of hardening is extremely rapid and usually accounts for up to 70% of the overall hardness increment within the first 60 s of ageing, whereas the sec- ond hardness increase takes much longer, often many days, depending on the ageing temperature [4–14]. This rapid hardening (RH) phenomenon occurs in Al–1.1Cu–xMg alloys that contain at least 0.5 Mg (at.%), but not in binary Al–Cu and Al–Mg alloys or ternary Al–Cu–Mg alloys with phase compositions in the a + h field (high Cu:Mg ratio), even though they are all age-hardenable [15,16]. Rapid early hardening is not only restricted to the Al–Cu–Mg sys- tem, but also occurs in other systems such as Al–Zn–Mg alloys with small additions of Cu [17–19], Al–Cu–Mg (a + S) alloys microalloyed with Ag [2,6,20–22] and has more recently been observed in certain maraging steels [23]. Understanding the origin of the RH phenomenon in the Al–Cu–Mg system has attracted significant research interest [5–13,20,22,24–36] and various techniques have been employed to characterise the various processes of micro- structural evolution in order to reveal the underlying mech- anisms. Several different mechanisms have been proposed. Following Silcock (1961) [37], it was widely accepted that the first stage was due to the formation of fully-coherent precipitates called Gunier–Preston–Bagaryatsky (GPB) zones, and the second stage was associated with the precip- itation of the S phase. In 1997, Ringer et al. [5] used the technique of atom probe together with field ion microscopy to discover pre-precipitate atomic co-clusters of Cu and 1359-6454/$36.00 Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2010.05.020 * Corresponding author at: Australian Centre for Microscopy & Micro- analysis, The University of Sydney, Madsen Building F09, NSW 2006, Australia. Tel.: +61 2 9036 9107; fax: +61 2 9351 7682. E-mail address: ross.marceau@sydney.edu.au (R.K.W. Marceau). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 58 (2010) 4923–4939