Papers by Author: Pierre Lhuissier

Abstract: The formability of Aluminum 2050 alloy is critical for manufacturing large and thick components while maintaining its outstanding performance. To link the damage development during high-temperature loading with the alloy microstructure evolution, a time resolved tensile loading experiment at 480 °C was performed on this alloy using synchrotron diffraction and tomography, i.e. diffraction contrast tomography (DCT) to provide 3D grain maps and phase contrast tomography (PCT) to characterize pores and intermetallics. The evolution of both was quantified as a function of macroscopic strain up to 20.15%. Three pore formation mechanisms were identified: growth from pre-existing pores, fracture of the intermetallics, and nucleation of new pores. The characteristics of the pore evolution are linked with the grain structure characterized by DCT. Additionally, the grain maps reconstructed for initial and final strained states show newly recrystallized grains, indicating the presence of dynamic recrystallization. To exclude the possible explanation by annealing recrystallization, an extra annealing experiment was performed and no recrystallized grains were observed. A comprehensive insight into linking the damage development with the microstructure evolution under high-temperature deformation has been obtained by using synchrotron grain mapping techniques and tomography.

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: A way to overcome the low deformability of magnesium alloys at room temperature is toincrease the temperature of forming operations. The stress exponent n, which is known to be a keyparameter in the control of plastic stability, generally decreases when temperature increases.Nevertheless, low n-values are not enough to ensure large capacity of deformation since fracturecan also result from strain induced cavitation. In the present investigation, both the mechanisms ofhigh temperature deformation and damage were studied in selected Mg alloys. Since damage datacan also give information on the deformation mechanisms, the strain induce cavitation behaviourwas mainly studied thanks to X-ray micro tomography which provides 3D information like thecavity shapes or the variation with strain of the number of cavities. Moreover, additionally toconventional post mortem analyses, it was attempted to perform the 3D damage characterisation inin situ conditions, namely directly during high temperature deformation tests.