Tailored Mg and Cu contents affecting the microstructures and mechanical properties of high-strength Al-Zn-Mg-Cu alloys

The effects of systematic variation of Mg and Cu contents (Mg: similar to 1.5, 2.0, and 2.5, Cu: similar to 1.5, 2.0, 2.5, and 2.9 wt%) on the microstructures and mechanical properties of high-Zn (8.5 wt%) Al-Zn-Mg-Cu alloys are investigated. Fracture toughness is experimentally approached by the Kahn tear test. Results showed that, under same ageing condition, the conductivity, hardness, strength and toughness of the designed alloys are primarily determined by Mg content: the higher the Mg content, the higher the hardness and strength, but the lower the conductivity and toughness. Increasing Cu content can produce a similar phenomenon, but with weak effects compared with Mg. The experiments and thermodynamic/kinetic simulation indicate that, increasing Mg/Cu content can improve the volume fraction of matrix precipitates, so as to improve the strength and hardness, and the effects of Mg are stronger than Cu. Additionally, increasing Mg content can somewhat reduce the sizes of the matrix precipitates especially in overaged condition, which is also good for the strength and hardness. However, with increasing Mg content the area fraction of the grain boundary precipitates (GBPs) and the yield stress contrast between grain interiors and precipitate free zones (PFZs) at grain boundary can be increased greatly, consequently promoting intergranular fracture and decreasing toughness. For the alloys with low/middle Mg content (e.g., 1.5/2.0 wt%), increasing Cu content will improve the yield stress contrast between grain interiors and PFZs as well as the recrystallization degree, so that intergranular fracture will be promoted for toughness reduction. For the alloys with high Mg content (e.g., 2.5 wt%), the increased undissolved phases induced by high Cu content will promote fracture at/near coarse constituent particles, favoring further toughness reduction. (C) 2016 Elsevier B.V. All rights reserved.