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9 Décembre 2011 - C. Mariora - Effects of Cu and Ag additions to the precipitates in the Al-Mg-Si system

par LABGPM - publié le

9 Décembre 2011

Title : Effects of Cu and Ag additions to the precipitates in the Al-Mg-Si system

Abstract : Adding Cu to the Al-Mg-Si system increases precipitate number density and material peak hardness. Cu has a profound effect on metastable precipitate structures, changing the precipitation sequence from pre-b”, b” (main hardening phases) -> b’, U1, U2, B’ (overaging) -> b, Si (stable) for the Cu-free system, to (pre-b”, b” ), S, QC, L, C (main hardening phases) -> Q’ (overaging) -> Q (stable) for the Cu-containing alloys. Although these metastable phases have different crystal structures, they have all been found to be based on a common near-hexagonal network of Si atomic columns with projected dimensions a = b ≈ 0.4 nm when viewed along its c-direction that has dimension c = n*0.405 nm (n = 1, 2,…). The c-direction of the network is parallel to the precipitate’s needle/rod/lath/plate directions having highest coherency with the Al matrix. Cu, Al and Mg atomic columns occupy positions in-between the Si columns of this network. In the case of pre-b” and b” the network deviates most from the hexagonal symmetry, probably due to the high coherency these phases have with the Al matrix. The network’s base-plane is oriented along <100>Al directions for the L and C phases, while for the rest the orientation is along <510>Al. Ag was found not to form fundamentally new phases, but it can partly replace Si in the network of some precipitates. Both Cu and Ag atomic columns represent centers of local symmetries in the crystal. Ag has a strong tendency to precipitate at the particles interfaces with the Al matrix.