Extraordinary Solidification Mechanism of Liquid Alloys Under Acoustic Levitation State

Abstract

The acoustic levitation of various materials can be realized by highly intensive ultrasound, which provides a free surface and containerless state for materials processing under space simulation conditions. The nonlinear effects such as acoustic radiation pressure, acoustic streaming, and ultrasonic cavitation open up special access to modulate the fluid dynamics and solidification mechanisms of liquid materials. Here, the physical characteristics of liquid flow, undercooling capability, phase separation, and crystal nucleation and growth within acoustically levitated droplets are explored comprehensively to reveal the extraordinary solidification kinetics of liquid alloys. The sectorial shape oscillations of the 2nd to 10th order modes accompanying internal potential flow are observed for water droplets with modulated ultrasound amplitudes, while the enhanced ultrasound intensity promotes ice nucleation and thus reduces water undercooling. The migration of Sn‐rich globules during phase separation of immiscible Al–Cu–Sn alloy is dominated by the droplet deformation and rotation related to acoustic levitation. The high undercooling states of liquid Ag–Cu–Ge and Ni–Sn alloys during acoustic levitation result in the refinement of (Ag) dendrites and the formation of anomalous (Ni+Ni3Sn) eutectics. The ultrasound–liquid interaction also induces surface waves during the containerless solidification of Ag–Cu and Ni–Sn eutectic alloys.

The dynamics of vertical coalescence of acoustically levitated droplets

“…Among these techniques, acoustic levitation uses sound waves to levitate any material, including solids, liquids and small living animals independent of material properties (Andrade, Pérez, and Adamowski 2018). These versatilities of acoustic levitation have found applications in analytical chemistry (Santesson and Nilsson 2004), material science (Geng et al 2022), micro-assembly (Shi et al 2019) as well as in simulating microgravity on earth (Vashi et al 2023).…”

Freezing Shrinkage Dynamics and Surface Dendritic Growth of Floating Refractory Alloy Droplets in Outer Space

“…The liquid-solid phase transition has a strong relationship between the microstructure/performance and the physical fields of solidification, then resulting in the deep undercooling, rapid solidification dynamics, and multiple microstructure patterns. [1][2][3][4][5][6][7][8] In outer space where a stable microgravity condition is provided, the solidification of liquid alloy would not only produce new microstructures but also reveal various interesting scientific puzzles concealed by the earth's gravity. However, there are still many challenges to perform this kind of experiment due to the scarce chances and high temperatures, especially for refractory DOI: 10.1002/adma.202313162 alloys with liquidus temperature beyond 2000 K. Therefore, various methods were used to achieve the microgravity environment, measure thermophysical properties, obtain novel microstructures, and promote the development of solidification theories.…”

“…However, there are still many challenges to perform this kind of experiment due to the scarce chances and high temperatures, especially for refractory DOI: 10.1002/adma.202313162 alloys with liquidus temperature beyond 2000 K. Therefore, various methods were used to achieve the microgravity environment, measure thermophysical properties, obtain novel microstructures, and promote the development of solidification theories. [5,[9][10][11] China Space Station (CSS) was completely set up in November 2022, with a microgravity level of 10 −5 g 0 (g 0 is the acceleration of gravity on the ground). The buoyancy convection caused by gravity in the solidification process of the high-temperature droplets can be ignored at such a microgravity level.…”