A simple model revealing the evolution of mechanical properties in Al-Zn-Mg-Cu alloys with a rich Al angle based on CALPHAD

A simple model, namely the equivalence precipitation model of η-type precipitate, has been established based on CALPHAD for the Al-Zn-Mg-1.5Cu alloy with a rich Al angle. The relationship of the theoretical mass fraction of η-type precipitate, the total content of Zn and Mg, and the Zn/Mg ratio is disclosed through the equivalence precipitation model. Moreover, the evolution of microstructure and mechanical properties in alloys with different theoretical mass fractions of η-type precipitate are explored. The findings imply that the fluctuation of theoretical mass fraction of η-type precipitate in the alloy primarily impacts the precipitation behavior of η-type precipitate. The increase of theoretical mass fraction of η-type precipitate leads to a higher volume fraction of η' phase, resulting in an improvement in strength. However, alloys with a higher theoretical mass fraction of η-type precipitate are inclined to form the quench-induced η phase, showing higher quench sensitivity. These results are attributed to the regulation of the precipitation behavior by the total content of Zn and Mg and the Zn/Mg ratio with different theoretical mass fractions of η-type precipitate. Experimental verification has demonstrated that the equivalence precipitation model can effectively predict precipitation strengthening and evaluate the quench sensitivity of Al-Zn-Mg-1.5Cu alloys.