Precipitation in Al-Cu-Mg-Ag Alloys

A new phase (designated 0) which precipitates in Al-Cu-Mg-Ag alloys having high Cu:Mg ratios, is known to contribute significantly to the overall strengthening of these alloys. Indeed the conunercial alloy 201, in which this phase forms, has the highest tensile and yield strengths of all aluminium casting alloys, and may be cost effective in replacing high strength wrought alloys for certain applications. The n phase precipitates on the {111} matrix planes as hexagonal plates which may develop very high aspect ratios (up to 400:1). Li ttle is known, however, about the nucleation and growth of this phase and one of the aims of this thesis was to investigate the mechanism of precipitation of 0.A series of alloys with 4 wt% copper and varying magnesium and silver contents was produced in order to study the characteristics of the 0 phase and determine its range of stability with respect to other precipitates that may form on ageing. The presence of 0 was detected by Transmission Electron Microscopy and selected area diffraction techniques in alloys with Cu :Mg ratios of between 15:1 and 1:1 at % although it was only found that 0 was the dominant precipitate in an alloy with a Mg:Ag ratio of 3:1 at ~ 0. Differential Scanning Calorimetry was used to fur ther investigate these alloys and one important result was that, during heating, then phase nucleates at a significantly lower temperature than other phases (8' and S') that form in Al-Cu-Mg alloys. This result (i) suggests that the 0 precipitates are energetically easier to nucleate