AbstractThe precipitation behaviour in Al–1·52Cu–0·75Mg alloys containing 0, 0·23, 0·49, and 1·03%Si has been investigated by scanning electron microscopy, transmission electron microscopy, and differential scanning calorimetry. In the solution treated condition, the Si free and 0·23%Si alloys had a single phase structure, but on aging precipitation of the S′(Al2CuMg) phase was observed. In the higher Si alloys, completely different precipitation behaviour and the presence of other phases, such asθ′(CuAl2), were observed. These alloys contained up to 1·34 vol.-% of the insoluble phase Q(Al5Cu2Mg8Si6). The formation of the insoluble phase was found to change the solute contents of the matrix such that the Cu/Mg ratio changed from 2 in 0·23%Si alloy to 23 in 1·03%Si alloy. In the Al–1·52Cu–0·75Mg alloy, which retains up to ~0·3%Si in solid solution, precipitation of S′phase was observed. At concentrations above ~0·3%Si, the matrix becomes depleted in solute as a result of the formation of the insoluble Al, Cu, Mg, Si bearing quarternary Q phase. The significant change in precipitation behaviour due to small changes in the Si content of the alloys is attributed to this rapid depletion of the matrix in solutes as a result of the formation of insolubles.MST/714