Papers by Author: Nathalie Bergeon

Abstract: The columnar to equiaxed transition (CET) has been widely studied for many years [1] because this phenomenon is observed in metallurgical applications like castings. In non refined alloys, detachment of dendrite fragments is the most probable mechanism responsible for the formation of an equiaxed microstructure [1]. In this frame, melt convection influences the grain structure evolution by playing a role in the fragmentation phenomena [2].

Abstract: In the present study, we report on an image analysis procedure, which enables to extract from synchrotron radiographs the long range solute profiles in the whole sample and in both phases (solid and liquid). This image analysis is based on the measurement of local density differences, and is applied to study the directional solidification of Al - 4wt% Cu alloy, from planar to onset of the initial instability. Dedicated experiments were carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). In order to validate this analysis the value of a key solidification parameter, namely the partition coefficient, was experimentally determined during the planar solidification, and a very good agreement was found with value found usually in the literature. On a further step, the evolution of the microstructure and solute profile during the initial transient of solidification was analysed in detail.

Abstract: The properties of structural materials are to a large extent determined by the solid microstructure so that the understanding of the fundamental physics of microstructure formation is critical in the field of materials engineering. A directional solidification facility dedicated to the characterization of solid-liquid interface morphology by means of optical methods has been developed by CNES in the frame of the DECLIC project. This device enables in situ and real time studies on bulk transparent materials. The aim of the project is to perform experiments in microgravity to eliminate the complex couplings between solidification and convection and to get reliable benchmark data to validate and calibrate theoretical modeling and numerical simulations. Presently, ground experiments are performed to finalize the design and the experimental procedures and to guarantee the accuracy of the measurements. These experiments also provide reference data for the study of solidification microstructure dynamics in the presence of buoyancy-driven natural convection. Recent progress is presented concerning the control of the interface shape (critical for pattern analysis), the selection of single crystal of defined orientation (critical for dendritic growth) and the analysis of the dendrite shape.