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Toward a Microgravity Research Strategy (Appendix F) Toward a Microgravity Research Strategy F Metals and Alloys STATUS NASA's current terrestrial microgravity program in metals and alloys involves approximately a dozen investigators working exclusively at universities and national (NASA or government-supported) laboratories. Metals and alloys constitute a critically important category of engineered materials encompassing structural materials, composites, conductors, magnetic materials, solders, brazes, catalysts, and the like. Their economic value is far and away the greatest of any class of materials used in technology. Currently the following areas are receiving attention and support from NASA's Microgravity Science and Applications Division. REPORT MENU Containerless Science NOTICE MEMBERSHIP SUMMARY Containerless science deals with experimental and theoretical studies of CHAPTER 1 competitive nucleation and growth kinetics of undercooled systems, especially in CHAPTER 2 the molten state when removed from physical contact with containers. Among CHAPTER 3 such studies are thermophysical measurements, x-ray diffraction, and heat flow CHAPTER 4 modeling to predict and control microstructural development. The techniques and APPENDIX A equipment used in the ground-based program include electromagnetic and/or APPENDIX B acoustic positioning devices and drop tubes that permit solidification or glass APPENDIX C formation at deep undercoolings or by the rapid removal of heat through gas APPENDIX D and/or piston quenching. APPENDIX E APPENDIX F Directional Solidification The responses of microstructures to thermal gradients, freezing rates, and convection are the topics of directional solidification. Experiments and computer modeling of well-characterized polyphase alloys are performed to file:///C|/SSB_old_web/cmgr92appendf.htm (1 of 7) [6/18/2004 11:10:11 AM]
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Toward a Microgravity Research Strategy (Appendix F) demonstrate the interaction of convective flow on solute distribution, phase scaling, interface morphology, and particle and/or bubble migration. Experiments designed to differentiate between theories and to improve the understanding of solidification (e.g., via coupled flow, spiral flow, etc.) are conducted both terrestrially on model transparent systems and in parabolic trajectories on KC- 135 aircraft. A well-designed apparatus has been constructed to measure Soret coefficients and diffusion coefficients in order to compare them with computed solutions that are valid in the absence of convection. Casting, Solidification, and Microstructure Understanding how convection and sedimentation affect cast and composite materials is the subject of studies in casting, solidification, and microstructure Alloy dendritic growth and coarsening behavior in response to a change in gravity vector are major objectives of the program. Computer modeling, which examines thermosolutal convection and microsegregation phenomena in dendritically solidifying alloys (by reducing the magnitude of gravity and changing its direction), is also a major focus. A goal of many ground- based calculations is to assist investigators in designing equipment and experiments for low-gravity environments. A major area of scientific and technological progress in solidification processing entails the development of quantitative, predictive, microstructural "scaling laws." For dendritic growth under diffusion control, scaling laws predict the growth speed, tip radius, and branch spacing as functions of the material parameters. It is estimated that reducing the acceleration of gravity to 0.001 its Earth value would permit almost an order-of-magnitude gain in the useful range of supersaturation for testing diffusion-controlled dendritic growth. In reduced gravity, not only are dendritic crystals less influenced by convection, but the increased size scale with decreased driving force also enables more accurate morphological measurements. Thermophysical Properties and Sintering Phenomena The measurement of such qualities as surface tension, heat of fusion, heat capacity, and gravitational effects during phase sintering of high-melting- point liquid phases is the objective in studies of thermophysical properties and sintering phenomena. Highly successful ground-based, millisecond-resolution, pulse heating experiments have allowed accurate measurements of the thermophysical properties of melts (T >1,500 K) for electrically conducting materials. Calculations of surface tension, entropy, and thermal emissivity on supercomputers are essential to interpreting contactless temperature measurements. The theoretical basis for predicting limiting compositions that can be sintered under normal gravity is being established. file:///C|/SSB_old_web/cmgr92appendf.htm (2 of 7) [6/18/2004 11:10:11 AM]
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Toward a Microgravity Research Strategy (Appendix F) MAJOR RESEARCH ACCOMPLISHMENTS For numerous and controversial reasons, significant results, mainly from Spacelab and sounding rocket missions, have been infrequent, not reproduced, and few. Results outlined below were obtained primarily from sounding rocket SPAR, Spacelab D-1, and Shuttle missions. Three main areas are addressed: solidification, diffusion coefficients, and modeling of solidification. Solidification The principal refinement of the eutectic spacing for the MnBi-Bi system was observed in microgravity on three separate occasions. This trend in reduced, inter-rod spacing was substantiated in similar terrestrial experiments in which melt convection was dampened by the use of a magnetic field. Several studies showed that alloys prepared in space have morphologies that differ from those of their terrestrial counterparts; for example, the patterns of Pb/TI and Co2Sm17/Co were more regular under microgravity, but distorted by convection on Earth, and the dendritic pattern of an Al/Cu space sample showed no radial or longitudinal segregation (indicative of diffusion-controlled growth), higher regularity, a larger size of dendritic array, and a five-times-larger arm spacing. Numerous attempts to prepare finely dispersed monotectic alloys (Al/In, Zn/Bi) by cooling through the miscibility gap showed clearly that, in addition to sedimentation, other phase- separation mechanisms exist, one of them probably surface-tension-driven migration. Directional solidification of composites (Al2O3 particles in copper) showed that below a critical volume fraction (<10 percent) for which the particles are isolated, a homogeneous distribution could be maintained under microgravity, even for particles larger than 1 um. A clear example of the dominating influence of surface energy effects with respect to microgravity occurs in monotectic alloy systems as the temperature falls below a critical point. Over a range of temperatures below the critical point, one of the liquids exhibits perfect welting behavior so that it essentially encapsulates the other liquid and will coat any container. If final freezing occurs within the temperature range of perfect welting, massive segregation will be produced in the microstructure. Alternatively, if the temperature for the monotectic reaction, which produces a solid and a liquid phase, is below the critical range, a regular well-aligned microstructure is possible by directional solidification. Diffusion Coefficients file:///C|/SSB_old_web/cmgr92appendf.htm (3 of 7) [6/18/2004 11:10:11 AM]
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Toward a Microgravity Research Strategy (Appendix F) Measurements of diffusion coefficients in liquid Sn in space were obtained with accuracies of better than 0.5 percent (10 to 50 times better than under Earth's gravity). Such data, which approach in accuracy those for diffusion in solids, cannot be reached in liquids on Earth because of connective disturbances. Furthermore, it was found that the temperature dependence of the diffusion coefficient is proportional to T2 and does not obey n Arrhenius law. In addition, for the first time evidence was obtained for an isotope effect in a self-diffusion experiment. The Soret coefficient measured in eutectic MnBi-Bi at 100°C/cm indicated that transport in microgravity was comparable to normal diffusion. Solutions of cobalt in the cold ends of the cylindrical samples differed markedly from those in the hot ends—that is, thermotransport (not measurable o Earth because of convection) had taken place—and the concentrations of the isotope Sn112 in the cold and hot ends differed also—that is, under convection-free conditions, thermodiffusion in liquids seems to lead to isotope separation. Modeling of Solidification Great strides have been made in comparing calculated microsegregation and experimental results of vertical Bridgman-Stockbarger microgravity experiments. Obtaining the proper experimental definition of metals-processing operations was shown to require collaboration with both fluids and transport communities. Codes have been produced to simulate nonlinear convection that leads to the formation of microsegregation defects. The consequences of reducing the magnitude of gravity, changing its direction and time dependence on thermosolutal convection, and subsequent segregation effects have been studied in detail. Frequently, theoretical models are based on the assumptions of steady state, axisymmetric flow, and constant heater temperature, whereas computer and analytical models have been developed for the diffusional decay of compositional or doping striations during solidification. Theoretical modeling is playing an important role in designing useful experiments to be performed in space. RESEARCH PROSPECTS AND OPPORTUNITIES General The future of NASA's microgravity program lies in the continuation of its Spacelab flight program up to and beyond the commissioning of Space Station Freedom, scheduled for the year 2000. To ensure the success of this ambitious program, an expansion of the ground-based, basic research infrastructure should he undertaken at centers of excellence in universities. The conditions necessary for microgravity's practical utilization should be defined, even though this field of file:///C|/SSB_old_web/cmgr92appendf.htm (4 of 7) [6/18/2004 11:10:11 AM]
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Toward a Microgravity Research Strategy (Appendix F) experimentation is still in its infancy and subject to delays and the risks of experimental failures in orbit. An alternative experimental program involving an unmanned orbital carrier should be put in place in case the space station system is not realized and to provide other, less expensive alternatives. Specific It is clear from previous experiments in space that microgravity can be viewed as a reference environment in the sense that it eliminates or reduces greatly the influence of one parameter that can play a multiple, but not completely understood, role in many metallurgical experiments. Even quite familiar terrestrial situations transferred to an apparently simple space environment require an extensive development program on Earth with theoretical, numerical, and experimental modeling and an open mind to cope with the unpredictable. For example, current models of the instability at solid and/or liquid interfaces have demonstrated the complexity of this problem and the various methods of treatment. The simplest models involve 7 parameters, some of them with considerable margins of uncertainty; the most elaborate involve more than 20 such parameters, which does not make comparative analysis easy! By reducing the imprecision resulting from many of these parameters (by means of measurements made under ground-based conditions), the yield of space experiments will be increased, and theoretical modeling will be facilitated. Any solidification process entails numerous geometrical, thermal, chemical, physical, and hydrodynamic parameters, which must be known if an accurate forecast of instability conditions under both terrestrial and space conditions is to be made. Improvements in the accuracy of some of these parameters are needed and are made possible by taking advantage of the microgravity environment. The improvements derived from such experimental work will benefit other situations of practical interest. Careful, preliminary, ground- based experiments performed concurrently would be worth carrying out since they will make it possible to approach the critical conditions for the onset of interface instability that will prevail in space. By varying the orientation of the growth rate with respect to the gravity vector (i.e., parallel, antiparallel, or perpendicular), one can introduce significant changes in the values of convective flow. Another approach would be to study certain systems enclosed in capillary tubes, which makes it possible to reduce the values of the Rayleigh number, as does reduction of the gravity level. In the case of an electrically conducting fluid, a strong damping of convection action can be imposed by using magnetic fields. The effect of such a magnetic field may be to delay the occurrence of convection by increasing the viscosity of the liquid, which introduces a resistance to hydrodynamic instabilities. Organized solute fluxes are then reduced drastically, but the random fluctuations of chemical, and especially of thermal, fluxes are maintained. Additionally, it would be valuable to study, on Earth, systems with minimum values of fractional file:///C|/SSB_old_web/cmgr92appendf.htm (5 of 7) [6/18/2004 11:10:11 AM]
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Toward a Microgravity Research Strategy (Appendix F) density change and of thermal and solutal coefficients of expansion, both parameters being the driving forces for convective fluxes. Freezing a dispersion (gas bubble or particle) with the aim of obtaining a stable material that combines the properties of two different components is desirable. A crucial and theoretically unsolved problem concerns the conditions for incorporation of particles into the advancing solid. Experiments with both immiscible alloys and metal matrix composites would be beneficial. Often, bubbles will be a nuisance in microgravity experiments if they occur unintentionally. The automatic elimination of bubbles by buoyancy does not occur in space. However, the application of a controlled artificial transport creates new problems, usually requiring additional time because the motions that can be induced are slow. During orbiter accelerations or crew maneuvers, time-dependent variations in acceleration forces ("gravity-fitter") may occur. Their effects on certain phenomena, such as the distribution or redistribution of impurities and/or defects in alloy crystal growth, should be evaluated. Also, because electromagnetic levitation is one of the current techniques employed in containerless processing, it is important to assess quantitatively the role played by residual electromagnetic forces in producing convection and the decay rate of such convection. Many metals-processing operations involve the simultaneous transfer of matter and thermal energy and the coupling of other transport phenomena. Thermal diffusion (e.g., the Soret effect and the Dufour effect) and electromigration are examples. By carrying out microgravity experiments, the role of convection in modifying these phenomena may be minimized, possibly providing ideal conditions for determining the appropriate transport coefficients for these coupled flux phenomena. Since 1973, over 1,000 experimental hours have been devoted to microgravity research. These experiments have covered a broad selection of metals and alloys. Many were "firsts" and involved severe restrictions in weight, size of equipment, power, and time; repetition and/or iterations were infrequently allowed. Although microgravity provides a new environment for metals processing, much more fundamental research is needed to use its full potential, as indicated above. The following general fields of metallurgy are recommended for continued microgravity research: Gravity-related aspects of nucleation and growth; Accurate measurements of material parameters, for example, thermophysical and diffusion parameters, especially at elevated temperatures; Development of microgravity techniques for preparing alloys, for example, containerless processing and skin technology; and file:///C|/SSB_old_web/cmgr92appendf.htm (6 of 7) [6/18/2004 11:10:11 AM]
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Toward a Microgravity Research Strategy (Appendix F) Preparation of "benchmark" research samples for use as terrestrial standards. Last update 7/13/00 at 4:24 pm Site managed by Anne Simmons, Space Studies Board The National Academies Current Projects Publications Directories Search Site Map Feedback file:///C|/SSB_old_web/cmgr92appendf.htm (7 of 7) [6/18/2004 11:10:11 AM]