A trade-off between mechanical strength and electrical conductivity of Al–Zn–Mg–Cu alloy via Ag alloying and retrogression re-aging heat treatment

Abstract

The effects of silver (Ag) alloying and a retrogression re-aging heat treatment on the mechanical properties and electrical conductivity of the Al–Zn–Mg–Cu aluminum alloy were studied by means of tensile test, electrical conductivity test, and transmission electron microscopy (TEM). A trade-off between the mechanical strength and electrical conductivity was achieved, with the former reaching 558 MPa and the latter reaching 45.1 IACS. The alloy was prepared through a retrogression re-aging heat treatment (120 °C/6 h + 160 °C/7 h + 220 °C/0.5 h + 120 °C/24 h) performed on the solution and quenched samples. Furthermore, through TEM analysis of the sample solution treated at Temperature Ⅰ (420 °C/3 h + 470 °C/2 h) and Temperature II (420 °C/3 h + 480 °C/2 h), indicates that the Ag interacts with zinc or magnesium to form small clusters, which can act as precursors for η′ precipitates. η–type precipitates were the dominant precipitated phase, and the precipitation sequence was supersaturated solid solution (SSS)→ small clusters → Guinier–Preston (GP) zone → η′ → η. Furthermore, after the retrogression re-aging heat treatment, a large amount of GP (Ⅱ) and η′ precipitated in the matrix. GP (Ⅱ) and the η′ phase are related to the precipitation of {111} Al crystal plane, it is thus speculated that the interaction between the η′ phase and GP (Ⅱ) or nanosized Al3Zr dispersoids contributes to the strength of the alloy. Moreover, the retrogression re-aging heat treatment promotes the precipitation of the second phase, which can purify the matrix and reduce lattice distortion, thus improving the conductivity of the alloy.