The influence of alloy composition on precipitates of the Al-Mg-Si system
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The Influence of Alloy Composition on Precipitates of the Al-Mg-Si System C.D. MARIOARA, S.J. ANDERSEN, H.W. ZANDBERGEN, and R. HOLMESTAD To study how changes in solute elements affect precipitation, six Al-Mg-Si alloys aged at 175 °C were investigated by transmission electron microscopy (TEM). In alloys with 1.3 at. pct solute, when the Si/Mg ratio exceeds 5/6, a sharp hardness peak appears after 3 hours that correlates with a high density of fine Guinier–Preston (GP) zones. A second, broader peak correlates with precipitates and U phases. With high Si/Mg ratios, GP zones survive for long aging times. The -Mg5Si6 phase becomes very stable in the alloy with its Si/Mg ratio closest to 6/5. Deviation from this ratio increases fractions of , U-phases and disordered precipitates. In Mg-rich alloys less GP zones form and the first peak is suppressed. A coarse precipitate microstructure of and develops, the volume fraction being much higher than in Si-rich alloys. The Mg-rich alloys overage faster. Reducing the content of solutes causes alloys with high Si/Mg ratios to have a more Mg-rich behavior.
I. INTRODUCTION
THE Al-Mg-Si heat-treatable alloys are characterized by a large increase in hardness upon aging. The strength increase is caused by interface strain between the matrix and small, semicoherent, metastable precipitates forming from solid solution. In the Al-Mg-Si system, many high-quality alloys have been developed having attractive properties for manufacturers of a variety of goods. Factors such as low price, low density, high weldability, medium strength, high extrudability, and good corrosive properties have supported a continuous growth in the usage and production of these alloys. Today more than 90 pct of the extruded Al goods in Western Europe are based on the 6xxx series.[1] For a long time, transmission electron microscopy (TEM) has been among the most significant techniques in the investigation of the microstructure in these materials. A considerable amount of research has been invested on studies of the precipitates and how they influence the material properties. Even with the newest generation of high-resolution electron microscopes (HREMs), many of the questions have been difficult to answer. In part, this is caused by the complex thermal processing of these alloy systems in general, with several processing steps that each may influence the type of precipitate, its distribution, and size. The smallness of the individual precipitates makes them difficult to study. Until quite recently, very few details concerning precipitate structures were available. A deeper level of understanding is now made possible as more phases have been discovered and the crystal structures of many of the phases of the precipitation sequence have been solved. SSSS→ atomic clusters[2,3,4]→GP zones→b¿¿→ b¿/ B¿U→b/Si C.D. MARIOARA, Research Scientist, and S.J. ANDERSEN, Senior Scientist, are with SINTEF Materials and Chemistry, N-7465 Trondheim, Norway. Contact e-mail: calin.d.marioara@si
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