Effects of silicon additions on aging behaviour of Al–1·52Cu–0·75Mg alloy

Abstract

The 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 as θ′ (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