Improving mechanical properties of extruded Al-Zn-Mg-Cu alloys under secondary aging by Yb-doped microalloying

To investigate the influence of ytterbium (Yb) on precipitation behavior and mechanical property evolution in AA7150 Al-Zn-Mg-Cu alloys during two-stage aging, we systematically compared Yb-modified and Yb-free alloys using transmission electron microscopy (TEM), atom probe tomography (APT), and room-temperature tensile testing. The results revealed that The addition of Yb did not significantly promote the formation of intermetallic phases in the Al-Zn-Mg-Cu alloy, but effectively induced grain refinement and suppressed recrystallization behavior during thermomechanical processing. The Yb element participates in Zn-Mg cluster formation during the first-stage aging (120 °C), promoting the generation of vacancy-rich clusters. However, the low aging temperature impedes vacancy dissolution, resulting in reduced free vacancy concentration, fewer nucleation sites, and lower precipitate number density, ultimately diminishing the mechanical performance of Yb-containing alloys at this stage. In contrast, during the second-stage aging (160 °C), elevated temperature facilitates the dissolution of Yb-associated vacancy-rich clusters, significantly increasing free vacancy concentration and accelerating atomic diffusion. This mechanism enhances precipitate number density and improves mechanical properties, with the Yb-modified alloy achieving a peak yield strength (YS) of 681 MPa at peak-aged condition. Our findings elucidate the critical role of Yb in modulating vacancy dynamics and precipitation kinetics during multi-step aging, providing insights into microstructure-property relationships in advanced age-hardenable aluminum alloys.