A HRTEM study of metastable phase formation in Al-Mg-Cu alloys during artificial aging

Microstructure evolution of an age hardenable Al-3Mg-0.4Cu-0.12Si (wt%) alloy has been studied during artificial aging at 180 degreesC prior to the formation of the stable S-phase. The primary investigation method used in this study was high-resolution transmission electron microscopy (HRTEM), coupled with image processing and image simulation. After 1 h of aging, the presence of super-lattice reflections was detected in the Fourier spectra of the HRTEM images, Suggesting an L1(0) type ordering of Mg and Cu atoms in the Al matrix. After 4 and 8 h of aging, coherent particles were observed in the microstructure. These particles give rise to diffraction spots that in previous literature have been considered to be characteristic of the S-phase in the Cu-lean Al-Mg-Cu alloys. It is shown that these diffraction spots can be indexed in terms of a crystal structure that is closely related to the L1(0) ordering formed at the shorter aging times. The crystal structure is orthorhombic with lattice parameters a = 1.2 nm, b = 0.4 nm, c = 0.4 nm and space group Cmmm. We propose to identify these coherent particles Lis GPB-II zones, and the ordering that precedes them as GPB zones. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.