Microstructure Solidification Maps for Al-10 Wt Pct Si Alloys
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hypo-eutectic Al-Si alloy system is known for its strong corrosion resistance, good castability and relatively high strength-to-weight ratio.[1] These characteristics make it an important alloy system and has led to its widespread usage in both the automotive and aerospace industries.[2] However, even with these desirable properties, hypo-eutectic Al-Si alloys have limited usage as structural materials, due to the inherent characteristics of the Si phase that forms within its eutectic structure. In the as-cast state, eutectic Si forms a flaky lamellar morphology that, combined with the inherent brittle nature of Si, significantly reduces the ductility and mechanical property performance. It is possible to
WILLIAM HEARN, ABDOUL-AZIZ BOGNO, JONAS VALLOTON, and HANI HENEIN are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2G6 Canada. Contact e-mail: [email protected] JOSE SPINELLI is with the Department of Chemical and Materials Engineering, University of Alberta and also with the Department of Materials Engineering, Federal University of Sa˜o Carlos, Sa˜o Carlos, SP 13565-905 Brazil. Manuscript submitted June 19, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
modify the Si into a fibrous and rod-like shape, which can yield a 50 pct improvement in the tensile strength, and a three-fold improvement in the ductility.[3] To achieve this refinement, the typical methods are the use of alloy additions or the control of the solidification conditions.[3] Alloying additions modify the Si by restricting its nucleation or by restricting its growth.[4–7] While the solidification conditions achieve refinement by controlling the cooling rate, where an increase in the cooling rate will make the Si more fibrous and rod-like.[8,9] Even though Si can be modified using either technique how this modification occurs is poorly understood, especially the refinement caused by high cooling rates. It is reported in the work of Khan and Elliott[10] that the transition in Si growth from bulky/faceted plates to smooth/globular fibers is accompanied by a drop in undercooling. This suggests that fibrous growth is a departure from the normal growth of broad faceted flakes toward continuous growth of a non-faceted phase. However, currently no mechanism can explain how Si morphology may be refined, which makes it difficult to reliably predict or design solidification processes to produce Al-Si alloys with the desired Si morphology. Solidification studies related to the high cooling rate production of Al-Si alloys have been conducted previously. Trivedi[11] and Kalay[12] developed a map of the Al-Si system that described the influence of alloy composition and undercooling. Pierantoni et al.[13]
defined the Al-rich boundary of the coupled eutectic zone, at Si compositions between 15.5 and 26 wt pct Si. Although both were beneficial to our understanding, the works only examined the components that would form, rather than the morphology of the eutectic structure. Moreover, they dealt with eutectic/hypereutectic alloy co
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