Structures of solid and liquid during melting and solidification of indium.

During melting and solidification of polycrystalline indium the structures of the solid and liquid phases were investigated by X-ray diffractometry (XRD) in 1g conditions. The experiments showed that melting is immediately preceded and accompanied by an abnormal increase of vacancy concentration in the solid that enhances the diffusion. At the melting point (T(M)) the lattice planes facing the first formed liquid appear to be [002] and [101]; that is, those planes allocating first and second neighbors around a given atom, with shell radii very close to the mean distance of nearest neighbors in liquid as obtained from the radial distribution function (RDF). On this basis some types of solid-liquid interface (sharp or diffuse) are discussed; the possibility of a sudden texture change in the solid is also considered. On the other hand, the evolution of RDF curve of liquid indium, cooled down through T(M), shows that there are correlations between the structure of the liquid and of the forming solid during solidification. To avoid convective motions in the liquid and to achieve the best experimental conditions, we discuss the possibility of repeating the measurements in microgravity (microg) using thin oxide films as containers. This technique was already successfully tested by one of the investigators in the experiment ES 311 A-B carried out during the mission SPACELAB-1.