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Open AccessArticle

Metallurgical Parameters Controlling the Eutectic Silicon Charateristics in Be-Treated Al-Si-Mg Alloys

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Département des Sciences appliquées, Université du Québec à Chicoutimi, Chicoutimi, QC G7H 2B1, Canada
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Corporativo Nemak, S.A. de C.V., P.O. Box 100, Garza Garcia, Nuevo León 66221, Mexico
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Materials Engineering, General Motors, 823 Joslyn Avenue, Pontiac, MI 48340, USA
*
Author to whom correspondence should be addressed.
Academic Editor: Peter J. Uggowitzer
Materials 2016, 9(2), 78; https://doi.org/10.3390/ma9020078
Received: 20 December 2015 / Revised: 8 January 2016 / Accepted: 19 January 2016 / Published: 27 January 2016
The present work was carried out on Al-7%Si-0.4%Mg-X alloy (where X = Mg, Fe, Sr or Be), where the effect of solidification rate on the eutectic silicon characteristics was investigated. Two solidification rates corresponding to dendrite arm spacings (DAS) of 24 and 65 μm were employed. Samples with 24 μm DAS were solution heat-treated at 540 °C for 5 and 12 h prior to quenching in warm water at 65 °C. Eutectic Si particle charateristics were measured using an image analyzer. The results show that the addition of 0.05% Be leads to partial modification of the Si particles. Full modification was only obtained when Sr was added in an amount of 150–200 ppm, depending on the applied solidification rate. Increasing the amount of Mg to 0.8% in Sr-modified alloys leads to a reduction in the effectiveness of Sr as the main modifier. Similar observations were made when the Fe content was increased in Be-treated alloys due to the Be-Fe interaction. Over-modification results in the precipitation of hard Sr-rich particles, mainly Al4SrSi2, whereas overheating causes incipient melting of the Al-Cu eutectic and hence the surrounding matrix. Both factors lead to a deterioration in the alloy mechanical properties. Furthermore, the presence of long, acicular Si particles accelerates the occurrence of fracture and, as a result, yields poor ductility. In low iron (less than 0.1 wt%) Al-Si-Mg alloys, the mechanical properties in the as cast, as well as heat treated conditions, are mainly controlled by the eutectic Si charatersitics. Increasing the iron content and, hence, the volume fraction of Fe-based intermetallics leads to a complex fracture mode. View Full-Text
Keywords: aluminum alloys; additives; eutectic structure; fractography; tensile properties; image analyses aluminum alloys; additives; eutectic structure; fractography; tensile properties; image analyses
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Ibrahim, M.F.; Elgallad, E.M.; Valtierra, S.; Doty, H.W.; Samuel, F.H. Metallurgical Parameters Controlling the Eutectic Silicon Charateristics in Be-Treated Al-Si-Mg Alloys. Materials 2016, 9, 78.

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