Study of the Precipitation Kinetics, Microstructures, and Mechanical Properties of Al-Zn-Mg-xCu Alloys

Microstructures and mechanical properties of Al-5Zn-2.6Mg alloys with 0.24, 0.43, and 0.91 wt.% Cu were studied and the precipitation rate and activation energy at 378, 393, and 408 K were calculated using the Arrhenius equation in this work. The aging reaction rate k increased and the precipitation activation energy Ea decreased from 25.7 to 15.0 kJ/mol. The η’ distribution density of the precipitates clearly increased with increasing Cu content. However, the size and number of coarse second phase with Fe and Mn impurities also increased, which increased the tendency for crack initiation and propagation at the grain boundary, resulting in a decrease in dimple area. The fracture morphology transformed from plastic transgranular fracture to brittle intergranular fracture and the elongation of the alloys decreased by 3.8%. The contribution of Cu content to yield strength was predominantly due to precipitation strengthening rather than grain boundary strengthening and solid solution strengthening. The tensile strength of the Al-5Zn-2.6Mg alloys with 0.91 wt.% Cu subject to peak aging at 393 K increased by 10.2%