Experimental investigation and numerical simulation on liquid phase separation of ternary Fe-Sn-Si/Ge monotectic alloy
Authors: Wu Yu-Hao  , Wang Wei-Li  , Wei Bing-Bo  ,   Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072, China
Published: December 2020

The liquid phase separation of small Fe-Sn-Si/Ge alloy droplets under reduced-gravity condition is investigated experimentally by free fall technique and theoretically by lattice Boltzmann method. In the drop tube experiments, the Fe-Sn-Si/Ge monotectic alloys are heated by induction heating in an ultrahigh vacuum chamber and further overheated to 200 K above their liquid temperatures for a few seconds. Finally, the molten alloy melt is ejected out from the small orifice of a quartz tube by high pressure jetting gas of He and dispersed into numerous tiny droplets, which are rapidly solidified during free fall in a protecting He gas environment. These droplets benefit from the combined advantages of high undercooling, containerless state and rapid cooling, which can provide an efficient way to study the liquid phase separation of high-temperature alloys in microgravity. In order to efficiently reproduce the dynamic process of phase separation inside drop tube equipment, the effects of surface segregation and Marangoni convection are introduced into the interaction potential of different liquids within lattice Boltzmann theory. Based on this modified model, the dynamic mechanism of phase separation can be sufficiently analyzed and the phase separation patterns can be realistically simulated. Experimental results demonstrate that conspicuous liquid phase separations have taken place for both Fe-Sn-Si and Fe-Sn-Ge alloy droplets and the corresponding morphologies are mainly characterized by core-shell and dispersed structures. The phase separation process can be modulated by the third-element addition. As the Si element of Fe-Sn-Si alloy is replaced by the Ge element with the same fraction, the distribution order of Fe-rich and Sn-rich zones is reversed within core-shell structure. A core-shell structure composed of a Fe-rich core and a Sn-rich shell is frequently observed in Fe-Sn-Si alloy droplets whereas the Fe-Sn-Ge alloy droplets tend to form a core-shell structure consisting of a Sn-rich core and a Fe-rich shell. Theoretical calculations show that the droplet cooling rate is closely related to droplet size: a smaller alloy droplet has a higher cooling rate. The liquid L2(Sn) phase always nucleates preferentially and forms tiny globules prior to solid Fe phase. Stokes motion can be greatly weakened in this experiment and the Marangoni migration dominates the globule movement in the process of liquid phase separation. Furthermore, the intensity of Marangoni convection within Fe-Sn-Ge alloy droplets is significantly stronger than that inside Fe-Sn-Si alloy droplets. Numerical simulations reveal that the cooling rate, Marangoni convection and surface segregation play the important roles in determining the selection of core-shell configurations and the formation of dispersed structures. Ultrahigh cooling rate contributes to forming the dispersed structures. When the Marangoni convection proceeds more drastically than the surface segregation, the minor liquid phase with a smaller surface free energy migrates to droplet center and occupies the interior of droplet, otherwise most of the minor phases appear around the periphery of droplet.