Demonstration of the effect of stirring on nucleation from experiments on the International Space Station using the ISS-EML facility

Abstract

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 Zr57Cu15.4Ni12.6Al10Nb5 (Vit106), as well as Cu50Zr50 and the quasicrystal-forming Ti39.5Zr39.5Ni21 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.

Modeling the Fluid Flow for Recent Solidification Experiments in Microgravity Electromagnetic Levitation

“…The fluid flow conditions were used to analyze the changes that stirring has on the nucleation rate, I(T), through changes in the prefactor, A * , which contains kinetic parameters, such as the diffusion coefficient, atomic jump distance, and the number of possible sites for nucleation, used in calculating the homogeneous nucleation with classical nucleation theory. The prefactor was found to correlate with both the maximum velocity and shear-strain rate in the sample in [5]. By comparing the maximum shear-strain rate in the flow to the prefactor, it was calculated that an order of magnitude increase in the fluid flow corresponds to an increase in the prefactor of approximately 5 order s of magnitude in Vit106 [5].…”

“…The prefactor was found to correlate with both the maximum velocity and shear-strain rate in the sample in [5]. By comparing the maximum shear-strain rate in the flow to the prefactor, it was calculated that an order of magnitude increase in the fluid flow corresponds to an increase in the prefactor of approximately 5 order s of magnitude in Vit106 [5].…”

“…However, in moving liquids, the fluid flow transports of solute throughout the melt and is expected in to increase the nucleation rate. [5], [6].…”

“…In Cu50Zr50 the prefactor increased by approximately 4 times over a 40% increase in the fluid flow. [5] Meanwhile in the Ti39.5Zr39.5Ni21, the prefactor was calculated to decrease by a factor of 33 with a factor of 4 increase in the fluid flow [5].…”

“…The results from this work suggest that the fluid flow effects are much smaller in alloys like Cu50Zr50 in which the compositional changes during solidification are smaller. [5] 3. Dynamic Nucleation and Pressure within the Sample During electromagnetic levitation experiments, subcritically undercooled melts are expected to be very stable and to remain liquid for extended periods of time.…”