Characterization of precipitate evolution in an artificially aged Al–Zn–Mg–Sc–Zr alloy

The precipitates of an age-hardenable Al–Zn–Mg–Sc–Zr alloy aged at 140 °C have been studied by transmission electron microscopy (TEM). The results are interpreted in terms of structural transformations that involve two types of Guinier-Preston (GP) zones (I and II), the metastable η′ hardening precipitate and the stable phase η-phase (MgZn2). At early stages of ageing, the GPI zones act as nucleation sites for the plate-like η′ phase which precipitates on the {1 1 1}Al plane. The transformation from η′ to η occurs gradually at longer ageing times. The η particles mainly consist of η1 and η2 phase, with small amounts of η4 type and minor quantities of η′ type structures. Furthermore, while a fine dispersion of coherent Al3(Sc,Zr) precipitates is known to have a beneficial effect on the strengthening of the alloy, the formation of GPII zones is suppressed in the presence of zirconium that leads to a reduction in the amount of η′ formed during subsequent ageing.