Metastable Phases in Al-Zn-Mg Ternary Based Alloys

In order to investigate the nature of metastable phase appeared in super-high strength aluminium alloys (Mesoalite), several alloys were prepared by changing the Zn/Mg composition ratio, while Cu, Mn and Ag solute concentrations were kept constant. The effective Zn/Mg ratio dependence of the volume fraction (Vf) of metastable precipitates and the electron density difference (Δρ) between precipitate and matrix was determined from the small angle scattering intensity. The change of solute atom partition within the grains was previously reported to take place mainly for Zn and Mg during isothermal aging at 383 K. Based on the previous result, the phase diagrams of Al-Zn-Mg ternary systems were calculated by using a computer soft, ThermoCalc. In order to consider the coherent interface between matrix and metastable precipitates, the elastic strain energy was introduced into the chemical free energy. Two phases equilibria were calculated for α′+T′ and α′+η′ systems. The calculated results of Vf and Δρ could explain well the experimental data. Consequently, the metastable T′ phase is suggested to precipitate in the specimens with effective Zn/Mg ratio less than unity, while the η′ metastable phase appears in the specimen with the ratio of about 2.