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)(79.8)Mg4.7Cu3Cr5.5(Er,Ti)(7), with a size of approximately 10 mu m and an aluminum solid solution. Following two-stage homogenization heat treatment, the Al8Cu4Er and Mg2Si phases exhibit minimal morphological changes, with the theta-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 degrees 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 mu A/cm(2)) is optimized by retrogression and re-aging.