Effect of Zn content and Sc、Zr addition on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys

For aluminum alloys in modern aerospace, it is more and more important for synchronous improvement of strength and toughness by elaborated microstructural and compositional design. In this work, advanced electron microscopy techniques, XRD, tensile tests and quantitative calculations were used to study high Zn content and effects of adding Sc and Zr on aging precipitation behavior and mechanical properties in Al-Zn-Mg-Cu alloys. Al-9.2Zn-2.2Mg-2.3Cu-0.2Sc-0.15Zr (7E36) alloy has higher peak strength as tensile strength (UTS), yield strength (YS) and elongation ( e f ) reach 773 MPa, 758 MPa and 11.5%, respectively. Moreover, compared with Al-9.2Zn-2.2Mg-2.3Cu-0.15Zr (7036) alloy and Al-8.0Zn-2.2Mg-2.3Cu-0.2Sc-0.15Zr(7E55), YS of 7E36 alloy is increased by 11.5% and 14.6%. In addition, 7E55 alloy has better mechanical properties than 7036 alloy, especially e f as high as 17.5%. The increasing Zn content leads to a weaker solid solution strengthening effect, but promotes the precipitation of η` phases whose increment to YS is 55 MPa for 7036 and 35 MPa for 7E36 higher than that of 7E55 alloy, respectively. Both 7E55 and 7E36 alloys can form nano-structured Al <sub>3</sub> (Sc,Zr) precipitates with Sc-rich core and Zr-rich shell, and YS increase was estimated by Orowan mechanism. The Heterostructures have been found in 7E55 and 7E36 alloys, and dislocation densities inside the grains are analyzed, and YS increments were calculated by corrected Hall-Petch correlation and by dislocation strengthening. This work proposed a new way of alloy design with appropriate Zn increase and compound addition of Sc and Zr, based on strengthening of grain refinement, Al <sub>3</sub> (Sc,Zr) phase dispersion and η` phase precipitation to improve strength and plasticity.