Microstructure evolution and phase constituent of as-cast and homogenized Al-9.1Zn-1.9Mg1.6Cu alloy were investigated by means of optical microscope（OM）

SEM（scanning electron microscope）

EDS（energy dispersive spectrometer）and XRD（X-ray diffraction）and DSC（differential scanning calorimeter）.The results reveal that a lot of nonequilibrium low-melting eutectic phase exist in the ascast microstructure of the alloy

and initial melting temperature reaches 475℃.As-cast microstructure is composed ofα-Al phase

η-MgZn2phase and minor Al7Cu2Fe phase.The alloy is totally composed ofα-Al phase after homogenized at above 460℃

which shows that the non-equilibrium low-melting eutectic phase can dissolve into the matrix entirely.Desirable effects of the alloy can be observed with homogenizing at 465℃for 24hours.