Abstract:By employing atomic-resolution imaging and first principles energy calculations, the growthbehavior of S-phase precipitates in a high strength Al-Cu-Mg alloy was investigated. It is demonstrated that the nucleation and growth of theS-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 °C. 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 precipitates with lath-like morphology.

Key words: aluminum alloy; precipitation; age hardening; anisotropy; crystal growth