Precipitation and solute distribution in an interrupted-aged Al-Mg-Si-Cu alloy

• Published in: Philosophical magazine (Abingdon, England) England), 2008-01, Vol.88 (3), p.373-390
• Main Authors: Buha, J., Lumley, R. N., Crosky, A. G.
• Format: Article
• Language: English
• Subjects: Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Fatigue, embrittlement, and fracture; Materials science; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Precipitation; Treatment of materials and its effects on microstructure and properties
• ISSN: 1478-6435; 1478-6443
• DOI: 10.1080/14786430701847949

Quantitative analysis of the precipitate species and solute distribution was carried out on Al-Mg-Si-Cu alloy 6061 aged to peak hardness using a conventional T6 heat treatment and the so-called T6I6 heat treatments. In this latter, a dwell period at reduced temperature (65°C) is introduced into the T6 ageing cycle (at 177°C or 150°C) which modifies the microstructure and results in the simultaneous improvement of both tensile properties and fracture toughness. Analysis of three-dimensional atom probe data reveals that the superior mechanical properties of the T6I6/177 temper are achieved by a combined effect of a greater consumption of solute atoms by precipitates, an increased number density of fine precipitates and the presence of greater fractions of the effective strengthening precipitates in the final microstructure. Three types of precipitates were found to be characteristic of the peak aged conditions: β′′ precipitates, Guinier-Preston zones and Mg-Si(-Cu) co-clusters. The composition of the strengthening precipitates was found to vary over a wide range for the different heat treatment schedules, corresponding to a variation in the number density of stable nuclei, without any accompanying change in their morphology. All precipitates were found to contain substantial quantities of aluminium. The results also indicate that the strengthening precipitates are preferentially formed from Si-rich nuclei that contain Cu atoms, as opposed to Cu-free nuclei.