Phases and microstructures of high Zn-containing Al-Zn-Mg-Cu alloys

Phases and microstructures of three high Zn-containing Al-Zn-Mg-Cu alloys were investigated by means of thermodynamic calculation method, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) analysis. The results indicate that similar dendritic network morphologies are found in these three Al-Zn-Mg-Cu alloys. The as-cast 7056 aluminum alloy consists of aluminum solid solution, coarse Al/Mg(Cu, Zn, Al)(2) eutectic phases, and fine intermetallic compounds eta(MgZn2). Both of as-cast 7095 and 7136 aluminum alloys involve alpha(Al), eutectic Al/Mg(Cu, Zn, Al)(2), intermetallic eta(MgZn2), and theta(Al2Cu). During homogenization at 450 degrees C, fine eta(MgZn2) can dissolve into matrix absolutely. After homogenization at 450 A degrees C for 24 h, Mg(Cu, Zn, Al)(2) phase in 7136 alloy transforms into S(Al2CuMg) while no change is found in 7056 and 7095 alloys. The thermodynamic calculation can be used to predict the phases in high Zn-containing Al-Zn-Mg-Cu alloys.