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Aluminum-silicon alloys are an important class of commercial non-ferrous alloys having wide ranging applications in the automotive and aerospace industries. Typical aluminum-silicon alloys have two major microstructural components, namely primary aluminum and an aluminum-silicon eutectic. While nucleation and growth of the primary aluminum in the form of dendrites have been well understood, the understanding of the evolution of the Al-Si eutectic is still incomplete. The microstructural changes caused by the addition of strontium to these alloys is another important phenomenon that still puzzles the scientific community. In this thesis, an effort has been made to understand the evolution of the Al-Si eutectic in the presence and absence of strontium through two sets of experiments: (1) Quench experiments, and (2) sessile drop experiments. The quench experiments were designed to freeze the evolution of the eutectic after various time intervals along the eutectic plateau. The sessile drop experiments were designed to study the role of surface energy in the formation of the eutectic in the presence and absence of strontium. Both experiments were conducted on high purity alloys. Using observations from these experiments, possible mechanis(s) for the evolution of the Al-Si eutectic and the effects of strontium on modifying the eutectic morphology are proposed.

Thesis (B.S.)--University of Missouri, School of Mines and Metallurgy, 1933. / The entire thesis text is included in file. Typescript. Title from title screen of thesis/dissertation PDF file (viewed July 12, 2010) Includes bibliographical references (p. 26).

As três famílias de ligas de maior importância comercial para a indústria de fundição apresentam a formação de eutéticos do tipo facetado/não-facetado durante a solidificação: os ferros fundidos com grafita (eutético austenita/grafita), os ferros fundidos brancos (eutético austenita/carboneto) e as ligas Al-Si (eutético alfa/silício). No caso dos ferros fundidos brancos, a morfologia dos carbonetos eutéticos exerce influência decisiva sobre o comportamento mecânico e a resistência à degradação (em função de solicitações como abrasão ou ciclos térmicos e/ou mecânicos) das ligas resistentes a desgaste pertencentes ao sistema Fe-C-X (em que X representa um ou mais dos elementos Cr, V, Mo, W e, mais raramente, Nb). Investigou-se o efeito do potencial nucleante do banho metálico sobre o eutético de ligas do sistema Fe-C-V, e conseqüentemente sobre a morfologia do carboneto. As alterações de morfologia do eutético austentita/carboneto VC em função da adição de alumínio ou titânio foram avaliadas por meio de microscopias óptica e eletrônica de varredura. Observou-se que a adição de alumínio ou titânio, em ligas com teores residuais de oxigênio ou nitrogênio, promoveu alteração do eutético VC de irregular para fibroso (no caso da adição de alumínio) e para divorciado (no caso da adição de titânio). Conclui-se que a presença de partículas nucleantes induziu a nucleação seletiva da fase facetada (carboneto VC) a temperaturas mais elevadas, diminuindo o grau de cooperação durante o crescimento do eutético. / The three most commercially important families of alloys for the foundry industry show the formation of faceted/non-faceted eutectics type during solidification: cast irons with graphite (austenite/graphite eutectic), white cast irons (austenite/carbide eutectic) and Al-Si alloys (alfa/silicon eutectic). In the case of white cast irons, the eutectic carbide morphology has decisive influence on mechanical behavior and on degradation resistance (due to solicitation such as abrasion or thermal and/or mechanical cycling) of wearing resistant alloys of the Fe-C-X system (where X stands for one or more of the following elements: Cr, V, Mo, W and, seldom, Nb). The metallic bath nucleant potential on the eutectic of the Fe-C-V system and, thus, on the carbide morphology, were investigated. It was evaluated the nucleant potential for aluminum and titanium addition to the bath. The morphologies changes for the austenite/carbide eutectic were evaluated by optical and scanning electronic microscopy. It was observed that aluminum or titanium addition in alloys with residual content of oxygen or nitrogen promoted VC eutectic morphology change from irregular to fibrous (when aluminum is added) and to divorced (when titanium is added). It is concluded that the presence of nucleant agents in the metallic bath induced the preferred nucleation of the faceted phase at higher temperatures, decreasing the cooperation during the eutectic growth.