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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
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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.
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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
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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
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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
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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
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Preparation of "benchmark" research samples for use as terrestrial
standards.
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