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

• Published in: Acta materialia Vol. 52; no. 9; pp. 2509 - 2520
• Main Authors: Kovarik, L., Gouma, P.I., Kisielowski, C., Court, S.A., Mills, M.J.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 17-05-2004; Elsevier Science
• Subjects: Age hardening; Al–Mg–Cu alloys; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; GPB zones; HRTEM; Image reconstruction; Materials science; Physics; Solid solution hardening, precipitation hardening, and dispersion hardening; aging; Solid solution, precipitation, and dispersion hardening; aging; Treatment of materials and its effects on microstructure and properties; Al–Mg–Cu alloys; GPB zones; Age hardening; Image reconstruction; HRTEM; Aluminium base alloys; Metastable phases; Electron diffraction; Magnesium alloys; Coherent precipitate; Copper additions; Al-Mg-Cu alloys; Experimental study; Heat treatments; Silicon additions; Binary alloys; Precipitation; Transmission electron microscopy; Solid solutions; Microstructure; Guinier-Preston zones
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2004.01.041

Microstructure evolution of an age hardenable Al–3Mg–0.4Cu–0.12Si (wt%) alloy has been studied during artificial aging at 180 °C 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 as GPB-II zones, and the ordering that precedes them as GPB zones.