The effect of silver on microstructural evolution in two 2xxx series Al-alloys with a high Cu:Mg ratio during ageing to a T8 temper

The precipitation behaviour in two high Cu:Mg ratio 2xxx series Al-alloys, 2022 and 2139, has been investigated in detail during artificial ageing to a T8 temper; the main difference in composition between the two alloys being a similar to 0.3% silver addition in 2139 and a slightly higher Mg level. The addition of Ag in 2139 lead to enhanced ageing kinetics and a significant increase in strength, due to promotion of the Omega phase, which became the dominant age hardening precipitate. Some Omega was also observed in the Ag-free 2022 alloy, although Omega precipitation has not been previously reported in 2xxx series alloys with such low Mg levels. The S and sigma phase were also found to nucleate heterogeneously as minor constituents in both alloys, although the size of the sigma phase was reduced in the Ag bearing alloy. Finally, the weak remaining residual stresses in thick plate material, after stretch ng, were found to bias the habit plane selection of Cu GP zones and the theta '' metastable phases in the 2022 material, but had no effect on the Omega phase predominantly present in 2139. (C) 2008 Elsevier B.V. All rights reserved.