  The effect of composition on microstructure both at the length scale of the
secondary dendrite arm spacing and nano-sized dispersoids during homogenization
of Al-Zn-Cu-Mg-Zr alloys has been studied. A comprehensive model that can
predict the microstructure at both the length scales has been used for the
study. The microstructure predicted has been compared to that for two
homogenized samples from a directionally solidified AA7050 sample and a
reasonable match has been found. The initial as-cast microstructure for
different compositions is calculated using Scheil type solidification from
Thermo-Calc. The initial microstructure has a considerable influence on
microstructural evolution during homogenization. To take advantage of the
decreased solid solubility of Zr in {\alpha}-fcc, cooling rates during
solidification must be high enough to prevent precipitation of primary Al3Zr.
Under solidification with industrial cooling conditions solute rich alloys
leads to fewer dispersoids. Based on the study, an improved composition range
of 6-8%Zn, 1-2%Cu, 1-2%Mg and 0.1-0.15%Zr for 7XXX alloys has been proposed.
