“…For the (Cu + Mg) content higher than this critical value or the alloys in (a-Al + S) phase field (i.e., the alloys 2-2, 1-2 and 3-3), there are still some residual coarse phases after hightemperature treatment (i.e., 475 °C) when the alloys lie outside the solubility limit at 490 °C (i.e., the alloys 1-2 and 3-3). However, these coarse phases can be completely dissolved after specific treatments (i.e., the two-step high-temperature treatment) when the alloys lie between the solubility limits at 475 °C and 490 °C (i.e., the alloy 2-2), but they may exhibit high quenching sensitivity [4,13,14]. Also, the Zn:Mg ratio can affect the fraction of the S phase, i.e., as the Zn:Mg ratio increases at a constant Cu content, the alloy will move from the two-phase field into a single-phase field [43]; high Zn:Mg ratio may increase the Cu solubility and more S phase can be dissolved [14], i.e., the AA7050 alloy.…”