Evolution of microstructure and properties in a new type 2 mm Al–Zn–Mg–Sc–Zr alloy sheet

Abstract

The microstructural and property evolution of a novel Al–Zn–Mg–Sc–Zr sheet during its preparation were investigated in detail by tensile tests and electron microscopy methods. The results show that severe segregation exists in the ingot. After homogenization treatment at 470°C for 12h, dissoluble Zn and Mg enriched non-equilibrium phases dissolve into matrix completely and only little indissoluble impurity phases containing Fe and Si elements remain. At the same time, precipitation of nanometer-scaled coherent secondary Al3(Sc, Zr) particles from a supersaturated solid solution occurs. The proper homogenization process is 470°C×12h. After solution treatment at 470°C for 1h, a lot of non-equilibrium T(Mg32(Al,Zn)49) phases formed during hot rolling dissolve into matrix. Aged at 120°C, the precipitates gradually transform from coherent GP zones to semi-coherent η′ phase and incoherent η phase for the duration of aging, exhibiting a typical behavior of aging strengthening. The optimal solution-aging process is solution treated at 470°C for 1h, followed by water quenching and then aged at 120°C for 24h (peak-aged). Under this condition, the ultimate tensile strength, yield strength and elongation reach 555±2MPa, 524±4MPa and 12.3±0.6% respectively. The main strengthening mechanisms of Al–Zn–Mg–Sc–Zr aged sheets are precipitation strengthening derived from η′ precipitates, and dispersion strengthening and sub-grain strengthening caused by coherent secondary Al3(Sc, Zr) particles.