Effects of Cu addition on the microstructure and mechanical properties of as-cast and heat treated Mg-6Zn-4Al magnesium alloy

The microstructure and mechanical properties of as-cast and heat treated Mg-6Zn-4Al-xCu (x=0, 0.5, 1.0 and 1.5 wt%) alloys were investigated in this work. The results show that the grain sizes of the alloys decrease considerably with increasing Cu addition, and the eutectics are refined with 0.5% Cu addition. The icosahedral quasi-crystalline phase is observed in Mg-6Zn-4Al and Cu-containing alloys，and the MgAlCu phase forms in the Cu-containing alloys because of Cu addition. Compared with Mg-6Zn-4Al alloy, the Cu-containing alloys exhibit improved age-hardening response during single-aging treatment. In addition, high-density fine precipitates are formed in the matrix during double-aging treatment, resulting in remarkable improvement of the tensile strength. The double aged Mg-6Zn-4Al-0.5Cu alloy shows the relatively optimal tensile properties. The yield strength, ultimate tensile strength and elongation are 202 MPa, 312 MPa and 7%, respectively, which is attributed to the combined effects of fine grains and the uniform distribution of high-density fine precipitates. It is noticed that the ductility of as-aged Cu-containing alloys is lower than that of the as-aged Mg-6Zn-4Al alloy due to the stress concentration during the tensile stress caused by the residual phases distributed along the grain boundaries after solution treatment.