Confirmation of Anomalous Nucleation in Zirconium
- PDF / 1,206,890 Bytes
- 7 Pages / 593.972 x 792 pts Page_size
- 9 Downloads / 177 Views
https://doi.org/10.1007/s11837-020-04257-7 2020 The Minerals, Metals & Materials Society
MATERIALS RESEARCH IN REDUCED GRAVITY
Confirmation of Anomalous Nucleation in Zirconium G.P. BRACKER ,1,4 S. SCHNEIDER,2 R. WUNDERLICH,3 H. FECHT,3 J. ZHAO,1 and R.W. HYERS1 1.—University of Massachusetts Amherst, Amherst, USA. 2.—Deutsches Zentrum fu¨r Luft- und Raumfahrt, Cologne, Germany. 3.—Ulm Universita¨t, Ulm, Germany. 4.—email: [email protected]
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.
INTRODUCTION The transformation between the noncrystalline melt structure and the crystalline solid has been well studied in liquid metals. The solidification begins at a nucleation site from which a crystalline solid grows. These nucleation sites can originate either homogeneously in the melt or heterogeneously on a site that reduces the critical volume of the nucleus to initiate growth. The rate of formation of these nucleation sites depends on the free energy of the system, temperature, atomic vibration frequency, activation energy, and other factors. In non-glass-forming liquid metals, the rate of nucleation is heavily dependent on temperature, increasing rapidly from functionally zero to nearly instantaneous over a very narrow temperature range.1 The specifics of free energy and temperature at which this increase occurs depends on the properties of the melt and solid crystal. During solidification, heterogeneous nucleation typically dominates the formation of nuclei because the heterogeneous nucleation sites reduce the free energy required to form a nuclei with a supercritical radius to allow for growth.1 However, heterogeneous nucleation can be minimized using levitation techniques in a vacuum. Electromagnetic levitation (EML) is one such
technique that utilizes a magnetic field to control the position of the sample and to induce heating in the sample. During electromagnetic levitation in vacuum, the magnetic levitation eliminates contact with the container and a gas, minimizing heterogeneous nucleation sites. This allows access to deep undercoolings, often on the order of 200C–300C or more.2 During electromagnetic levitation (EML) processing, it is typically expected that a subcritically undercooled melt can be held at constant, subcritical, unde
Data Loading...
