Undercooling Behavior of Immiscible Metal Alloys in the Absence of Container-Induced Wetting and Nucleation

When an immiscible alloy is cooled, it will separate from a single solution into a mixture of the lighter component lying on top of the heavier one. This is very similar to a mixture of vinegar and oil at room temperature in which the two components separate from each other after having been shook into a near single solution. The difference between the alloy and the salad dressing, besides the materials, is that the latter is already below the temperature at which the separation process occurs spontaneously. Thus, all the shaking you want to perform will only break the droplets into smaller droplets, but will not break-up the droplets to the size of an atom which is required to have a solution mixture. However, as an immiscible alloy metal is cooled, it reaches the (consolute) temperature where the atoms begin to glob together to form droplets. If the mixture is in a container, the droplets will form at the container walls whereupon gravity will pull the denser droplets to the bottom or allow the less-denser drops to float to the top. Without gravity, the droplets will only move when other, less severe forces act upon them such as thermal or density gradients. The lack of gravity could cause a more uniform distribution of the droplets and thus provide unique properties to the alloy after it solidifies.

 Another interesting phenomena in fluid physics is the undercooling of liquids. This is the lowering of the temperature of a liquid beyond the freezing temperature and still maintaining a liquid form. Normal freezing occurs when the atoms of the container walls impose an ordering on the liquid atoms causing them to arrange themselves into a crystalline structure and begin to grow. Without the container, the onset of freezing (called nucleation) has a good chance of not happening. But, at some lower temperature nucleation will occur and very rapid freezing takes place (called recalescence). One question that arises when undercooling is considered with immisicibles is what happens if there were no container? Would the droplets still form at the surface? Even if they did, would the kinetics of the whole separation process be the same as in a container? In particular, would the undercooling phenomena still occur and would it be independent of whether the "vinegar or the oil" came to the surface first?

 Previous tests of classical nucleation theory applied to liquid-liquid gap miscibility systems found a discrepancy between experiment and theory in the ability to undercool either of the liquids before the L1-L2 separated [1,2].   To model this initial separation process, free-energy gradient [3] and density gradient [4] theories have been put forth.  If there is a large enough interaction between the critical liquid and the crucible, both models predict a wetting temperature (Tw) above which the minority liquid perfectly wets and forms layers at the crucible interface, but only on one side of the immiscibility dome.  Materials with compositions on the other side of the dome will have simple surface adsorption by the minority liquid before bulk separation occurs when the coexistence line is reached.  If the interaction between the critical liquid and the crucible were to decrease, Tw would increase, eventually approaching the critical consolute temperature (Tcc).  At this point, large composition ranges would exist in which non-perfect wetting conditions prevail, resulting in undercooling of the phase-separation transition across the miscibility gap.  The contra-positive of this argument has recently been demonstrated in a metastable immiscible system [5] using different oxide layers as crucibles. The bulk fluid flows and resulting microstructure will then depend on what has happened at the surface and the subsequent processing conditions.

 In the past several decades, many experiments in space [6-8] have been performed on liquid metal binary immiscible systems for the purpose of determining the effects that crucibles of different materials may have on the wetting and separation process of the liquids.  Several other studies have been performed on immiscibles in a semi-container environment using an emulsion technique [9,10]. Only one previous study on stable immiscible systems was performed using a completely containerless processing technique [11] and the results of that investigation are similar to the emulsion studies.  In all cases, surface wetting was attributed as the cause for the similar microstructures or the asymmetry in the ability to undercool the liquid below the binoidal on one side of the immiscibility dome.

 By removing the container completely, the loss of the crucible/liquid interaction should produce a large shift in Tw and thus change the wetting characteristics at the surface.  By investigating various compositions across the miscibility gap, a change in the liquid wetting potential at the surface of a containerless droplet should change the nucleating behavior of the droplet – whether it be the first order transition of liquid-liquid surface wetting or of bulk liquid-to-solid. Either of these transitions will change the undercooling capability and the subsequent degree of rapid solidification that occurs before the separation of the two liquid phases proceeds to completion.  Containerless processing eliminates external nucleants allowing the undercooling of the non-perfect wetting single-phase liquid into the metastable region which may produce significant differences in the separation process and the microstructure upon solidification.  An additional benefit of containerless processing is the removal of a large source of contamination that a container provides for these reactive materials.  In this study, we attempt to determine the amount of undercooling that either the liquid-liquid or liquid-solid transitions undergo by monitoring the temperature of the sample with optical pyrometry.  Another purpose of this study is to examine the effects of weightlessness on the separation process of the immiscible liquids.  Microstructural analysis will correlate the degree of undercooling and the separation mechanisms involved.

 The first objective of this project is to find a high temperatue (HT) immiscible binary system that has (1) a monotectic temperature (the temperature where one of the two liquids freezes) above 1000C, (2) a low vapor pressure, (3) a wide miscibility gap (i.e., concentration range in which liquid-liquid separation occurs), (4) low oxygen affinity, (5) available thermophysical properties, and (6) an experimentally verifiable phase diagram. Most of these conditions were found to be mutually exclusive from each other: HT immiscibles which have high consolute temperatures do not have low vapor pressures; the HT immiscibles that did have low vapor pressures had very high oxygen affinities (i.e., rare earth elements); experimentally determined phase diagrams were not readily available - only theoretically calculated diagrams. The necessity of a HT system is due to the short free fall cooling time (4.6 seconds) provided by the MSFC Drop Tube Facility.

Electromagnetic levitation was used as the processing technique in the Drop Tube Facility. The figure below shows the arrangement of the coil and sample carousel.  Initial processing of these alloys was not successful. Upon melting, they immediately began to vaporize creating a plume which coated and blocked the pyrometer viewport, or arced the levitation coil if processed in a vacuum.
Besides the detremental effects of rapid concentration changes, the vaporization also effects the surface wetting kinetics, nucleation, etc.  In an attempt to reduce the vaporization problem the Ti-Ce system was used.

 Cerium is a rare earth metal with an equilibrium oxygen partial pressure of less than 10-30 atmospheres at 1000K and even smaller value at room temperature. Conversations with the Rare-Earth Information Center also suggested that just a small addition of oxygen (~500 ppm) into the binary system could eliminate the immiscibility of the phase diagram. The Ti-Ce droplet starting material was created using the same techniques as with the previous systems. Great care was taken to use a glovebox and to keep the Cerium and alloys under ethanol to prevent oxidation. These alloys did perform much better in the levitation coil with regards to evaporation. Recalescence due to undercooling was seen on two of the Ti-Ce drops. However, oxidation on the surface of the droplets, both before, during, and after dropping could not be prevented ever with the use of a hydrogen atmosphere.

 Summary

 A much-higher value of 1831°K for the monotectic temperature and a higher solid phase composition for the Ti-Ce system has been measured.  The liquid-to-solid undercoolings that were observed relative to this temperature were larger in a low gravity, quiescent environment than in a 1-g, convectively stirred condition.  The tendency of the convection in the 1-g samples to coalesce the droplets may have contributed to the lack of undercooling compared to the relatively slower Marangoni flows experienced in the 0-g samples.  Wetting of one liquid phase by the other was not observed due to the lack of instrument sensitivity.  The Ce-rich liquid was found to be the outer shell for all compositions across the miscibility gap.

 The containerless, 0-g environment promotes the establishment of equilibrium conditions for monotectic systems which can be beneficial for constructing phase diagrams for high temperature, reactive monotectic systems.  Because of this, the 0-g equilibrium composition of the rapidly solidified Ti-rich liquid of 7 w/o is higher than that reported in other studies.  The microstructural morphology of the phases observed within the low-gravity samples was always spherically shaped droplets that had secondary precipitation of Ce within the a-Ti that also was spherical.  In contrast, the 1-g samples had a-Ti that was non-symmetrically shaped caused by the stirring of the EM field of the levitation coil.  Any secondary Ce precipitation was trapped interdendritically.  A quiescent, 0-g environment such as in space with accurate temperature measurements could help eliminate some of the questions regarding the effects on phase composition and microstructural effects due to stirring, undercooling, and quench rate.

References

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4. Gary F. Teletzke, L. E. Scriven, and H. Ted Davis, J. Colloid Interface Sci. 87(2)  (1982) 550.
5. G. Wilde and J. H. Perepezko, Acta. Mat., in press.
6. C. Potard, AIAA paper no. 79-0173 (1979).
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8. T. Carlberg and H. Fredriksson, Met. Trans. 11A (1980) 1665.
9. J. H. Perepezko, C. Galup, K. P. Cooper in Materials Processing in the Reduced Gravity Environment of Space (Guy Rindone, ed., Elsevier Science Pub. Co., Amsterdam, 1982) p. 491.
10. N. Uebber and L. Ratke, Scripta Metall. 25 (1991) 1133.
11.   J. B. Andrews in Immiscible Liquid Metals and Organics (L. Ratke, ed., DGM Informationsgesellschaft, Verlag, 1993) p. 199.

Curator: Tom Rathz/UAH tom.rathz@msfc.nasa.gov

 Principal Investigator: Dr. Michael Robinson/MSFC mike.robinson@msfc.nasa.gov