Ultra-high strength Al-Zn-Mg-Cu alloys with high zinc content prepared via powder hot extrusion

In this paper, two kinds of Al-Zn-Mg-Cu alloys with high Zn content (Al-8.1Zn-2.0Mg-2.3Cu-0.12Zr-0.12Sc and Al-10.5Zn-2.5Mg-1.5Cu-0.12Zr-0.12Sc) are fabricated by powder hot extrusion. Microstructure and mechanical properties are investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and tensile tests. The results show that the formation of ultrafine grains and homogeneous second phases in as-extruded alloys is achieved by perfect metallurgical bonding during severe plastic deformation. After the optimal solution treatment and peak aging treatment, the strength of the two alloys reaches up to 734 MPa and 802 MPa, and the elongation is 9.8 % and 5.3 %, respectively. The excellent mechanical properties are attributed to the grain boundary strengthening, precipitation strengthening and homogenous microstructure. The element content has little effect on the grain size of the powder hot-extruded Al-Zn-Mg-Cu alloy, but high Zn and Mg contents can improve the density of strengthening phases while low Cu content can reduce the difficulty of solution treatment by inhibiting the precipitation of S(Al₂CuMg) phase.