PRECITITATE STABILITY IN AL-CU-MG-AG ALLOYS AGED AT HIGH-TEMPERATURES

A study has been made of the thermal stability of the OMEGA phase in Al-Cu-Mg-Ag alloys aged at high temperatures (200 to 350-degrees-C). This phase, which precipitates as thin plates on the {111}alpha planes, has been shown to be replaced by the equilibrium precipitate theta (Al2Cu) after prolonged ageing (e.g. 2400 h at 250-degrees-C). Measurements have been made of the thickening behaviour of the OMEGA plates and the various orientations and morphologies of the theta phase have been characterised. Whilst there is some evidence for the direct allotropic transformation of OMEGA to theta, it is concluded that a gradual dissolution/re-precipitation mechanism dominates the changes to microstructure at these high temperatures. Although magnesium and silver are known to segregate to the OMEGA phase, they were not detected in association with theta. Rather they were found to partition to sites of the S phase (Al2CuMg) which forms as a minor precipitate under these ageing conditions.