Navegando por Autor "Gomes, Leonardo Fernandes"
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Artigo Assessing microstructures and mechanical resistances of as-atomized and as-extruded samples of Al-1wt%Fe-1wt%Ni alloy(Elsevier, 2017-01-15) Dessi, João Guilherme; Gomes, Leonardo Fernandes; Peres, Maurício Mirdhaui; Canté, Manuel Venceslau; Spinelli, José Eduardo; Silva, Bismarck LuizCurrent applications of Al–Fe–Ni alloys include Alnico permanent magnets, industrial furnaces, and cladding of nuclear fuel plates. In spite of industrial interest, limited knowledge regarding to inter-relations between microstructure and mechanical resistance can be noted to date. Thus, the aim of the present contribution is, firstly, to analyze the microstructure features of α-Al phase (size and morphology) during atomization of the ternary Al-1wt%Fe-1wt%Ni alloy, including determination of cooling rates and hardness of the obtained powders. Secondly, the nature, size and distribution of intermetallic compounds (IMC), strength and ductility of hot consolidated bulks by extrusion from two different ranges of Al-Fe-Ni powder size (powder size between 75 and 106 μm and powder size up 106 μm and less 180 μm) are examined. The sequence of processes includes nitrogen gas atomization followed by compaction and hot extrusion consolidation at both 350 °C and 400 °C. The procedures to characterize the samples involve X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), Vickers hardness and mechanical tensile tests. Al-rich cells prevailed for either smaller or larger Al-1wt%Fe-1wt%Ni atomized powders with formation of IMCs not only in the cell walls but also precipitated within the α-Al matrix. Strength and ductility of as-extruded samples are found to be consistent with their microstructuresArtigo Dendritic growth, solidification thermal parameters, and Mg content affecting the tensile properties of Al-Mg-1.5 Wt Pct Fe alloys(Springer, 2017-02-08) Gomes, Leonardo Fernandes; Silva, Bismarck Luiz; Garcia, Amauri; Spinelli, José EduardoAl-Mg-Fe alloys are appointed as favorable ones with respect to the costs and all the required properties for successful vessel service. However, the experimental inter-relations of solidification thermal parameters, microstructure, and mechanical strength are still undetermined. In the present research work, the dependences of tensile properties on the length scale of the dendritic morphology of ternary Al-1.2 wt pct Mg-1.5 wt pct Fe and Al-7 wt pct Mg-1.5 wt pct Fe alloys are examined. Transient heat flow conditions during solidification have been achieved by the use of a directional solidification system, thus permitting a comprehensive characterization of the dendritic microstructures to be performed. Thermo-Calc computations, X-ray diffraction, and scanning electron microscopy analyses are carried out to give support to the extensive microstructural evaluation performed with both ternary Al-Mg-Fe alloys. Experimental growth relations of primary, λ 1, and secondary, λ 2, dendrite arm spacings with cooling rate (T˙L) and of tensile properties with λ 2 are proposed. For both alloys examined, Hall–Petch type formulas show that the tensile strength increases with the decrease in λ 2. The soundest strength–ductility balance is exhibited by the Al-7 wt pct Mg-1.5 wt pct Fe alloy specimen with refined microstructure. This is shown to be due to a more homogeneous distribution of intermetallic particles in connection with solid solution strengthening propitiated by Mg. Functional experimental inter-relations of tensile properties with growth (V L) and cooling rates (T˙L) for both ternary Al-Mg-Fe alloys have also been derivedArtigo Is it possible to simplify environmental monitoring? Approaches with zooplankton in a hydroelectric reservoir(Acta Limnologica Brasiliensia, 2017-07-20) Missias, Ana Caroline de Alcântara; Gomes, Leonardo Fernandes; Pereira, Hasley Rodrigo; Silva, Leo Caetano Fernandes da; Angelini, Ronaldo; Vieira, Ludgero Cardoso GalliObjetivo: A fim de contribuir para o conhecimento a respeito da simplificação de levantamentos biológicos, o presente estudo avaliou o uso de grupos substitutos, resolução taxonômica e numérica para os três principais grupos da comunidade zooplanctônica (cladóceros, copépodes e rotíferos) em uma Usina Hidrelétrica (UHE). As seguintes questões foram abordadas: (i) Os padrões de ordenação espacial e/ou temporal gerados entre cada grupo zooplanctônico são concordantes? (ii) A concordância se mantém utilizando dados de presença e ausência em substituição a dados de densidade? (iii) A identificação dos organismos em nível de espécie pode ser substituída por gênero ou família? Métodos: Foram realizadas coletas em sete unidades amostrais ao longo de cinco campanhas entre os anos de 2009 e 2010 na UHE de Serra da Mesa (Goiás, Brasil). Para avaliar a correlação entre cada par de matrizes foi utilizado o teste de Mantel. Resultados:Não se deve realizar substituições entre os grupos zooplanctônicos, sendo necessário o monitoramento dos três grupos (copépodes, cladóceros e rotíferos). Além disso, sugerimos a utilização de dados de densidade de indivíduos ao invés de apenas dados de presença/ausência de espécies. Por fim, os resultados deste estudo indicam a possibilidade de dados em nível de espécies serem substituídos por dados em nível de gênero ou família. Conclusão: Para fins de monitoramento da comunidade zooplanctônica, apenas o uso da resolução taxonômica mostrou-se eficiente para esta área de estudo, não sendo recomendado o uso de substitutos nem resolução numérica entre os gruposArtigo Microstructure characterization and tensile properties of directionally solidified Sn-52 wt% Bi-1wt% Sb and Sn-52wt% Bi-2wt% Sb alloys(Elsevier, 2020-08) Paixão, Jeverton Laureano; Gomes, Leonardo Fernandes; Reyes, Rodrigo Valenzuela; Garcia, Amauri; Spinelli, José Eduardo; Silva, Bismarck LuizSn-Bi-based Thermal Interface Materials (TIM) are adequate alloys to promote heat dissipation in power electronics. However, despite the necessary thermal connection, mechanical support for different components and substrates are of prime importance in microelectronic devices. In this framework, the effects of Antimony (Sb) additions on the microstructure and tensile properties of the Sn-52 wt% Bi alloy are investigated. Various Sn-Bi(-Sb) samples solidified at different cooling rates and two levels of Sb-containing alloys allow a comprehensive examination of length scales of either dendritic or eutectic microstructures. A number of experimental techniques are used here to permit a sound analyses of the ternary Sn-Bi(-Sb) alloys: transient directional solidification, optical microscopy (OM), triangle and intercept quantification methods, scanning electron microscopy (SEM), x-ray fluorescence (XRF), x-ray diffraction (XRD), tensile tests and fractography. The addition of Sb enhances the nucleation of primary dendritic trunks, which resulted in a decrease in the primary dendritic arm spacing (λ1) by about 5 times for the Sn-52 wt% Bi-2 wt% Sb alloy as compared to the results for the binary Sn-Bi alloy. The relationships found for tensile properties as a function of the secondary dendritic arm spacing (λ2) demonstrate that Sb additions increase the alloy strength while preserving the ductility. This is due to very thin SnSb intermetallic particles formed in the Sn-rich dendritic matrix. The influence of λ2 variation on both the yield and ultimate strengths is roughly insignificant while the ductility varies strongly between 14.4% and 52% for samples solidified from 0.05 °C/s to 5.0 °C/s respectively. When 2.0 wt% Sb is added, there is a maintenance in the levels of ductility as those found for the binary Sn-Bi alloy. This occurs especially for very refined dendritic and eutectic microstructures samples, which also exhibit a ductile fracture modeArtigo The role of eutectic colonies in the tensile properties of a Sn–Zn eutectic solder alloy(Elsevier, 2020-01-01) Ramos, Lidiane Silva; Reyes, Rodrigo Valenzuela; Gomes, Leonardo Fernandes; Garcia, Amauri; Spinelli, José Eduardo; Silva, Bismarck LuizThe growth of eutectic colonies in Sn–Cu, Sn–Zn and Sn–Ag–Cu eutectic alloys has already been reported in the literature. However, relationships between this kind of microstructure and mechanical properties remain undetermined for solders. The use of water-cooled copper (Cu) and AISI 1020 low-C steel molds and the eutectic Sn-9 wt.%Zn alloy make it possible to address this matter. The samples grown in the Cu mold demonstrated higher solidification rates than those developed in the low-C steel mold. Overall, the microstructure is constituted by Zn-lamellae embedded in a Sn-rich matrix. The Zn lamellae are not only uneven in thickness but also irregularly perforated. Due to Cu dissolution into the alloy, a small fraction of Cu5Zn8 intermetallic particles formed during solidification of the Sn-9 wt.%Zn alloy in the Cu mold. The contamination with Cu appears to be responsible for the improvement in the distribution of Zn-lamellae. The decrease in spacing between broken lamellae measured from SEM images, as well as a higher number of Zn particles per area, explain such occurrence. Ductility and tensile strength of different samples could allow the establishment of relationships among properties vs. eutectic colony spacing. For the Cu mold, the motion of Cu towards the alloy as well as higher solidification rates, allowed microstructures to be formed combining 60% of strain to fracture and 52 MPa of ultimate tensile strength. These achievements are mainly due to the finest spacings of both the eutectic colony (λχ = 36 μm) and the Zn lamellae (λL=0.9 μm), besides homogeneous distribution of Cu across the resulting microstructure