Effect of retrograssion and re-aging on microstructure and properties of Al–Zn–Mg–Cu–Zr–Er alloy

The study investigated the effect of retrogression and re-aging (RRA) on the structure and properties of the new Al–3.5Zn–3.5Mg–3.5Cu–1.6Er–0.2Zr–0.2Cr alloy through the use of scanning electron microscopy, thermodynamic calculations, hardness tests, current density, and corrosion potential. During the crystallization process, chromium is distributed between primary intermetallic compounds with an approximate composition of (Al,Zn)_79.8Mg_4.7Cu₃Cr_5.5(Er,Ti)₇, with a size of approximately 10 μm and an aluminum solid solution. Following two-stage homogenization heat treatment, the Al₈Cu₄Er and Mg₂Si phases exhibit minimal morphological changes, with the θ‑phase (Al₂Cu) being completely dissolved and the T‑phase (Al,Zn,Mg,Cu) transformed into the S‑phase (Al₂CuMg). Thermodynamic calculations indicate that the alloy should also contain the Al₃Zr and Al₄₅Cr₇ phases, which precipitate from the supersaturated solid solution during homogenization. Age hardening in the temperature range of 150–210 °C occurs due to the release of metastable modifications of the T‑phase. The combination of hardness (140 HV) and corrosion resistance (minimum corrosion current density 1 μA/cm²) is optimized by retrogression and re-aging.