Solute clustering and precipitation in an Al-Cu-Mg-Ag-Si model alloy

Solute clustering and precipitation in an Al-Cu-Mg-Ag-Si model alloy has been investigated by atom probe tomography (APT) as well as high-angle annular dark-field (HAADF) imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM). Nine types of solute clusters (Cu, Ag, Mg-Cu, Mg-Ag, Mg-Cu-Si, Mg-Ag-Si, Mg-Ag-Cu, Cu-Ag-Si and MgAgCuSi) were observed by APT in both the as-quenched alloy and after ageing the alloy at 180 degrees C for 1 h. Three types of precipitates (Omega (AlCuMgAg), theta (Al2Cu) and Mg2Si) were observed by APT and HAADF-STEM after further ageing at 180 degrees C for 24 h and 100 h. We propose that MgAgCu and MgAgCuSi clusters are likely to be responsible for the formation of the Omega (AlCuMgAg) phase. Furthermore, we also suggest that the theta (Al2Cu) phase forms from Cu clusters and the Mg2Si phase forms from the decomposition of MgAgSi and MgAgCuSi clusters by losing Ag to Omega phase growth. Many early binary clusters (Mg-Cu, Mg-Ag) do not seem to undergo a significant further growth during ageing; these are more likely to be transformed into complex ternary and quaternary clusters and be subsequently consumed during the growth of large clusters/precipitates. Furthermore, it is proposed that the plate-like Omega (AlCuMgAg) precipitates evolve continuously from the MgAgCu and MgAgCuSi clusters, rather than via heterogeneous nucleation on their precursors (i.e. MgAgCu and MgAgCuSi clusters). More interestingly, even after ageing at 180 degrees C for 100 h, the Omega (AlCuMgAg) precipitates remain coherent with the alpha-Al matrix, indicating that these precipitates have a high thermal stability. This can mainly be attributed to the presence of a single Mg-Ag-rich monolayer observed at the interface between the Omega precipitate and the alpha-Al matrix, significantly improving the coarsening resistance of the Omega (AlCuMgAg) precipitates. Our results thus reveal links between a variety of solute clusters and the different types of precipitates in the Al-Cu-Mg-Ag-Si model alloy. Such information can in the future be used to control the precipitation by tailoring solute clustering.