Confirmation of Anomalous Nucleation in Zirconium

Abstract

Pure, low-oxygen zirconium samples have been observed to nucleate a solid phase under conditions during which the sample was expected to remain liquid. This phenomenon was first seen during Spacelab Mission MSL-1R (materials science laboratory) experiments and has since also been observed in the International Space Station (ISS) electromagnetic levitation (EML) facility on a different sample. Current work has been able to replicate these anomalous solidification events under a range of conditions in the ISS MSL-EML facility. The solidification events are not well explained by classical homogeneous or heterogeneous nucleation. The current theory is that collapsing voids in the melt create a local region of high pressure that results in local material being deeply undercooled and a strong driving force for solidification.

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

“…These solidification events were attributed to dynamic nucleation induced by the collapse of cavities in the fluid [10], [11] similar to the theories presented by Hunt and Jackson [12] and Childs and Frawley [13]. Work presented in [14] replicated the solidification events reported by Hofmeister [10] and further explored hypotheses explaining the anomalous solidification events.…”

“…If this were to be the case, it is expected that the sample would be unable to reach the very deep undercoolings expected for pure zirconium. However, the zirconium sample used in [14] was allowed to free cool and achieved undercoolings of 321.7±5.7°C below the melting temperature before, during and after the experiments in which the sample solidified at subcritical undercoolings. A subset of this hypothesis, is that the sample may have been contaminated with oxygen, providing for interstitial atoms to increase subcritical nuclei beyond the critical size.…”

“…There are two forms of solid material in the sample chamber that could possibly accomplish this: "dust" particles and "flakes" of material. While larger "flakes" of material could, in theory, trigger heterogeneous nucleation, the flakes of material are rare in EML and are unlikely to account for the large number of solidification events observed in [14]. Solid "dust" particles have also been suggested to provide heterogeneous nucleation sites for solidification in the sample.…”

“…Further experiments were done in 2018 on the same zirconium sample in the ISS-EML to confirm the anomalous solidification event. These experiments are described in detail in [14]. The 2018 experiment series, held the molten zirconium sample at subcritial undercoolings from 45° to 290° below the melting temperature.…”

Electromagnetic levitation containerless processing of metallic materials in microgravity: thermophysical properties

“…Project USTIP was initiated to bring in magnetohydrodynamic models to support other experiments on ISS-EML. These models have been applied to a wide range of experiments on topics including nucleation 80 , 81 , thermophysical property measurements 82 , and forthcoming experiments on the transition to turbulent flow in EML drops. The models rely on the calculated magnetic field to determine the resulting magnitude and pattern of the flow.…”