Al-4.5Zn-1.0Mg-0.8Cu alloys containing different RE content were prepared by metal mould casting.Effects of RE on microstructure and mechanical properties of Al-4.5Zn-1.0Mg-0.8Cu alloy were investigated by tensile testing

OM（optical microscovpe）

SEM（scanning eldctron microscope）and EDS（energy dispersive spectrometer）.The results show that the tensile strength and elongation are increased firstly

and then decreased with the increase of RE content.The alloy with 0.12% RE（mass fraction）exhibits the desriable mechanical properties.With proper RE

secondary dendrite arm spacing can be decreased

and RE is distributes mainly at grain boundaries and dendrite boundaries in form of AlZnMgCuRE phase.Excessive RE can cause over-modification effect

increasing secondary dendrite arm spacing.With a lower RE content

AlZnMgCuRE phase exhibits rod-like shape

and with excessive RE

AlZnMgCuRE phase becomes coarse block structure

which can weaken the mechanical properties as a result of cutting effects.In addition

RE can promote the solid solution of Mg and Zn in Al melt

improving the solid solution strengthening effects.