Metastable precipitation in 6XXX Al alloys as revealed by STEM-ADF at the atomic level

Precipitation hardening remains an important and efficient way to improve the mechanical resistance of aluminium alloys. In the case of the Al-Mg-Si system (6XXX series), a complex precipitation sequence occurs, which is greatly affected by the presence of a minor addition of copper. We focus here on a Al-Mg 1 wt. %-Si 0.6 wt. %-Cu 0.25 wt. % 6061 alloy. At 300 C, closely related metastable phases, labelled QP, QC and Q(′) co-exist during ageing between a few minutes and one hour. Although numerous (HR)TEM studies have been published on this particular alloy, all atomic structures are not accurately known. This has motivated the present study by means of corrected STEM-ADF imaging, which contrarily to conventional HRTEM imaging, is essentially not sensitive to coherent diffraction and should thus provide a better atomic description of these phases. We demonstrate that the Q-based phase occuring at longer ageing times is actually identical to the stable Q-AlxCu 2Mg12-xSi7 phase. The QC phase has also been refined: a structural model has been proposed (P-6 space group), which is very similar to the P-62m hexagonal phase identified previously by one of the present authors (C.C.). We finally discuss the segregation of Cu at the periphery of the precipitates, and the structural relationship between the QP and the Q(′) phase. © 2013 EDP Sciences.

Published in:
Revue de Metallurgie. Cahiers D'Informations Techniques, 109, 393-407

 Record created 2014-11-14, last modified 2018-04-20

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