Microgravity studies of solidification patterns in model transparent alloys onboard the International Space Station

We review recent in situ solidification experiments using nonfaceted model transparent alloys in science-in-microgravity facilities
onboard the International Space Station (ISS), namely the Transparent Alloys (TA) apparatus and the Directional Solidification Insert
of the DEvice for the study of Critical Liquids and Crystallization (DECLIC-DSI). These directional-solidification devices use innovative
optical videomicroscopy imaging techniques to observe the spatiotemporal dynamics of solidification patterns in real time in large
samples. In contrast to laboratory conditions on ground, microgravity guarantees the absence or a reduction of convective motion
in the liquid, thus ensuring a purely diffusion-controlled growth of the crystalline solid(s). This makes it possible to perform a direct
theoretical analysis of the formation process of solidification microstructures with comparisons to quantitative numerical
simulations. Important questions that concern multiphase growth patterns in eutectic and peritectic alloys on the one hand and
single-phased, cellular and dendritic structures on the other hand have been addressed, and unprecedented results have been
obtained. Complex self-organizing phenomena during steady-state and transient coupled growth in eutectics and peritectics,
interfacial-anisotropy effects in cellular arrays, and promising insights into the columnar-to-equiaxed transition are highlighted.