Analysis of the metastable precipitates in peak-hardness aged Al-Mg-Si(-Cu) alloys with differing Si contents

Abstract

The peak-hardness aged (T6) precipitates in 6061 (Mg:Si ≈ 2:1) and Si-enriched 6061 (Mg:Si ≈ 1:1.1) alloys have been characterized using transmission electron microscopy and atom probe tomography. Mg-Si-Cu rich solute clusters/GP-zones, needle-shaped precipitates and lath-shaped precipitates coexist in both alloys at the T6 condition. The needle-shaped precipitates were dominant in both alloys. The alloy's overall composition strongly influences the needle-shaped precipitate's Mg:Si ratio and crystal structure. In the Si-enriched 6061 alloy these precipitates typically have an average Mg:Si ratio of 1.25:1 and monoclinic crystal structures in good agreement with the pre-β″ and β″ phases. The standard 6061 alloy's higher overall Mg:Si ratio increases the needle-shaped precipitate's average Mg:Si ratio to 1.65:1. This high ratio was a result of a reduction in the precipitate's Si content rather than an increase in the Mg content, which is believed to be enabled by the formation of needles with mixed β″/disordered structures. The alloy's overall composition also influences the lath-shaped precipitate's crystal structure with Q′ laths identified in the Si-enriched 6061 alloy while disordered L-phase laths were identified in the standard 6061 alloy. The current work shows that high Mg:Si ratio Al-Mg-Si(-Cu) alloys promote more disordered precipitate distributions.