Solute clustering, segregation and microstructure in high strength low alloy Al–Cu–Mg alloys

In this work, atom probe field ion microscopy and transmission electron microscopy were used to study solute clustering, segregation and microstructure in two Al–Cu–Mg alloy systems, having different Cu:Mg ratios. The microstructural phases in a Ge modified Al–Cu–Mg alloy having a base composition in the α+θ field of the Al–Cu–Mg phase diagram exhibited an internal structure similar to a twinning type contrast, suggesting that the initial clustering and segregation of microalloying elements may modify the stacking fault energy of the resulting precipitate phases. The effect of Si in combination with Ge did not produce internal precipitate structures, although a complex series of clustering and precipitation processes were identified. The results of nanoscale compositional analyses of precipitation in an Al–Cu–Mg alloy containing 0.1 at.% Ag and having a base composition lying in the α+T phase field are also reported. This system is of interest since Ag has been shown to stimulate the formation of a new precipitate phase in this alloy, recently designated Z. The present work shows that this phase is rich in Mg, Cu, Al and contains up to ∼5.0 at.% Ag within the bulk of the phase, suggesting that the nucleation of this phase may be similar to a number of other metastable precipitates which form in related Al–Cu–Mg–Ag alloys.