The microstructures and mechanical properties of a highly alloyed Al-Zn-Mg-Cu alloy: The role of Cu concentration

The influence of Cu content on microstructures and mechanical properties of a highly alloyed Al-10.0Zn-2.6Mg-xCu-0.15Zr alloy with the Cu content in the range of 0 to 3.0 was studied by OM, SEM, TEM, tensile tests, XRD, and DSC. We determined that the Mg(Zn,Al,Cu)2 (σ) and Al2Mg3Zn3 (T) phases presented in all as-cast alloys, and Al2Cu (θ) phase was found in the alloys with Cu higher than 2.5 wt.%. Also, it is found that the volume fraction of the primary phase increased with the Cu increase. Further, the volume fraction of the residual eutectic phase (REP) increased gradually with the increase of Cu after solid solution, mainly attributed to the low diffusion coefficient of Cu and thus suppressed the dissolution of the second phase. The tensile testing of the aged alloys indicates that the strength of the studied alloys increased significantly with the Cu increase when Cu is lower than 1.0 wt.%, whereas further increase Cu content it decreases. The elongation increases to a peak value fast at low Cu content. After a critical Cu content (1.5 wt.%), a slight decrease is followed. The elongation of highly alloyed Al-Zn-Mg-Cu alloys should consider the coupling effect of grain refinement and REP.