To address these questions, members of this research group are in the flight definition phase of an experimental investigation to be made on the International Space Station (ISS), using an electromagnetic levitation facility currently under construction by the European Space Agency (ESA). Convection decreases the impact of long-range diffusion in terrestrial undercooling experiments for low viscosity liquids.  The high viscosity of quasicrystal melts, particularly the Ti-TM-Si-O ones, and the reduced gravity, on the ISS will allow the investigation of diffusion on nucleation during solidification.  Maximum undercoolings and growth velocities for TiZrNi and TiFeSiO liquids that form quasicrystals and crystal approximants will be measured.  Also, the specific heat, viscosity, density and electrical resistivity will be measured in the liquid as a function of temperature.

To supplement the flight experiment results, ground-based undercooling and structural studies are already underway, using the electrostatic levitation facility (ESL) at Marshall Space Flight Center, Alabama, and the Advanced Photon Source (APS) at Argonne National Laboratory, Illinois.  The terrestrial undercooling studies will allow an evaluation of the role of convection on the nucleation and growth of complicated crystal phases.  The x-ray studies on the APS will allow a quantification of the character of the developing short-range order in the liquid.  We are completing the determination of the TiFeSiO and TiZrNi phase diagrams and have made the first measurements of the undercooling of a polytetrahedral crystal phase, the C14 hexagonal Laves Phase.

The scientific impact of the proposed studies is clear, leading to an improved understanding of the structures of undercooled liquids and polytetrahedral solid phases, the growth behavior of complex periodic and ordered nonperiodic phases, and the mechanism by which oxygen stabilizes the icosahedral order in Ti-TM-Si-O alloys.  They will also expand our understanding of nucleation when the concentrations of the initial and final phases are different, and when diffusion kinetics due to partitioning are competitive with interfacial attachment kinetics.  This improved knowledge will lead to better process control and the design of processing parameters for technological materials development.