The microstructural evolution during low temperature ageing of two commercial purity alloys (Al-1.2Cu-1.2Mg-0.2Mn and Al-1.9Cu-1.6Mg-0.2Mn at.%) was investigated. The initial stage of hardening in these alloys is very rapid, with the alloys nearly doubling in hardness during 20 h ageing at room temperature. The microstructural evolution during this stage of hardening was investigated using differential scanning calorimetry (DSC), isothermal calorimetry and threedimensional atom probe analysis (3DAP). It is found that during the hardening a substantial exothermic heat evolution occurs and that the only microstructural change involves the formation of Cu-Mg co-clusters. The kinetics of cluster formation is analysed and the magnitude of the hardening is discussed on the basis of a model incorporating solid solution hardening and modulus hardening originating from the difference in modulus between Al and clusters.

Abstract. The hardening and microstructural evolution during ageing of a Al-1.2Cu-0.5Mg and a Al-1.2Cu-1.2Mg (at.%) alloy has been investigated. Artificial ageing at 150°C of stretched and naturally aged samples initially (up to about 48 h) leads to very limited further strengthening, but ageing at 190°C results a quick increase in strength. Detailed microstructural investigation using differential scanning calorimetry, transmission electron microscopy and three-dimensional atom probe demonstrated that hardening at 150ºC is mostly dominated by the formation of solute clusters with varying compositions and plate-like zones rich in copper at early stages of ageing (t<24h) and by the formation of S phase at the later stages of ageing. At higher 190ºC no zones or clusters form and the ageing is dominated by the formation of S precipitates.

The maximum separation method is a useful technique for the identification of solute clusters within 3-dimensional atom probe (3DAP) data. However, the method requires the selection of appropriate parameters that will correctly identify solute clusters without incorrectly identifying random variation in solute atom separations as being due to cluster formation. A simple analytical approach has been used to estimate the apparent number of clusters that would be expected using the maximum separation method in a random solute distribution of a given overall composition. Results of the analytical model have been found to give a good match with tests where the maximum separation method has been applied to randomised experimental 3DAP data. The model allows calculation of suitable parameters for accurate identification of clusters under a range of different conditions. A Poisson probability distribution has been used to formalise the errors involved in the measurement of the number densities of clusters or precipitates within 3DAP data. This approach also allows an upper limit for the precipitate density to be defined in cases where no precipitates are observed within a volume of analysis.

Hot rolled Al-6Li-1Cu-1Mg-0.2Mn (at.%) (Al-1.6Li-2.2Cu-0.9Mg-0.4Mn, wt.%) and Al-6Li-1Cu-1Mg-0.03Zr (at.%) (Al-1.6Li-2.3Cu-1Mg-0.1Zr, wt.%) alloys developed for age forming were studied by tensile testing, electron backscatter diffraction (EBSD), three-dimensional atom probe (3DAP), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). For both alloys, DSC analysis shows that ageing at 150°C leads initially to formation of zones/clusters, which are later gradually replaced by S phase. On ageing at 190°C, S phase formation is completed within 12 h. The precipitates identified by 3D atom probe and TEM can be classified into (a) Li-rich clusters containing Cu and Mg, (b) a plate-shaped metastable precipitate (similar to GPB2 zones/S''), (c) S phase and (d) δ' spherical particles rich in Li. The Zr containing alloy also contains β' (Al 3 Zr) precipitates and composite β'/δ' particles. The β' precipitates reduces recrystallisation and grain growth leading to fine grains and subgrains.

The 3D atom probe has been used to characterise the precipitates in two Licontaining 2xxx series aluminium alloys: Al-6Li-Cu-Mg-0.2Mn (Alloy A) and Al6Li-Cu-Mg-0.03Zr (Alloy B). The alloys have been heat-treated at 150°C for a range of times varying from 6h to 24h. In Alloy A, the particles identified can be classified into two categories: those rich in Cu and Mg (similar to GPB zones and S' precipitates) and δ', spherical particles rich in Li. Clusters rich in Li, are observed in addition to these particles. The same phases are observable in the Alloy B, though the growth of the δ' phase precipitates is accompanied by the formation of a Zr-rich phase similar to β'.