A major reason to conduct solidification experiments in space is that the unique conditions accessible in reduced-gravity allow investigation of fundamental questions while limiting the influence of sedimentation or buoyancy-induced convection. When processing metallic alloys using containerless electromagnetic levitation, convection may be controlled over a wide range, spanning the laminar-turbulent transition, by proper selection of facility operating conditions. By measuring key thermophysical properties such as density, viscosity, and electrical resistivity on-orbit, the specific sample being processed may be characterized and the results used to update pre-mission magnetohydrodynamic model predictions of induced stirring within the droplet. Thus, convection becomes a controlled experimental parameter that can be applied to an investigation of how stirring influences the metastable-to-stable transformation during rapid solidification of FeCrNi alloys. For these alloys, the incubation or delay time is observed to be a weak function of undercooling and a strong function of applied convection.

Space levitation processing allows researchers to conduct benchmark tests in an effort to understand the physical phenomena involved in rapid solidification processing, including alloy thermodynamics, nucleation and growth, heat and mass transfer, solid/liquid interface dynamics, macro- and microstructural evolution, and defect formation. Supported by ground-based investigations, a major thrust is to develop and refine robust computational tools based on theoretical and applied approaches. This work is accomplished in conjunction with experiments designed for precise model validation with application to a broad range of industrial processes.

Surface tension is an essential thermophysical property of liquids, and the oscillating droplet method is particularly effective for investigations involving reactive molten alloys. The Rayleigh equation is commonly used to evaluate surface tension from measurements of the damping frequency response of an oscillating droplet with small deformation, but non-linear effects are expected to arise for larger deformation. This work describes an improved method for interpreting frequency analysis and validates previous numerical simulation and theoretical analyses which predict a decrease in observed frequency at moderate deformation amplitude. Experimental results from microgravity tests are used to determine a correction of the Rayleigh equation to eliminate the influence of finite deformation.