The evolution of microstructures and mechanical properties of Al-Mg-Si-Mn-Cu-Fe alloys with different Mn contents during the homogenization heat treatment were investigated by SEM and in-situ TEM. Nucleation

growth and coarsening of α-Al（FeMn）Si dispersoids were thus quantitatively described. The peak shaped evolution law that dispersoid size is increased gradually and its volume fraction is increased firstly and then decreased during the homogenization heat treatment were measured with high precision

based on ultra-low penetration depth imaging and statistical analysis using low voltage and high resolution. The drastic influence of dispersoids on mechanical properties of alloys was clarified.The results indicate that homogenization heat treatment at 580 ℃ for about 8 h generates the highest volume fraction of dispersoids with adequate size and thus the alloy exhibits optimal comprehensive mechanical properties

while longer treatment leads to coarsening of dispersoids and degradation of mechanical properties. Moreover

the tensile strength and elongation of the alloy can be significantly improved by adding appropriate amount of Mn（0.5%） with homogenization treatment

however

excessive addition of Mn（0.8%） results in serious coarsening of the primary phase

degrading the mechanical properties of the alloy.