By employing atomic-resolution imaging and first principles energy calculations, the growth behavior of S-phase precipitates in a high strength A1-Cu-Mg alloy was investigated. It is demonstrated that the nucleation and growth of the S-phase precipitate are rather anisotropic and temperature-dependent companying with low dimensional phase transformation. There are actually two types of Guinier-Preston （GP） zones that determine the formation mechanism of S-phase at high aging temperatures higher than 180 ℃. One is the precursors of the S-phase itself, the other is the structural units or the precursors of the well-known Guinier-Preston-Bagaryatsky （GPB） zones. At high temperatures the later GPB zone units may form around S-phase precipitate and cease its growth in the width direction, leading to the formation of rod-like S-phase crystals; whereas at low temperatures the S-phase precipitates develop without the interference with GPB zones, resulting in S-phase orecioitates with lath-like momhology.
The A1-Cu-Li-（Mg） alloy is a high-performance lightweight material strengthened by complex coexisting precipitates that form in the alloy upon thermal ageing. Using high-resolution （scanning） transmission electron microscopy in association with first-principles energy calculations, we systematically studied the complex coexisting precipitates in the alloys and correlated their precipitation sequences with thermal ageing processes applied. The principal results are the following： （1） eight types of precipitates can be observed in the alloy; （2） of these precipitates, the Tl-phase is most stable. The S-phase precipitates with segregated Li atoms at their interfacial edges are unexpectedly more stable than the a-phase; （3） the Tl-phase has a characteristic precursor that plays the key role in its nucleation and growth.
