Nanostructure of aluminium alloy 2024: Segregation, clustering and precipitation processes

Variations in solute element distribution occurring in a commercial 2024 aluminium alloy during isothermal ageing treatments at 170 °C for up to 120 h have been characterized using atom probe tomography. An early (0.5 h at 170 °C) rapid increase in hardness was correlated with the formation of fine scale (average 24 atom) solute clusters, comprising principally Mg and Cu, but with minor concentrations of Si and Zn. There was, in addition, evidence of significant segregation of Mg, Cu and Si to at least some fraction of grain boundaries and existing matrix dislocations. At peak hardness (80 h at 170 °C) the microstructure comprised coarse precipitates of S phase, with a composition approaching stoichiometric Al₂CuMg, a dense distribution of Guinier-Preston-Bagaryatsky zones elongated parallel to 〈1 0 0〉 in a matrix of α-Al and a residual distribution of smaller equiaxed solute clusters. Both the clusters and zones contained predominantly Mg and Cu, with minor concentrations of Si and Zn. The S phase contained small but significant (0.5-1.8 at.%) concentrations of Si, which was non-uniformly distributed in elongated domains within the laths of the S phase. In overaged samples (114 h at 170 °C) the microstructure comprised almost exclusively coarse S phase, Al₂Mg(Cu,Si), in assemblies suggestive of a combination of precipitate coarsening and coalescence.