Analysis of gravity effects during binary alloy directional solidification by comparison of microgravity and Earth experiments with in situ observation.

Under terrestrial conditions, solidification processes are influenced to a large degree by the gravity effects such as natural convection or buoyancy force, which can dramatically modify the final characteristics of the grown solid. In the last decades, the coupling of in situ observation of growth from the melt, that enables the study of microstructure formation dynamics, and microgravity experimentation, that allows to approach diffusive conditions, has been implemented for both transparent and metallic materials. The results of these investigations enable to test the validity of advanced solidification theories, to validate or develop numerical models and sometimes to reveal unexpected phenomena. The aim of this paper is to present a selection of conclusive experiments obtained with this combined approach in our group to highlight the gravity effects by a comparative study of experiments carried out on earth and in microgravity conditions.

Neutron diffraction studies for realtime leaching of catalytic Ni.

The leaching of Al from intermetallic samples of Nickel Aluminium alloys to form Raney-type nickel catalysts is widely used in the hydrogenation industry, however, little is known of the leaching process itself. In this study, the leaching of Al was measured in realtime, in situ, using the high-flux powder neutron diffractometer, D20, at the Institut Laue-Langevin. Despite the liberation of hydrogen and effervescent nature of the reaction the transformation of the dry powder phases into Raney-type Ni was determined. Samples produced by gas-atomisation were found to leach faster than those produced using the cast and crushed technique. Regardless of processing route of the precursor powder, the formation of spongy-Ni occurs almost immediately, while Ni2Al3 and NiAl3 continue to transform over longer periods of time. Small-angle scattering and broadening of the diffraction peaks is an evidence for the formation of the smaller Ni particles. Understanding the kinetics of the leaching process will allow industry to refine production of catalysts for optimum manufacturing time while knowledge of leaching dynamics of powders produced by different manufacturing techniques will allow further tailoring of catalytic materials.