Effects of Cu on precipitation in Al-Zn-Mg alloys

Several effects of Cu on the hardening and microstructure of Al-Zn-Mg alloys were observed in this work which are dependent on the ageing temperature and alloy composition. At higher ageing temperatures (greater than or equal to 190 degrees C), Cu additions can stimulate the formation of GPB zones and S (Al2CuMg) phase. At lower ageing temperatures, Ctl additions promote the rapid nucleation of eta' and eta (MgZn2) precipitates. During isothermal ageing at 150 degrees C, the addition of Cu produces rapid early hardening similar to that observed in Al-Cu-Mg alloys. Since the number density of eta' and eta (MgZn2) precipitates increases significantly during further ageing with no corresponding increase in hardness, it seems unlikely that these precipitates are solely responsible for the initial rapid hardening reaction. This suggests that Cu introduces clustering or precipitation processes which are additional to those observed in ternary Al-Zn-Mg alloys.