Papers by Author: Alexandre Velhinho

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Authors: Alexandre Velhinho, P.D. Sequeira, Francisco Manuel Braz Fernandes, José D. Botas, L.A. Rocha
Authors: Alexandre Velhinho, Francisco Manuel Braz Fernandes, S.C. Ferreira, L.A. Rocha, Gerard Vignoles, P. Cloetens
Abstract: This paper provides a brief overview of the possibilities offered by X-ray computed microtomography, and particularly synchrotron radiation X-ray microtomography, regarding metal matrix composite characterization, emphasis being placed in the case of Al-based functionally graded materials. Examples are provided concerning the characterization of the reinforcement population, interfacial properties, in-situ transformation and damage evolution. The specific needs of the technique and limitations to its widespread use are mentioned.
Authors: Nelson Silva, Alexandre Velhinho
Abstract: In the present paper, work of a preliminary nature pertaining to the development of a simple tool for easy identification of reinforcement clustering in particle-reinforced MMCs is reported. The tool consists of an image processing and analysis routine performed with the aid of a public-domain software. Results obtained were in the form of distribution curves pertaining to the Euclidian distances between particle centroids within sample images. Analysis of that distribution allowed the identification of cluster occurrence. The technique was successfully applied to computer-generated images of virtual MMCs, as well as to micrographs of rheocast MMCs. Apart from particle clustering, the distribution curves were found useful to roughly assess the matrix cell size, for cases where this is significantly higher that the size of the reinforcing particles.
Authors: José D. Botas, Alexandre Velhinho, Rui Jorge C. Silva
Abstract: Current technology provides means of fabrication of spherical micro-particles, either hollow or compact, for all engineering materials. Such spherical particles can be further embedded into another material to build-up either random dispersions or close-packed arrays, according to the production route and the degree of anisotropy intended for the ultimate composite material. In this study, a simple analytical formula for the composite stiffness is derived from an early micromechanics model, to describe the actual reinforcement of ductile matrices by a random dispersion of uniform spherical ceramic particles. Predictions from this model are checked against some other relevant models, and specific features arising from its theoretical derivation are pointed out.
Authors: Alexandre Velhinho, P.D. Sequeira, Rui Miguel S. Martins, Gerard Vignoles, Francisco Manuel Braz Fernandes, José D. Botas, L.A. Rocha
Authors: Alexandre Velhinho, Gerard Vignoles, P. Cloetens, X. Thibault, Elodie Boller, Francisco Manuel Braz Fernandes, L.A. Rocha, José D. Botas
Abstract: Reliability of functionally graded metal matrix composites (FGMMCs) for automotive components is still dependent on the detailed knowledge of the mechanisms of the microstructural build-up, for instance on the mechanisms leading to the distribution and relative positions of the reinforcing particles. In order to assess the influence of the SiC particle size on the 3-D inter-particle connectivity in functionally graded Al/SiCp composites produced by centrifugal casting, X-ray microtomography experiments were performed at the ID19 beamline in ESRF (European Synchrotron Radiation Facility). The FGMMCs consisted of an Al-10Si-2Mg alloy matrix, reinforced by an average SiC particle volume fraction of 0.10; two different average sizes were used: 37 µm and 12 µm. The holographic modification of the X-ray CMT (Computer Micro- Tomography) method allowed to obtain neatly contrasted images, as opposed to classical CMT.Good agreement was found between the particle size evaluated by CMT and by laser interferometry. Particle clustering has been evaluated in number and volume, showing that a lower mean particle size is related to more clustering. Such an adverse effect relies on the importance of particle/liquid alloy surface tension. Also, the mean particle size has been evaluated as a function of particle number within a cluster: as expected, the larger a cluster, the larger the particles inside it.
Authors: Alexandre Velhinho, Francisco Manuel Braz Fernandes, José D. Botas
Authors: S.C. Ferreira, Alexandre Velhinho, L.A. Rocha, Francisco Manuel Braz Fernandes
Abstract: Syntactic functionally graded metal matrix composites (SFGMMC) are a class of metallic foams in which closed porosity results from the presence of hollow ceramic microspheres (microballoons), whose spatial distribution varies continuously between the inner and the outer section of the part, thus resulting in a continuous variation in properties. In this work, aluminiumbased SFGMMC rings were fabricated by radial centrifugal casting. The graded composition along the radial direction is controlled mainly by the difference in the centrifugal forces which act on the molten metal matrix and the ceramic particles, due to their dissimilar densities. In this case where the density of the SiO2-Al2O3 microballoons is lower than that of molten aluminium, the particles show a tendency to remain closer to the inner periphery of the ring. Thus the microballoon volume fraction increases along the radial direction of the ring from the outer to the inner periphery; in other words, the particle-rich zone is limited to an inner layer of the ring. Precursor conventional MMCs were prepared by stir-casting from the constituent materials, by homogeneously dispersing commercial SiO2-Al2O3 microballoons (particle size: 50 µm; particle volume fraction: 5 and 10 %) within a molten commercial Al-7Si-0.3Mg (A356) alloy. The resulting MMCs were then re-melt and centrifugally cast in order to produce the functionally graded composites. Particle gradients in the centrifugally cast composites were investigated by quantitative image analysis of optical micrographs (for the estimation of the particle volume fraction, mean particle diameter and porosity volume fraction).
Authors: Alexandre Velhinho, José D. Botas, Edith Ariza Avila, J.R. Gomes, L.A. Rocha
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