X-ray diffractometry(XRD),optical microscopy(OM),scanning electron microscopy(SEM)were used to study the fatigue fracture of the T7451 Al-Zn-Mg-Cu alloy(470℃,60 min+115℃,8 h+165℃,16 h).The study reveals mainly the microscopic structure of the alloy in the process of crack formation and crack growth.The fatigue fracture is characterized by three zones:fatigue crack source zone,fatigue crack propagation zone and fatigue fracture zone.The fatigue damage preferably incubates at the fractured inclusion particles at or near(about 25μm)the specimen free surfaces,and these brittle Fe-rich intermetallic inclusion particles are (7-10)μm×(11-14)μm in size.Some features such as"feather-like","river and range"and boundary extrusions can be observed in the fatigue propagation zone,and in the fatigue fracture zone the surface is rough and uneven.
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