Remarkable undercooling capability and metastable thermophysical properties of liquid Nb84.1Si15.9 alloy revealed by electrostatic levitation in outer space.

The stable manipulation, high undercooling, and thermophysical property measurement of the liquid Nb84.1Si15.9 refractory alloy were successfully achieved by the electrostatic levitation technique on board the China Space Station. By controlling the superheating temperature, a maximum liquid undercooling up to 421 K (0.18 TL) was obtained in the space environment, and two distinct solidification paths with different recalescence features were realized at metastable undercooled states. The liquid density and the ratio of specific heat to emissivity were measured in a wide temperature range from 1841 to 2346 K, which displayed linear and quadratic relations vs temperature, respectively. The liquid emissivity was further deduced from the specific heat of the liquid alloy calculated by molecular dynamics simulation. In addition, both the density and structural characteristics of the undercooled liquid alloy were also analyzed by MD calculations.

Thermophysical properties and atomic structure of liquid Zr-Nb alloys investigated by electrostatic levitation and molecular dynamics simulation.

The investigation of the thermophysical properties of liquid Zr-Nb alloys holds great significance for theoretical research and technical application in liquid physics. However, the high temperatures involved make their experimental measurement challenging. In this study, the densities of liquid Zr-xwt.% Nb (x= 1.0, 2.5, 6.0) alloys were examined by electrostatic levitation and molecular dynamics calculation. Remarkably, the alloys achieved maximum undercooling of 335 K, 311 K and 326 K, respectively. Correspondingly, the densities are 6.20, 6.22 and 6.26 g·cm-3at the liquidus temperatures (TL), respectively. The corresponding temperature coefficients are 2.61 × 10-4, 2.75 × 10-4and 2.84 × 10-4g·cm-3·K-1, respectively. Notably, the experimental density results align well with the simulated results. Moreover, the molar volume (Vm), thermal expansion coefficient (α) and diffusion coefficient (D) were derived based on the experimental data and simulations. The thermal expansion coefficients reduce linearly with decreasing temperature. The analysis of the pair distribution function, coordination number (CN) and the radial distribution function reveals the temperature-dependent evolution of the atomic structure. TheCNtotalandCNZr-Zrinitially increase and then decrease with decreasing temperature, while the change trends forCNZr-NbandCNNb-Nbvaried among the three alloys. The radial distribution function of three liquid alloys reveals that the atomic number density increases as the temperature drops. Additionally, the total diffusion coefficients decrease with the reduction of temperature and the rise of Nb content from 1.0 wt.% Nb to 6.0 wt.% Nb.