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 A l-Cu-Mg-Ag alloys aged at high temperatures (200 to 350-degrees-C). Th is phase, which precipitates as thin plates on the {111}alpha planes, has been shown to be replaced by the equilibrium precipitate theta (Al 2Cu) after prolonged ageing (e.g. 2400 h at 250-degrees-C). Measuremen ts 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 allotropi c transformation of OMEGA to theta, it is concluded that a gradual dis solution/re-precipitation mechanism dominates the changes to microstru cture at these high temperatures. Although magnesium and silver are kn own to segregate to the OMEGA phase, they were not detected in associa tion 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.