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In the frame of ESA-MAP (Microgravity Application Promotion) project entitled XRMON (In situ X-Ray MONitoring of advanced metallurgical processes under microgravity and terrestrial conditions), a microgravity (μg) experiment in the XRMON-GF (Gradient Furnace) setup was successfully launched in 2012 on board MASER 12 sounding rocket. During this experiment, in situ and real time observations of the formation of the solidification microstructures in diffusive conditions were carried out for the first time by using X-ray radiography. In addition, two reference experiments with the same control parameters but in ground-based conditions were performed to enable us a direct comparison with the μg experiment and therefore to enlighten the effects of gravity upon microstructure formation. This communication reports on fragmentation phenomenon observed during those experiments. For 1g upward solidification, fragmentations mainly take place in the upper part of the mushy zone. After their detachments, dendrite fragments are carried away by buoyancy force in the bulk liquid where they are gradually remelted. For μg experiment and horizontal solidification, this type of fragmentation is not observed. However, a great number of fragmentations are surprisingly revealed by in situ observation in the deep part of the mushy zone, when the liquid fraction is very small. Moreover, as soon as they are detached, the dendrite fragments move toward the cold part of the mushy zone, even in the case of μg experiment. The observations suggest that sample shrinkage may be at the origin of this fragment motion.

International audience; 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].We present here a comparative study of the fragmentation mechanism in Al-Ni alloys and Al-Si alloys during directional solidification.In-situ and real time X-ray imaging of the solidification has provided direct access to the dynamics of fragmentation mechanism. By decreasing the temperature gradient during the experiments, we have been able to approach conditions of a casting. These experiments have yielded a better understanding of fragmentation and have shown the existence of a cascade fragmentation phenomenon in non refined Al-based alloys. Moreover, by coupling these experiments with more classical directional solidification experiments in bulk samples, we have observed that fragmentation is easier in Al-Si alloys compared to Al-Ni alloys and we propose that this diverging behaviour is due to the microstructure morphology and extension of the mushy zone of both alloys.Additionally, we have performed directional solidification experiments with Accelerated Crucible Rotation Technique (ACRT). This study reveals that ACRT, by creating forced convection, can significantly modify the microstructure pattern. Using ACRT, we have been able to provoke CET in non refined Al - 7.0 wt% Si alloy.

This paper presents a summary of cellular and dendritic morphologies resulting from the upward directional solidification of Al – Ni alloys in a cylindrical crucible. We analysed the coupling of solid-liquid interface morphology with natural and forced convection. The influence of natural convection was first analyzed as a function of growth parameters (solute concentration, growth rate and thermal gradient). In a second step, the influence of axial vibrations on solidification microstructure was investigated by varying vibration parameters (amplitude and frequency). Experimental results were compared to preliminary numerical simulations and a good agreement is found for natural convection. In this study, the critical role of the mushy zone in the interaction between fluid flow and solidification microstructure is pointed out.