Abstract:The evolution of undissolved phases in the high-zinc content super-high strength aluminum alloy during ageing was investigated by means of SEM and EDS. The results show that undissolved phases of Cu-richM(AlZnMgCu) exist in the silver-free alloy at solid-solution state. With increasing the ageing time, the precipitation of age-hardening precipitates MgZn₂stimulates Zn atoms within the undissolved phases to diffuse into the matrix, and thus the Cu content in theM
(AlZnMgCu) phase increases relatively. For the silver-bearing alloy, small addition of Ag promotes the formation of AgrichM(AlZnMgCuAg) undissolved phases and deteriorates mechanical properties of the alloy. At the early stage of ageing, Ag content within theM(AlZnMgCuAg) phases greatly decreases due to rapid diffusing of Ag atoms into the matrix
and the co-clustering of Ag and Mg atoms. As the , ageing time prolonging, the precipitation of MgZn₂results in the decrease of Zn content in the undissolved phases, and the relative increase of Ag and Mg contents.

Key words: Al-Zn-Mg-Cu; undissolved particles; ageing; SEM; EDS