Dynamic nucleation in sub-critically undercooled melts during electromagnetic levitation

By classical nucleation theory, sub-critically undercooled melts in electromagnetic levitation are expected to be very stable for extended periods of time. While electromagnetic levitation experiments are typically consistent with classical nucleation theory, there have been several historical instances including experiments on zirconium during the MSL-1 campaign and during the IML-2 experiments where the molten sample solidified at sub-critical undercoolings. While different electromagnetic levitation conditions were present in these anomalous nucleation events, both sets of anomalous nucleation events were attributed to dynamic nucleation. The work presented here investigates more recent experiments in the International Space Station Electromagnetic Levitation facility in which both sets of anomalous nucleation events were replicated and further investigated. The conditions of the MSL-1 solidification events were replicated in the ISS-EML using a pure zirconium sample held in isothermal conditions between 45 °C and 290 °C below the melting temperature. The sample successfully solidified in 18 of these experiments in less than 600 s where classical nucleation theory predicts the melt to remain liquid for very long periods of time. The IML-2 result, was replicated in the ISS-EML in which a Zr64Ni36 sample was used to demonstrate pulse-triggered nucleation events. During these experiments, the excitation pulse triggered solidification in the sample which was held between 59.5 °C and 64.5 °C below the melting temperature. Again, classical nucleation predicts that under these conditions the sample will remain molten. The results of both sets of experiments are consistent with dynamic nucleation theory in which nucleation events are affected by the flow conditions within the drop.