The effect of fluid flow on crystal nucleation in supercooled liquids is not well understood. The variable density and temperature gradients in the liquid make it difficult to study this under terrestrial gravity conditions. Nucleation experiments were therefore made in a microgravity environment using the Electromagnetic Levitation Facility on the International Space Station on a bulk glass-forming Zr₅₇Cu_15.4Ni_12.6Al₁₀Nb₅ (Vit106), as well as Cu₅₀Zr₅₀ and the quasicrystal-forming Ti_39.5Zr_39.5Ni₂₁ liquids. The maximum supercooling temperatures for each alloy were measured as a function of controlled stirring by applying various combinations of radio-frequency positioner and heater voltages to the water-cooled copper coils. The flow patterns were simulated from the known parameters for the coil and the levitated samples. The maximum nucleation temperatures increased systematically with increased fluid flow in the liquids for Vit106, but stayed nearly unchanged for the other two. These results are consistent with the predictions from the Coupled-Flux model for nucleation.

Transitions from the liquid to the solid state of matter are omnipresent. They form a crucial step in the industrial solidification of metallic alloy melts and are greatly influenced by the thermophysical properties of the melt. Knowledge of the thermophysical properties of liquid metallic alloys is necessary in order to gain a tight control over the solidification pathway, and over the obtained material structure of the solid. Measurements of thermophysical properties on ground are often difficult, or even impossible, since liquids are strongly influenced by earth’s gravity. Another problem is the reactivity of melts with container materials, especially at high temperature. Finally, deep undercooling, necessary to understand nucleus formation and equilibrium as well as non-equilibrium solidification, can only be achieved in a containerless environment. Containerless experiments in microgravity allow precise benchmark measurements of thermophysical properties. The electromagnetic levitator ISS-EML on the International Space Station (ISS) offers perfect conditions for such experiments. This way, data for process simulations is obtained, and a deeper understanding of nucleation, crystal growth, microstructural evolution, and other details of the transformation from liquid to solid can be gained. Here, we address the scientific questions in detail, show highlights of recent achievements, and give an outlook on future work.

The thermophysical properties of liquid germanium were recently measured both in parabolic flight experiments and on the ISS in the ISS-EML facility. The viscosity measurements differed between the reduced gravity experiments and the literature values. Since the oscillating drop method has been widely used in EML, further exploration into this phenomenon was of interest. Models of the magnetohydrodynamic flow indicated that turbulence was present during the measurement in the ISS-EML facility, which accounts for the observed difference.

Oscillating drop experiments allow the surface tension and viscosity of high temperature and highly reactive melts to be measured without an interface contacting the surface of the molten sample. Surface oscillations are induced by varying the electromagnetic field. The oscillations are measured to determine the surface tension and viscosity from the frequency and damping of the oscillations, respectively. The damping of the oscillations is, however, sensitive to the flow conditions within the melt. Recent advances have allowed transient magnetohydrodynamic models to calculate changes in the internal flow in response to variations in the magnetic field, much like those used to induce surface oscillations. These models show that the excitation pulse drives rapid acceleration within the melt. While the fluid flow may accelerate to speeds above the laminar-turbulent transition, the flow speeds are not sustained for sufficient time periods to allow turbulent flow to develop. Following the excitation pulse, the flow rapidly slows and quickly returns to the conditions present before the excitation pulse.