Precipitation in Al-Mg-Si alloys with Cu additions and the role of the Q' and related phases

Application prospects in the automotive industry have led to extensive studies of Al-Mg-Si and Al-Mg-Si-Cu alloys. Ternary Al-Mg-Si alloys have been reported to go through the sequence of precipitation stages, GP -->beta" --> beta' -->beta, during artificial aging. Recent results indicate that the precipitation sequence is considerably more complex, and the precipitates more numerous and varied depending upon the relative amounts of the different solutes present, including Cu. Conflicting reports of the metastable phases and their sequences are compared and critically analyzed. All of these alloys have been reported to be strengthened primarily by the beta'' phase. The source of such strengthening in quaternary alloys is critically analyzed in light of conflicting reports on the ubiquity of beta'' and our studies on the role of the Q' phase.