Effects of co-addition of minor Sc and Zr on aging precipitates and mechanical properties of Al–Zn–Mg–Cu alloys

An Al–Zn–Mg–Cu alloy and an Al–Zn–Mg–Cu–Sc–Zr alloy are used to study effects of co-addition of minor Sc and Zr on aging precipitates and mechanical properties of Al–Zn–Mg–Cu alloys. After solid solution treatment, almost complete recrystallization occurs in the Al–Zn–Mg–Cu alloy, while recrystallization is significantly inhibited in the Al–Zn–Mg–Cu-Sc-Zr alloy, with a recrystallization fraction of 12.6%. The Al–Zn–Mg–Cu–Sc–Zr alloy has remarkably higher strength than the Al–Zn–Mg–Cu–Sc–Zr alloy in the solid solution-treated and water-quenched state, which is due to the Orwan strengthening of coherent Al3(Sc, Zr) particles and the grain boundary strengthening for finer grains. At the aging temperature of 120 °C，the co-addition of Sc and Zr accelerates the nucleation and growth of η′ phases in the grain interior, allowing the aged Al–Zn–Mg–Cu–Sc–Zr alloy to reach its peak strength earlier. At the aging temperature of 140 °C, the aging precipitate strengthening effect is weaker for the Al–Zn–Mg–Cu–Sc–Zr alloy than for the Al–Zn–Mg–Cu alloy because of the accelerated transformation from η′ phases to η phases and growth of η phases in the grain interior