Influência da convecção termossolutal na transição colunar/equiaxial em ligas Al-Si sob condições unidirecionais e transitórias de extração de calor

A relação entre macroestrutura e propriedades mecânicas de um material tem sido objeto de intensa investigação pois o tamanho dos grãos, a orientação cristalina e a distribuição dos mesmos exercem influência direta no comportamento mecânico dos produtos acabados. Assim, o entendimento dos fatores que influenciam a formação das zonas estruturais coquilhada, colunar e equiaxial nos materiais fundidos como, por exemplo, o sistema de liga, composição da liga, temperatura de vazamento, temperatura do molde, material do molde, coeficientes de transferência de calor na interface metal/molde, taxa de resfriamento, gradientes térmicos, dimensão da peça, presença de convecção no líquido, transporte de soluto, etc é de fundamental importância para a melhoria da eficiência do processo de fundição envolvido. Com base no conhecimento dos princípios termofísicos em que essas zonas são formadas, é possível manipular de forma bastante razoável a estrutura dos produtos fundidos e, conseqüentemente, as propriedades mecânicas dos mesmos. Tendo como principal foco a análise da mudança da zona colunar para a equiaxial, este trabalho apresenta um estudo teórico-experimental sobre a transição colunar/equiaxial (TCE) das ligas hipoeutéticas Al- 3%Si, Al-7%Si Al-9%Si solidificadas unidirecionalmente em um dispositivo horizontal refrigerado a água sob condições transientes de fluxo de calor. A condição de contato térmico na superfície de extração de calor foi padronizada como sendo polida. Os perfis térmicos foram medidos em diferentes posições do lingote e os dados foram armazenados automaticamente. Um método numérico é utilizado na determinação de variáveis térmicas de solidificação como coeficientes de transferência de calor na interface metal/molde (h_i), velocidades das isotermas liquidus (V_L), gradientes térmicos (G_L) e taxas de resfriamento (T_R) que influenciam diretamente a referida transição estrutural. Os resultados teóricos e experimentais apresentaram boa concordância. Um estudo comparativo entre os resultados obtidos neste trabalho e valores propostos na literatura para analisar a TCE durante a solidificação unidirecional vertical ascendente sob condições transientes de extração de calor das ligas investigadas, também é apresentado.

The columnar to equiaxed transition of horizontal unsteady-state directionally solidified Al-Si alloys

Experiments were conducted to investigate the influence of thermal parameters on the columnar to equiaxed transition during the horizontal unsteady-state directional solidification of Al-Si alloys. The parameters analyzed include the heat transfer coefficients, growth rates, cooling rates, temperature gradients and composition. A combined theoretical and experimental approach is developed to determine the solidification thermal variables considered. The increasing solute content in Al-Si alloys was not found to affect significantly the experimental position of the CET which occurred for cooling rates in the range between 0.35 and 0.64 K/s for any of three alloy compositions examined. A comparative analysis between the results of this work and those from the literature proposed to analyze the CET during upward vertical solidification of Al-Si alloys is reported and the results have shown that the end of the columnar region during horizontal directional solidification is abbreviated as a result of about six times higher thermal gradient than that verified during upward unidirectional solidification of alloys investigated.

Extraktion und nanoanalytische Charakterisierung refraktärer Nanometallpartikel frühester solarer Materie und Synthese metallischer Nanopartikel aus dotierten Ca-Mg-Al-Si-Schmelzen

Primitive kohlige Chondrite sind Meteorite, die seit ihrer Entstehung im frühen Sonnensystem kaum verändert wurden und dadurch einen Einblick in Prozesse geben, die zur Bildung und Veränderung der ersten festen Materie führten. Solche Prozesse können anhand von Bruchstücken dieser Meteorite detailliert im Labor studiert werden, sodass Rückschlüsse auf die Entwicklung unseres Sonnensystems im frühen Stadium getroffen werden können. Ca-, Al-reiche Einschlüsse (CAIs) aus chondritischen Meteoriten sind die ersten Festkörper des Sonnensystems und enthalten viele refraktäre Metallnuggets (RMNs), welche hauptsächlich aus den Elementen Os, Ir, Ru, Mo und Pt bestehen. Nach weit verbreiteter Ansicht sind diese Nuggets wahrscheinlich im Gleichgewicht mit dem solaren Nebel kondensiert, bereits früher oder gleichzeitig mit Oxiden und Silikaten. Die exakten Mechanismen, die zu ihren heute beobachteten Eigenschaften führten, sind allerdings unklar. Um frühere Arbeiten fortzuführen, wurde eine hohe Anzahl RMNs in vier unterschiedlichen Typen von Meteoriten detailliert studiert, darunter solche aus dem nahezu unveränderten Acfer 094, Allende (CV3ox), Leoville (CV3red) und Murchison (CM2). Die RMNs wurden in-situ, assoziiert mit ihren Wirtsmineralen und auch in Säurerückständen gefunden, deren Präparationsprozedur in dieser Arbeit speziell für RMNs durch eine zusätzliche Dichtetrennung verbessert wurde.rnDie Ergebnisse decken eine Reihe von Ungereimtheiten zwischen den beobachteten RMN-Eigenschaften und einer Kondensationsherkunft auf, sowohl für Kondensation in solarer Umgebung, als auch für Kondensation aus Material von Supernovae oder roten Riesen, für die die Kondensationssequenzen refraktärer Metalle speziell für diesen Vergleich berechnet wurden. Stattdessen wurden in dieser Arbeit neue Einblicke in die RMN-Entstehung und die Entwicklung der ersten Festkörper (CAIs) durch eine Kombination aus experimentellen, isotopischen, strukturellen und petrologischen Studien an RMNs gewonnen. Viele der beobachteten Eigenschaften sind mit Ausfällung der RMN aus einer CAI-Schmelze vereinbar. Ein solches Szenario wird durch entsprechende Untersuchungen an synthetisch hergestellten, mit refraktären Metallen im Gleichgewicht stehenden CAI-Schmelzen bestätigt. Es folgt aus den Ergebnissen, dass die Mehrzahl der RMNs isotopisch solar ist und alle untersuchten RMNs innerhalb von CAIs bei rascher Abkühlung (um bis zu 1000 °C/40 sek.) einer CAI-Schmelze gebildet wurden. rn

Evaluation of silicon twinning in hypo-eutectic Al-Si alloys

It is generally accepted that growth of eutectic silicon in aluminium-silicon alloys occurs by a twin plane re-entrant edge (TPRE) mechanism. It has been proposed that modification of eutectic silicon by trace additions occurs due to a massive increase in the twin density caused by atomic effects at the growth interface. In this study, eutectic microstructures and silicon twin densities in samples modified by elemental additions of barium (Ba), calcium (Ca), yttrium (Y) and ytterbium (Yb) (elements chosen due to a near-ideal atomic radii for twinning) in an A356.0 alloy have been determined by optical microscopy, thermal analysis, X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Addition of barium or calcium caused the silicon structure to transform to a fine fibrous morphology, while the addition of yttrium or ytterbium resulted in a refined plate-like eutectic structure. Twin densities in all modified samples are higher than in unmodified alloys, and there are no significant differences between fine fibrous modification (by Ba and Ca) and refined plate-like modification (by Y and Yb). The twin density in all modified samples is less than expected based on the predictions by the impurity induced twining model. Based on these results it is difficult to explain the modification with Ba, Ca, Y and Yb by altered twin densities alone.

Effects of boron on eutectic modification of hypoeutectic Al-Si alloys

The effects of boron on the eutectic modification and solidification mode of hypoeutectic Al-Si alloys have been studied adding different boride phases. The results show that boron does not cause modification of the eutectic silicon. Boron-containing samples display eutectic nucleation and growth characteristics similar to that of unmodified alloys. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Strength-ductility behaviour of Al-Si-Cu-Mg casting alloys in T6 temper

A comparative study of the mechanical properties of 20 experimental alloys has been carried out. The effect of different contents of Si, Cu, Mg, Fe and Mn, as well as solidification rate, has been assessed using a strength-ductility chart and a quality index-strength chart developed for the alloys. The charts show that the strength generally increases and the ductility decreases with an increasing content of Cu and Mg. Increased Fe (at Fe/Mn ratio 0.5) dramatically lowers the ductility and strength of low Si alloys. Increased Si content generally increases the strength and the ductility. The increase in ductility with increased Si is particularly significant when the Fe content is high. The charts are used to show that the cracking of second phase particles imposes a limit to the maximum achievable strength by limiting the ductility of strong alloys. The (Cu + Mg) content (at.%), which determines the precipitation strengthening and the volume fraction of Cu-rich and Mg-rich intermetallics, can be used to select the alloys for given strength and ductility, provided the Fe content stays below the Si-dependent critical level for the formation of pre-eutectic alpha-phase particles or beta-phase plates.

Eutectic modification of Al-Si alloys with rare earth metals

The effects of different concentrations of individual additions of rare earth metals (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) on eutectic modification in Al-10mass%Si has been studied by thermal analysis and optical microscopy. According to the twin-plane re-entrant edge (TPRE) and impurity induced twinning mechanism, rare earth metals with atomic radii of about 1.65 times larger than that of silicon, are possible candidates for eutectic modification. All of the rare earth elements caused a depression of the eutectic growth temperature, but only Eu modified the eutectic silicon to a fibrous morphology. At best, the remaining elements resulted in only a small degree of refinement of the plate-like silicon. The samples were also quenched during the eutectic arrest to examine the eutectic solidification modes. Many of the rare-earth additions significantly altered the eutectic solidification mode from that of the unmodified alloy. It is concluded that the impurity induced twinning model of modification, based on atomic radius alone, is inadequate and other mechanisms are essential for the modification process. Furthermore, modification and the eutectic nucleation and growth modes are controlled independently of each other.

Eutectic nucleation in Al-Si alloys

In addition to a change in silicon morphology, modification of aluminium-silicon alloys with strontium or sodium increases the size of the eutectic grains. To determine the mechanism responsible, eutectic solidification in commercial purity and ultra-high purity aluminium-si I icon alloys, with and without strontium additions, was examined by a quenching technique. In the commercial unmodified alloy, nucleation was prolific while in the high-purity unmodified alloy few eutectic grains nucleated. The addition of strontium to the commercial alloy reduced the number of eutectic grains that nucleated. Addition of strontium to the high-purity alloy did not significantly alter nucleation. It is concluded that commercial purity alloys contain a large number of potent nuclei that are susceptible to poisoning by impurity modification. The flake-to-fibre transition that occurs with impurity modification is shown to be independent of any change in eutectic nucleation mode and frequency. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.