INTRODUCTION Directional solidification of binary alloys, which allows the independent control of growth parameters (pulling velocity V, temperature gradient G, initial solute concentration C 0 ), is an experimental method of choice for the investigation of many fundamental problems (e.g. microstructure formation and selec- tion, segregation of chemical species) encountered in the pro- cessing of structural materials. Two major features should be noticed. First, solute is rejected in the liquid during the phase transition when the segregation coefficient k is less than unity (k is the ratio of the solute concentration in solid to that in liq- uid), or rejected when k is higher than unity. Consequently, solidification establishes an exponential solute distribution in the melt which accompanies the thermal profile imposed to drive the growth process. Thus, on Earth, the local liquid densi- ty depends on two fields and convection is almost always pres- ent in the melt [1]. Second, the planar solid-liquid interface may undergo the Mullins - Sekerka instability [2] which leads to the formation of cells and dendrites. Those patterns are described by shape parameters (primary and secondary spacings, tip radius …) and characteristic relationships have been proposed to relate the shape parameters to the processing conditions when the transport of heat and chemical species is dominated by dif- fusion [3]. Actually, natural buoyancy-driven convection often plays an important role in terrestrial solidification processing by interacting with the microstructure and modifying the relation- ships. Moreover, between the fully solid and liquid phases the solid microstructure forms a solid-liquid zone called “mushy zone”, where the phase transition begins and progressively gets complete. For fluid mechanics, this mushy zone can be consid- ered as an intermediate porous medium whose internal structure is composed of fine-scale crystals, through which residual melt flows [4]. In Bridgman solidification, one way to prevent thermosolutal convection is to solidify in a both thermal (i.e. vertical upward solidification) and solutal (i.e. rejected solute denser than sol- © Z-Tec Publishing, Bremen Microgravity sci. technol. XVI B. Billia, H. Nguyen Thi et al: Tailoring of Dendritic Microstructure in Solidification Processing by Crucible Vibration/ Rotation 15 ___________ Mail Address: 1) L2MP, University Aix-Marseille III, Marseille, France 2) National Microgravity Laboratory, CAS, Beijing, China 3) Institute of Continuous Media Mechanics UB RAS, Perm, Russia 4) Laboratoire de Modélisation en Mécanique, Marseille, France 5) Department of Chemical Engineering, National Taiwan University, Taiwan Tailoring of Dendritic Microstructure in Solidification Processing by Crucible Vibration / Rotation B. Billia 1 , H. Nguyen Thi 1 , G. Reinhart 1 , Y. Dabo 1 , B.H. Zhou 1,2 , Q.S. Liu 2 , T.Lyubimova 3 , B. Roux 4 , C.W. Lan 5 Directional solidification of alloys, which allows the independ- ent control of growth parameters (pulling velocity, temperature gradient), is an experimental method of choice for the investi- gation of many fundamental problems (e.g. microstructure for- mation and selection, segregation of chemical species) encoun- tered in the processing of structural materials. Upward direc- tional solidification is carried out on hypoeutectic Al-Ni alloys, under natural and controlled fluid-flow conditions. First, the influence of natural convection on solidified dendritic microstructure is analyzed as a function of growth parameters. Then, directional solidification experiments with axial vibration are performed. It results that crucible vibration can be used to either damp or control fluid flow in the melt, and thus tailor columnar or "equiaxed" dendritic mush. Advanced modeling and numerical simulation are essential to clarify and quantify the various physical effects. Microgravity benchmark experi- ments under diffusion transport, and possibly with crucible rotation, are foreseen using the Materials Science Laboratory of ESA on ISS.