Papers by Author: Luc Salvo

Abstract: Due to limited deformability at room temperature, high temperature forming of magnesium alloys appears as an interesting alternative. Superplastic properties can be obtained in the case of fine grained magnesium alloys and in this regime, due to significant damage sensitivity, fracture strain is mainly controlled by nucleation, growth and coalescence of cavities. Magnesium alloys with large grained alloys can also exhibit interesting deformabilities at high temperature since dislocation movements can be controlled by a solute drag effect promoting plastic stability. Examples of such situations are presented in the case of wrought magnesium alloys, the associated damage mechanisms being investigated thanks to 3D X-ray micro tomography performed in continuous mode, namely directly during high temperature deformation tests.

Abstract: As for aluminium alloys, magnesium alloys are generally sensitive to strain induced cavitation when they are deformed in superplastic conditions. It has been widely shown that X-ray micro tomography is a particularly efficient tool for studying in 3D damage mechanisms during superplastic deformation. However, such characterisations are generally performed in post mortem conditions, namely on samples first deformed up to given strains and then characterised. In the present investigation, thanks to particularly short acquisition times offered by ESRF, damage induced by superplastic deformation of a magnesium alloy is studied thanks to tomography analyses performed in 4D conditions, namely directly during high temperature deformation tests. Such conditions provide unique opportunities for investigating nucleation, growth and coalescence of cavities since it is thus possible to follow each cavity up to the fracture process.

Abstract: X-ray micro-tomography has been applied recently in a wide range of research fields (damage in materials, solidification …). Thanks to the high flux of synchrotrons and specific cameras the total time to acquire a scan was considerably reduced. The use of a specific camera based on CMOS technology allows dividing the acquisition time for a complete scan by a factor of 100. Therefore we have been able to perform in situ solidification of aluminium-copper alloys at high cooling rates (between 1 and 10°C/s) and we will show results concerning the evolution of the microstructure in 3D in the early stage of solidification, in particular the morphology of the solid phase and the kinetics of growth.

Abstract: The deformability of wrought magnesium alloys at room temperature is limited and a way to overcome this limit is to carry out forming operations in warm or hot conditions. In the case of fine grained alloys, superplastic properties can be generally achieved but in this regime, the Mg alloys are sensitive to strain induced cavitation. However, large grained alloys can also exhibit quite large deformabilities when they are deformed at high temperature. This can be due to the fact that on one hand, the Mg alloys may quite easily dynamically recrystallize and on the other hand, that dislocation movements may be controlled by a solute drag effect leading to significant strain rate sensitivity parameters. These various mechanisms of deformation will depend on the composition, the mean grain size and the conditions of deformation (i.e. temperature and strain rate). In this work, the high temperature deformation mechanisms as well as the associated damage mechanisms of two wrought magnesium alloys are discussed.

Abstract: This paper is concerned with an investigation of the deformation behaviour of an Al-Cu alloy during tensile testing in the semi-solid state. It was carried out by fast in-situ X-Ray microtomography at ESRF, Grenoble. Deformation was performed at constant velocity, which was chosen to be small enough so as not to affect the acquisition of the images. It is observed that deformation is accompanied, initially, by some liquid flow from the adjacent regions towards the deformed zone. Then pores form in the liquid films and grow until they occupy a significant part of the cross section of the specimen. Quantification of this phenomenon was carried out thus leading to a better understanding of pore formation in semi-solid mixtures.

Abstract: The aim of this work is to study by X-Ray microtomography carried out at ESRF Grenoble the microstructure of an Al-4wt%Cu alloy which was previously cold rolled to obtain globules of the solid phase upon heating in the semi-solid range. Since this process produced entrapped liquid in the globules, 3D quantification of this liquid was performed. Moreover, the influence of the addition to the alloy of a small amount of Ba, which has been shown to decrease the contiguity between the solid globules as a consequence of the decrease of the solid-liquid interfacial energy σsl, was investigated. It is in particular shown that the amount of entrapped liquid is much larger in the Ba-containing alloy in agreement with the reduction of σsl, whereas the size of the liquid pockets is similar. In addition characterization of the interglobular liquid shows that the interface area between this liquid and the solid per unit volume is larger for the alloys containing Ba in agreement with previous observations carried out on 2D sections. The influence of strain during cold rolling is also reported but it is shown to have a quite limited influence on the previous parameters.

Abstract: Entangled materials are similar to cellular materials, with regard to their low density and discrete architecture. In this work steel wool (sintered in a furnace for various time at two temperatures) and non sintered steel wool are investigated. Experimental mechanical compression tests were performed on both materials. Compression stress and Young’s modulus are extracted and compared with the time and temperature of sintering, and initial density. The results are analyzed using a classical Toll’s model. A special attention is paid to the value of the exponent which relates stress and Young’s modulus to density. This exponent ranges from 3 to 5 for non sintered wool, and is close to 3 for the stress law and 4 for the Young’s modulus law for sintered wool.

Abstract: The evolution of the microstructure of semi-solid Al-Cu alloys has been characterized during isothermal treatments in the semi-solid state by using in-situ X-ray microtomography experiments carried out at ESRF, Grenoble. This work is therefore a continuation of previous experiments reported at the 8th S2P Conference in Limassol. Quantitative data are presented dealing with changes with holding time of the solid volume fraction, of the solid-liquid interface area and of the mean and Gaussian curvatures of the solid phase. Discussion of the results is carried out on the basis of the mechanisms involved in semi-solid systems.

Abstract: The replication process is presented and discussed with emphasis on methods for microstructural tailoring of open-pore microcellular aluminium-based foams, highlighting methods it offers for control of principal foam mesostructural and microstructural parameters: pore volume fraction, pore shape, pore size(s), as well as the composition and microstructure of the metal making the foam.