[Multi shell sphere]
Study of liquid-liquid phase separation of undercooled liquid metals and forming process of multi shell sphere

RESEARCH OVERVIEW

• The phase separation of alloys is one of the most important subjects of metallurgy. Iron (Fe) and Copper (Cu) are popular metals, but the mixture of Fe and Cu show curious behavior because the two elements have a repulsive character in the undercooled state. The Study of Liquid-Liquid Phase Separation of Undercooled Liquid Metals and Forming Process of Multi Shell Sphere (ELF-Multi Shell Sphere) investigation reveals the hidden nature of this popular alloy.
• In ELF-Multi Shell Sphere, the cooling curve of a levitated Fe-Cu liquid alloy is observed along with its undercooling limit and solidification behavior under microgravity conditions. Based on the thermodynamics of materials, this research seeks to provide a better understanding of why these metals make mixtures and make phase separations.
• The applicability of the thermodynamics data in the undercooled liquid state can be verified from results of this research. This knowledge can extend the exploration range of novel materials.

DESCRIPTION

The Study of Liquid-Liquid Phase Separation of Undercooled Liquid Metals and Forming Process of Multi Shell Sphere (ELF-Multi Shell Sphere) investigation evaluates a mechanism of liquid-liquid phase separation in undercooled liquid metals and the formation of a multiple shell structure of alloy spheres by using the microgravity conditions aboard the International Space Station (ISS). The phase transition temperature of a spherical sample under containerless conditions is measured and the processed samples returned to Earth for analysis of the concentration distribution by X-ray imagery, microscopy, and and other methods to study how the characteristic structure is formed. In order to seek the influence of gravity on this phenomena, the microgravity experiment is combined with similar kinds of levitation experiments by using a gas-jet levitator in the research team’s laboratory. The size of samples is an important factor in the study of this phenomena, so the atomization experiment is carried out by using a short drop tube.

In previous research of the atomization of iron-copper alloys using a short drop tube, some characteristic structures of spherical alloys, whose cross sections look like boiled eggs, were obtained. The processing of the sample only consisited of thermal control, such as the melting and the solidification; however, the two alloys - whose concentrations are clearly different - separated completely and formed a double or triple multi-core structure in atomized samples. It has been suggested that the reason for the formation of the multi-core structure is the liquid-liquid phase separation in undercooled liquid iron-copper alloys. One of the separated liquid phases aggregates to the center of the spherical sample due to the Marangoni convection, which is induced by the surface or interface tension and temperature gradient. The other phase surrounds the outside of the sample and therefore forms the egg shape. For the verification of the suggested theory, the temperature measurement during solidification provides important information. However, measuring the temperature of small metal spheres in freefall in a drop tube is quite difficult.

Recently, containerless solidification of iron-copper alloys was performed using a gas-jet levitation apparatus with a high-power laser. The processed sample had a diameter of about 2 mm, and a similar multi-core was obtained. Also, solidification temperatures were measured using a pyrometer because of the stability of the sample position and the large size of the sample. The solidification temperatures were clearly lower than the liquid-solid equilibrium temperature of phase diagram. The multi-shell structure could be obtained when the deep undercooled liquid state was accomplished; however, the success ratio was quite low.

Under the gravitational environment, the density difference between two liquid phases induces a convection in the small spherical sample; therefore, the formation of multi-shell structures might be disturbed. In order to observe the formation of multi-spheres in detail, microgravity conditions are applicable because of the lack of convective flow.

The objective of the ELF-Multi Shell Sphere investigation is to track the temperature changes during the solidification process of iron-copper alloys under microgravity conditions. A molten alloy sample is processed under containerless conditions by using the Electrostatic Levitator Facility aboard the ISS, and the temperature change during the melting and solidification process is tracked by pyrometer. Different concentrations of samples in their initial states are prepared, and the concentration dependence of this phenomena is studied.

The temperature and concentration of the alloy provide important information about phase transition. The equilibrium phase diagram of the alloy can be estimated from the Gibbs’s free energy of alloys with thermodynamics treatments, which is well known as CALcultation of PHAse Diagrams (CALPHAD). This experimental result could extend to the estimation of the phase diagram in metastable states, which can provide clues for seeking novel materials.

The mechanism of this phase separation is suggested to be similar to partial immiscible solutions. These kinds of phase separations can be treated using the phase field model, in which the alloy structure can be derived from the minimum of interfacial energy and total energy. The aggregation of liquid phases also can be simulated via fluid dynamics; however, the free surface and moving interface of phases is a challenging subject. In addition, the simulation of nucleation in the liquid phase is almost impossible because of the lack of the information on size and frequency of embryo formation. Understanding the phenomena in this experiment contributes to solving these challenging subjects and improves the computer simulation modelling of materials processing.

SPACE APPLICATIONS

Iron-copper alloys form multi-core spheres in the absence of gravity. These spherical alloys are of interest to a variety of fields and could attract additional research on material processing in space.

EARTH APPLICATIONS

This investigation could provide basic and important data on the thermodynamics of metals. Results could make clear gravity’s effects on solidification and help improve computer simulations of phase separation and solidification for use in a variety of applications on Earth.

OPERATIONAL REQUIREMENTS AND PROTOCOLS

The ELF instrument is assembled and installed in the Multi-Purpose Small Payload Rack (MSPR/MSPR2) in Kibo. After setup, the experiment is operated from ground control as required by investigators at Space Station Integration and Promotion Center (SSIPC), Tsukuba Space Center. The experiment procedure is as follows:

1. A crew member prepares the experiment by inserting the Sample Holder into the Sample Cartridge then inserting the Sample Cartridge into the ELF chamber. The ELF is then activated and configured for operation.
2. To begin experiment operations, the sample is released into the Experiment Volume by the Sample Release Rod. The Sample is charged, its position controlled, and is heated and melted using electrodes and power lasers. During operations, the sample is measured through sensors and cameras.
3. At the completion of experiment operations, recorded video, pictures, and data are downlinked to Earth.
4. The experiment is closed out by the deactivation of ELF.

MASAKI Tadahiko [Shibaura Institute of Technology]