Effects of high Zn content on the microstructure and mechanical properties of Al-Zn-Cu gravity-cast alloys

In this paper, the effects of Zn on the microstructural evolution, hardness, and various characteristics of Al-20-45Zn-3Cu alloys were investigated. High-strength Al-Zn-Cu alloys (>450 MPa) were fabricated by gravity casting without melt modification or heat treatment (i.e. T4-T6). Al-based alloys containing more than 20% Zn were designed for the gravity-casting process. In terms of the microstructure, as the amount of Zn addition in the alloys increased, the α-phase size decreased and the α+η non-equilibrium solidification-phase fraction increased. A complex network of eutectoid α+η, supersaturated η, and Cu-related intermetallic particles formed in the grain boundary regions. In addition, increasing the Zn content improved the mechanical properties of the alloys but reduced their damping capacity and toughness. The fractographic examination of the fracture surfaces indicated that the Al-Zn-Cu alloys with high Zn addition had fewer ductile dimple surfaces and more brittle cleavage surfaces.