Aberration-corrected scanning transmission electron microscopy study of β′-like precipitates in an Al–Mg–Ge alloy

• Published in: Acta materialia 2012-04, Vol.60 (6-7), p.3239-3246
• Main Authors: Bjørge, R., Dwyer, C., Weyland, M., Nakashima, P.N.H., Marioara, C.D., Andersen, S.J., Etheridge, J., Holmestad, R.
• Format: Article
• Language: English
• Subjects: Aluminium alloys; Applied sciences; Exact sciences and technology; High angle annular dark field; Metals. Metallurgy; Precipitation; Scanning transmission electron microscopy
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2012.02.039

Precipitates in an Al–Mg–Ge alloy similar to the β′ phase in Al–Mg–Si alloys were investigated using qualitative and quantitative aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM). The needle-shaped β′-Ge precipitates are coherent with the Al matrix along the needle direction which is parallel to a 〈001〉Al direction, as well as one direction in the cross-section plane that is parallel to a 〈100〉Al direction. This is linked to a smaller lattice parameter in the needle cross-section plane than that of coarser, less coherent β′ precipitates in Al–Mg–Si alloys, despite Ge having a larger atomic radius than Si. Quantitative HAADF STEM results show that the nominal Ge columns of the β′-Ge precipitates are not fully occupied by Ge, the intensity of these columns being consistent with an Al concentration of 30±10%, or a vacancy concentration of 20±10%. The coherent interface is atomically smooth, but with matrix columns containing Ge spaced periodically along the interface.