Phase transformation and microstructure evolution of an ultra-high strength Al-Zn-Mg-Cu alloy during homogenization

Homogenization treatment of an ultra-high strength Al-Zn-Mg-Cu alloy was studied by SEM, TEM, DSC, hardness and electrical conductivity test. The second phases in as-cast 7A56 alloy are AlZnMgCu, Al2Cu and Al7Cu2Fe. AlZnMgCu phase has a similar lattice structure of MgZn2. After 380 °C homogenization treatment, transformation from AlZnMgCu to Al2CuMg occurred; after 470 °C homogenization treatment, AlZnMgCu were mainly directly dissolved into the matrix with no Al2CuMg discovered. That phenomenon was caused by the diffusion behavior of Zn and Cu at two different temperatures that Zn diffuses rapidly even at 380 °C while Cu shares a much lower diffusion rate. Much of the AlZnMgCu phase can be dissolved at 470 °C for a short time while at 380 °C it was seldom eliminated. After homogenized at 470 °C for 24 h, the dissolution process gradually reached into a balance with more than 95% had been eliminated and prolonging time imposes no evident influence on promoting the dissolution. Hardness and electrical conductivity are highly consistent with the vibration of AlZnMgCu phase, with the dissolution of which the hardness increases while electrical conductivity decreases.