Effect of Zn/Mg ratio on aging precipitates and mechanical property of high Mg content Al-Mg-Zn alloys with Sc and Zr additions

Abstract

To enhance the strength of traditional 5xxx series alloys, this work designed Al-Mg-Zn alloys with high Mg content and different Zn/Mg ratios (wt%, Zn/Mg ratio<1.0) drawing inspiration from aging precipitation strengthening of 2xxx or 7xxx series aluminum alloys and thermodynamic calculation. Measurement of density, microhardness and tensile property as well as electron backscatter diffraction (EBSD) and high-resolution transmission electron microscopy (HRTEM) were used to investigate the effects of four different Zn/Mg ratios on the aging precipitation behavior and mechanical property of the studied alloys. As Zn/Mg ratio increases, both N3 (Zn/Mg = 0.37) and N4 (Zn/Mg = 0.71) precipitate fine and dispersed distributed nano T-Mg32(Al,Zn)49 precipitates. After a two-step aging treatment process of 90 °C/24 h + 140 °C/8 h, N4 exhibits tensile strength of 595.5 MPa, yield strength of 484 MPa, and elongation of 10.2% in the peak state. N4 features higher specific strength and lower density compared to the popular 7075-T6. The TEM analysis reveals that the addition of trace Sc and Zr leads to the formation of Al3(Sc,Zr) precipitates, which significantly refines grain size thus enhancing strength. The primary strengthening mechanisms of Al-Mg-Zn-Sc-Zr alloy include grain boundary strengthening, solid solution strengthening, and precipitation strengthening. A yield strength model considering multiple strengthening mechanisms and thermodynamic calculation has been adopted to achieve accurate yield strength predictions. This work provides reference for composition design, microstructure and property regulation of lightweight high-strength aluminum alloys.