The Influence of Alloy Composition on Precipitates of the Al-Mg-Si System
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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 beta" precipitates and U phases. With high Si/Mg ratios, GP zones survive for long aging times. The beta" Mg5Si6 phase becomes very stable in the alloy with its Si/Mg ratio closest to 6/5. Deviation from this ratio increases fractions of beta" , U-phases and disordered precipitates. In Mg-rich alloys less GP zones form and the first peak is suppressed. A coarse precipitate microstructure of beta" and beta" 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. 6061 not mentioned, Al-Mg-Si always used.