Precipitate evolution and properties of an Al-Zn-Mg-Cu alloy processed by thermomechanical treatment

The precipitate evolution, strength and corrosion resistance of an Al-Zn-Mg-Cu alloy processed by cryorolling, warm rolling and final aging have been investigated. A large number of spherical Zn-rich GP zones formed after warm rolling. In-situ transmission electron microscopy (TEM) observation indicated that the GP zones are stable during aging treatment and can serve as nuclei for subsequent precipitates, leading to homogeneous distribution of precipitates in the matrix after aging. TEM results also showed that ultrafine subgrains were developed in the deformed microstructure during aging. The high density of precipitates and dislocations combined with grain refinement contribute to the higher strength of the thermo-mechanically treated alloy compared with the peak-aged alloy. On the other hand, the eta phase with C15 cubic structure having center dot center dot center dot RR center dot center dot center dot stacking is found in the thermo-mechanically treated alloy and is different from one with C14 hexagonal structure having center dot center dot center dot RR-1 center dot center dot center dot stacking that is commonly observed in conventionally aged alloys, where R and R-1 represent rhombic layer and the relatively 180 degrees -rotated rhombic layer, respectively. The transition from ill-defined stacking to C15 stacking and further to C14 stacking is observed in the structure of a eta phase. Besides, the combination of warm rolling and aging can avoid the formation of continuous grain boundary precipitates, which is beneficial to the corrosion resistance of the alloy.