In the current study, the evolution of S(Al2CuMg) phase in a commercial Al-Zn-Mg-Cu alloy during homogenization, deformation and solution treatments was investigated and the effect of residual S(Al2CuMg) phase on mechanical properties in finished plates was revealed. The results showed that S(Al2CuMg) phase formed during solidifications and existed in eutectic structure. Parts of AlZnMgCu phase transformed to S(Al2CuMg) phase after the first homogenization treatment with a regime of 470°C/24h. During the second homogenization treatment at 480°C, most of the S phase had gradually dissolved. However, a handful of large-sized S phase remained even after extending the second stage homogenization time to 48h. During deformation these remaining S(Al2CuMg) phase was broken, resulting in size reduction and apparent distribution along the rolling direction, which could be effectively eliminated by a two-stage solid solution regime of 470°C/1.5h+480°C/2h. Comparing the microstructure and properties of final plates treated by homogenization processes, it was evident that the alloy subjected to a single-stage homogenization treatment exhibited a significantly higher amount of residual S phase, which directly compromised fracture toughness but has a relatively minor impact on strength.
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