Papers by Author: Gerard Vignoles

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.

Abstract: The chemical vapour infiltration (CVI) process is used to fabricate the interphases and matrices of CMCs. This process involves complex physico-chemical phenomena such as the transport of precursor, carrier, and by-product gases in the reactor and inside a fibrous preform, chemical reactions (pyrolysis and deposition), and the structural evolution of the preform. It is able to provide high-quality materials because the process conditions are rather mild with respect to the fibres ; however it is expensive and sometimes difficult to optimize. Many variations of the basic concept have been proposed in the past decades, introducing thermal and pressure gradients, in order to increase the efficiency. This process has been the object of extensive modelling efforts, because of imperative optimization needs. The present work is an attempt to provide a synthetic view of these models, focusing on several features of CVI modelling : i) Modelling CVI requires a multi-scale strategy, with models ranging from process scale down to atomic scale, ii) Original physicochemical couplings are involved, which require the development of adequate treatments, iii) There is a hierarchy of model refinement, ranging from fully detailed models to quasi-analytical predictions.

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.