The damage and deformation behaviour of particle reinforced aluminum matrix composites can be highly sensitive to local variations in spatial distribution of reinforcement particles, which markedly depend on melt processing and solidification stages during production. The present study is aimed at understanding the mechanisms responsible for clustering of SiC particles in an Al-Si-Mg (A356) alloy composite during solidification process and establishing a model to predict the risk of cluster formation as a function of local solidification rate in a cast component. Special emphasis has been given to spatial characterization methods in terms of their suitability to characterize composite microstructures. Result indicate that methods that present a summary statistics on the global level of heterogeneity have limited application in quantitative analysis of discontinuously reinforced composites since the mechanical response of such materials are highly sensitive to dimensions, locations and spatial connectivities of clusters. The local density statistics, on the other hand, was observed to provide a satisfactory description of the microstructure, in terms of localization and quantification of clusters. A macrotransport - solidification kinetics model has been employed to simulate solidification microstructures for estimation of cluster formation tendency. Results show that the distribution of SiC particles is determined by the scale of secondary dendrite arms (SDAS). In order to attain the lowest amount of particle clustering, the arm spacings should be kept within the limit of 2dSiC &gt / SDAS &gt / dSiC, where dSiC is the average particle diameter.

[pt] Esta pesquisa teve como objetivos promover variações microestruturais em um ferro fundido nodular ferrítico tipo ASTM A536 60-40-18, pelo do uso de resfriadores durante a solidificação do material e pela adoção de tratamentos térmicos pós-fundição, bem como avaliar a influência de tais variações microestruturais sobre as propriedades mecânicas e vida em fadiga do material. Inicialmente, amostras do material foram fundidas em areia com e sem o uso de resfriadores. Após a fundição, algumas amostras de ambas as condições de resfriamento sofreram normalização, enquanto outras foram recozidas, perfazendo seis diferentes condições microestruturais: sem resfriador bruta de fundição (SRB); sem resfriador normalizada (SRN); sem resfriador recozida (SRR); com resfriador bruta de fundição (CRB); com resfriador normalizada (CRN) e com resfriador recozida (CRR). Em seqüência, corpos de prova de todas as condições mencionadas acima foram ensaiados em tração e, posteriormente, sofreram análises metalográficas qualitativas e quantitativas. Dando continuação a etapa experimental, levantaramse as curvas tensão versus número de ciclos para a falha das condições microestruturais SRB, SRR, CRB e CRR, por meio de ensaios de fadiga em flexão rotativa. A vida em fadiga do material também foi relacionada com às características metalúrgicas de cada condição. / [en] The objectives of the present work were to promote microstructural variations in an ASTM A 536 60-40-18 ferritic nodular cast iron, making use of cast coolers during and adopting heat treatments after casting, as well as to evaluate the influence of the microstructural variations on the mechanical properties and fatigue life of the material. Initially, samples of the material were cast in sand moulds with and without internal coolers. After casting, a number of samples in both cooling conditions were subjected to normalizing and annealing heat treatments and, therefore, six different microstructural conditions were obtained: as- cast without cooler (SRB), normalized without cooler (SRN), annealed without cooler (SRR), as-cast with cooler (CRB), normalized with cooler (CRN) and annealed with cooler (CRR). Tensile test were carried out on specimens representing the above mentioned conditions and their microstructures were analysed by qualitative and quantitative metallography. In the sequence of the experimental procedure, rotating bend fadigue test were performed in order to establish the stress-life curves for the SRB, SRR, CRB and CRR microstructural conditions. The fadigue life of the material was also related to the metallurgical characteristics of each condition. The metallographic analysis has indicated that the use of coolers increases the number of graphite nodules, reduces the graphite nodule size and otimizes the spherical morphology. However, these microstructural modifications seam to have no significant influence on the mechanical properties of the material. As to the effect of normalizing annealing, boht treatments not imply in significant changes in the mechanical properties of the as-cast conditions. Regarding the fadigue resistance of the material in the as-cast conditions, the CRB sample showed a longer fatigue life than the SRB samples and this performance under cyclic loading was attributed to the fact that the CRB condition is characterized by a higher number of graphite nodules of class VI. A comparison between the as-cast and annealed conditions indicated that the microstructural conditions SRR and CRR presented a lower fatigue resistance than the conditions SRB and CRB, respectively. This behaviour was associated with the microstructural features of the as-cast conditions, namely the smaller graphite nodule size, the higher microhardness of the matrix and the presence of the bull`s-eye structure. Finally, the experimental stress-life curves were modelled by means of Coffin-Manson law, which was considered efficient in the fadigue life data of ferritic nodular cast iron.

[pt] Esta pesquisa teve como objetivo avaliar a vida-fadiga de dois ferros fundidos nodulares modificados metalurgicamente, sendo uma classe predominantemente ferrítica e outra.perlítica. Inicialmente, amostra dos dois ferros fundidos nodulares ferrítico e perlítico foram fundidas adotando-se moldação em areia na geometria padrão Y-block. Em seqüência, corpos de prova para ensaios mecânicos e de fadiga foram usinados das amostras ferríticas e perlíticas. Após os ensaios de tração e dureza, realizaram-se análises metalográficas qualitativas e quantitativas em ambos os materiais, com o intuito de se determinar suas características metalúrgicas, tais como contagem, distribuição e classe dos nódulos de grafita, bem como quantidade da matriz ferrítica e perlítica. Dando continuidade a etapa experimental, as curvas tensão versus número de ciclos para a falha do ferro fundido nodular ferrítico e do ferro fundido nodular perlítico foram levantadas por meio de ensaios de flexão rotativa. A vida útil em fadiga dos dois materiais foi relacionada com as suas características metalúrgicas. Quanto a resistência à fadiga, as amostras do ferro fundido nodular perlítico tiveram um melhor comportamento sob carregamento cíclico do que as amostras do ferro fundido nodular ferrítico. Tal comportamento superior foi atribuído a maior microdureza da matriz e a presença da estrutura olho-de-boi. Finalmente, as curvas experimentais tensão versus número de ciclos para a falha dos ferros fundidos nodulares ferrítico e perlítico foram modeladas pela equação de Coffin-Manson, que se mostrou eficiente no tratamento de dados experimentais da vida em fadiga de ambos os materiais. / [en] The objective of the present work was to evaluate the fatigue life of two nodular cast irons with metallurgical modifications and resulting in ferritic and perlitic different classes of material. Initially, samples of both materials were cast in sand moulds adopting the standard Y-block geometry. In the sequence, tensile and fatigue specimens were machined from the ferritic and perlitic samples. After the tensile and hardness tests, the microstructure of the both materials were analyzed by qualitative and quantitative metallography, aiming to characterize their metallurgical aspects as content, distribution and class of graphite nodules, as well as the contents of the ferritic and perlitic matrix. Following the metallurgical characterization, rolating bend fatigue tests were performed in order to estabilish the stress-life curves of the ferritic and perlitic nodular cast irons. Regarding the fatigue resistance, the specimens machined from the perlitic nodular sample showed a longer fatigue life than that related to the ferritic nodular specimens. The longer fatigue life of the perlitic nodular specimens was associated with a higher microhardness of the perlitic matrix and the preserve of the bull`s-eye structure. Finally, the experimental stress-life curves of the ferritic and perlitic nodular cast irons were modeled adopting the Coffin-Manson law, which was considered efficient in fitting experimental fatigue life data of both materials.