* TMS Structural Materials Division. * a graduate student.

The precipitation characterization of an Al-Zn-Mg-Sc-Zr alloy aged at 120 ℃ has been studied by using high-resolution electron microscopy. The results are interpreted in terms of structural transformations that involve two types of GP zones (I and II), the metastable η ’ hardening precipitate and the stable phase η. At the early stages of aging, GPI zones act as the nucleation sites for the plate-like η ’ phase which is precipitated on the {111}Al plane. The η phases contain mainly η1, η2, and small amount of η4 precipitates. The HREM results showed that η1-plates lying on the {100}Al plane, η2-plates on the {111}Al plane and η4-rods along <110>Al direction. Furthermore, a fine dispersion of coherent Al3(Sc,Zr) precipitates is known to have beneficial effect on the strengthening of the alloy, while the formation of GPII zones is suppressed in the presence of zirconium and leads to a reduction in the amount of η’ formed during subsequent aging