The precipitates in Cu or Ag added excess Mg type Al–Mg–Si (ex.Mg) alloys at over aged condition have been observed by high resolution transmission electron microscope (HRTEM), classified according to characteristic of their HRTEM images, and compared with those in the ex.Mg alloy to understand the effect of Cu and Ag addition on their precipitation sequence. HRTEM images obtained for cross sections of precipitates in the Ag added ex.Mg (ex.Mg–Ag) alloy have been classified into five types according to the periodicity of bright dots in them; the random-type, parallelogram-type precipitates, parallelogram-type (60°) precipitate having interior angle about 60°, β″ and β′-phases. The β′-phase in this alloy was predominant in later stage of aging. HRTEM images obtained for cross sections of precipitates in the Cu added ex.Mg (ex.Mg–Cu) alloy have been also classified into the same five types of precipitates as those in ex.Mg–Ag alloy. In addition, the Q′ phase has been also observed in this alloy and it was predominant in later stage of aging. Different shape of precipitate has been also confirmed in both alloys. It is the lath shaped precipitate having elongated cross section, and it has been regarded as the elongated type precipitate. This elongated type precipitate in ex.Mg–Cu alloy was predominant in later stage of aging, although there were a few in ex.Mg–Ag alloy.
