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5 Materials Science Research Program INTRODUCTION Materials science provides the technical foundation for many of the nation's most vital industries: Semiconductors, optoelectronics, ceramics, and steel making are a just a few examples. Because the structure of a material determines its properties, the ability to produce a specific structure at will, and hence tailor a material's properties for a particular application, has been a continuing goal for materials scientists and engineers. However, understanding the fundamental relationship between process his- tory, structure, and properties is complicated by the fact that material processing often involves the liquid or vapor state, so that buoyancy-driven convection, surface tension effects, sedimentation, and container reactions can mask the critical effects under study. Materials scientists were among the first to recognize that the unique aspect of space experimenta- tion the absence of buoyant fluid effects and sedimentation could provide insights that would be difficult, or impossible, to gain by any other approach. Thus, the materials science program has been in existence longer than any of the other microgravity disciplines. The objective of this space-based, or microgravity, experimentation was to gain a deeper understanding of materials phenomena that could then be used to improve Earth-based science and technology. Early microgravity materials experiments were rudimentary and discovery-driven. In one of the first experiments, tiny lead-tin solder ingots solidified in a teacup-size furnace during an Apollo mis- sion, were examined for microstructural improvements expected from the low-gravity environment. The Skylab and Apollo-Soyuz test programs of the 1970s successfully demonstrated the processing of semiconductor, metal alloy, and halide crystals by solidification and vapor techniques. These platforms used a specially designed multipurpose furnace to achieve a variety of experimental conditions that took advantage of microgravity conditions to process different materials. Among the results of these early studies was a demonstration that the low-convection conditions of space eliminated the compositional variations due to oscillatory convection in semiconductor crystals these variations degraded the elec- tronic properties of Earth-grown material. (Later, magnetic suppression of convection became standard procedure in the silicon industry.) 50

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MATERIALS SCIENCE RESEARCH PROGRAM 5 The space shuttle provided access to space for performing microgravity material science experi- ments after Skylab. During the period of shuttle-based experimentation, NASA developed a cadre of ground-based and flight investigators and improved flight hardware; it also developed new analytical tools, computational modeling techniques, and advanced materials theories. Materials science experi- ments in space evolved to become driven more by hypothesis and theory and less by discovery-oriented goals. As is noted below, many of the successful experiments were directed at producing benchmark data sets. Within materials science, the present microgravity research program, described below, has strongly influenced scientific understanding in several technologically important areas, for example: Control of impurity segregation, interface stability, and dendrite formation in metals and semi- conductors; Control of coarsening, liquid-phase sintering, and grain structure in industrial alloys; and Measurement of accurate thermophysical properties, such as surface tension, viscosity, and diffu- sion, that are required for effective computational modeling. In the early 1990s NASA broadened its program significantly beyond just those experiments that were destined to be flown. The ground-based program now contains a number of experimental and theoretical investigations. The purpose of the Round-based experimental program is to provide the opportunity to establish conclusively the need for periormmg an experiment In mlcrogravlty and to develop the protocols necessary to perform the experiment prior to the development of spaceflight hardware. In addition, theoretical programs directed at modeling the processes under investigation are funded through the ground-based program. The current microgravity materials science program has a dual focus. Projects are conducted in each major materials system, but they are organized by research themes that are common to all materials systems. These research themes are fundamental in nature, but many also impact a range of practical problems. The materials systems being investigated currently are electronic and photonic materials, glasses and ceramics, metals and alloys, polymers, and more recently, biological materials. Common to these materials systems are the phenomena that form the key microgravity research themes: (1) nucle- ation and metastable states, (2) prediction and control of microstructures, (3) interracial and phase- separation phenomena, (4) transport phenomena, and (5) crystal growth and defect control. Addition- ally, work IS under way to improve the radiation resistance of shielding materials for use in the human exploration of space. .. . . . ~ . . _ IMPACT OF NASA'S MATERIALS RESEARCH The materials science program currently funds research across a broad spectrum of areas, ranging from crystal growth to materials for radiation shielding. Of the approximately 115 investigators who have been in the program over the past 2 years, 9 are members of the National Academies of Engineer- ing and Sciences, 5 are fellows of the Mining, Metals and Materials Society (TMS), a large materials society with 100 living fellows and 10,000 members, 11 are fellows of ASM International, another major materials society. and 17 are fellows of the American Physical Societv. These are large numbers ~ - , ~ ~ ~ . .. . - . ,% ,% .. ~ . ~ . . . . . .. . ... ~ .. given the comparatively narrow focus ot the NAbA materials program compared to the breadth ot the interests represented by each of these materials societies. NASA has funded outstanding materials science investigators during the formative period of their careers, two of whom have been subsequently elected to the National Academy of Engineering. A major focus of the materials science program is solidification and crystal growth. The Bruce Chalmers Award, sponsored by the TMS, is given to

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52 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA individuals who have made outstanding contributions to the science and technology of solidification processing. There have been only 12 winners of the award to date. Of the nine Americans who have received that award, eight have been, or are, in the NASA materials program. It is clear that the program has been successful in creating a community of high-quality investigators. Discussed below is the impact of the materials science program in the five research-theme areas mentioned previously. Not addressed are the new areas, since the research program in these areas is just beginning. Research was judged to have had a significant impact on the field if the paper received 50 or more citations or, for more recent publications, if it is being cited at a high rate. Other metrics were also used to assess impact; these are mentioned under the particular program. All of these papers were published as a result of NASA research funding. Nucleation and Metastable States Probing the thermophysical properties of liquids cooled significantly below their melting points, or deeply undercooled, poses a great challenge for those systems in which nucleation of the stable crystal occurs. By melting samples that are levitated it is possible in some cases to forestall nucleation of the crystal as the liquid cools, thus allowing the properties of the undercooled liquid to be measured. A recent example of this approach is provided by the study of alloys that form metallic glasses under slow cooling rates. Usually metals crystallize upon cooling significantly below their melting points. In contrast, in these "bulk metallic glasses" it is Possible to form a metallic class under very slow cooling 1 0 ~ O rates. These materials have garnered much attention recently because they display unusual properties such as strength (they are two to three times stronger than their crystalline counterparts) and the ability to withstand very high strains and still remain linearly elastic, while enabling standard casting processes to be employed to produce objects with large volumes. The thermodynamic properties of these novel bulk metallic glasses were investigated using electrostatic levitation (Kim et al., 1994; Busch et al., 1995). In these studies, the heat capacity of the glass, crystal as well as the levitated undercooled liquid, was measured. By comparing these measurements with those of more standard metallic glass alloys it was possible identify certain thermodynamic properties that favor bulk metallic glass formation, thus pointing the way to the development of other bulk metallic glass alloys. Flemings and coworkers have investigated phase selection within levitated liquid steel droplets for many years. It was found that through deep undercooling of the liquid below the liquidus it is possible to nucleate phases that are not normally found during near-equilibrium solidification, thus allowing a careful investigation of the thermodynamics and kinetics of metastable phase formation. Similar phases are found during strip casting of commercial steel alloys. This work thus has the potential to lead to better control of the processing parameters during strip casting and to improvement in the quality of the steel (DiMicco, 2000). Prediction and Control of Microstructure As a liquid is cooled below the melting point the solid phase forms and grows into the liquid phase. The growing solid displays a rich variety of morphologies, ranging from beelike objects called dendrites to undulating solid-liquid interfaces, termed cells. This process is of intense interest from both scientific and commercial viewpoints. Of scientific interest is the fact that these processes involve the formation of patterns governed by highly nonlinear, but simple, equations. Understanding the factors that control these patterns has been of great interest in the physics and materials communities. More practically,

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MATERIALS SCIENCE RESEARCH PROGRAM 53 these patterns control the distribution of chemical elements in the solid and thus the resulting properties of a casting. Since the solidification process occurs from a liquid phase, buoyancy-generated convec- tion of the liquid can strongly perturb these processes, and thus the focus of the work funded by NASA has been to understand the role of convection in these processes and to study the processes in the absence of convection, which is usually not possible on Earth even with the application of magnetic fields. An analysis of the citations clearly indicates that there has been a long history of important theoreti- cal work funded by NASA in this area. The effects of convection driven by composition and thermal gradients on the morphological stability of a solid-liquid interface during solidification were first inves- tigated by Coriell et al. (1980~. They showed that both morphological and convective instabilities can occur and that the ability of the solid-liquid interface to deform leads to conditions for the onset of instability that can be quite different from those in the absence of such a deformable boundary. Ungar and Brown (1984, 1985) examined the formation of deep cells during directional solidification. They did this through a direct numerical solution of the defining equations. They showed clearly the rich bifurcation structure of these nonlinear systems and the conditions for cellular interfaces to form drops of liquid at their roots like those observed experimentally. More recently, the numerical modeling of solidification processes has been revolutionized by the introduction of the phase-field method. The strength of this approach is that it is not necessary to track explicitly the location of the solid-liquid interface during the solidification process while still accounting for the thermodynamics of the interface. The phase field model thus enables numerical simulation of the myriad topologically complex solidifi- cation morphologies that are sensitive functions of the interracial thermodynamics. NASA researchers have been at the forefront of the development of this method by providing phase field models for systems with realistic anisotropic interracial energies (McFadden et al., 1993), thermodynamic con- straints on phase field models (Wang et al., 1993), and the first phase field model for the isothermal solidification of binary alloys (Wheeler et al., 1992~. Treelike structures known as dendrites are ubiquitous. They are found in processes ranging from crystallization from the vapor in this context they are called snowflakes to solidification of a liquid metal. An important advance (this paper received over 370 citations) in the study of dendritic solidifi- cation was made by Huang and Glicksman in their work on dendritic growth in succinonitrile (Huang and Glicksman, 1981~. They showed that dendrites grow into a liquid cooled below its melting point with a velocity and a tip radius that are unique for a given liquid temperature. In addition, they documented clearly the influence of the buoyancy-driven convection of the liquid that accompanies dendritic growth on the dendrite morphology and growth rate. This work stimulated significant theo- retical studies in the physics and mathematics communities in an attempt to understand why dendrites select the unique growth velocity and tip radius that was measured experimentally. The results of the experiments have been included in an undergraduate textbook on solidification (Kurz and Fisher, 1989) and in more advanced monographs (Davis, 2001~. To remove buoyancy-driven convection and produce a data set that can be compared directly to theory, the experiments were flown aboard USMP-2 in 1994 and two subsequent missions (Glicksman et al., 1994~. The experiments showed that the data on Earth are compromised by convection, and that once these convective effects are removed there are significant long-range thermal interactions between both neighboring dendrites and secondary dendrite arms that form back from the dendrite tip. Analysis of the data is continuing not only by the PI but by many others in the materials, physics, and mathematics communities, as evidenced by the numerous downloads of the spaceflight data. The experiments have produced a set of benchmark data that will serve as a test of existing and future theories of dendritic growth.

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54 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA Interfacial Effects and Phase Separation Liquid-phase sintering is a widely employed industrial process that can be used to produce an array of advanced materials, from cutting tools to automotive turbochargers. The process begins with pow- ders that are compacted into a desired shape and then heated to produce a two-phase, liquid-solid mixture. The liquid provides a pathway through which vapor pores can escape. The goal is to reduce the shape changes of the compacted powder that occur during sintering so that little machining is required following processing. Experiments performed in space by German show, surprisingly, that the shape distortion of samples processed in microgravity is considerably greater than that of terrestrially processed samples. This finding, coupled with other insights gained from ongoing research at Kennametal, Inc., enabled a better understanding of the underlying causes of the changes in shape of powder compacts during liquid-phase sintering. Kennametal, Inc., is a market leader in the metal- cutting tool industry, with annual sales revenues of $1.8 billion. Approximately 40 percent of the production cost of its cutting tools is associated with the post-sinter machining necessary to bring the dimensions of the tool into specification. Using this insight, it was possible to nearly eliminate the final, expensive grinding step in the fabrication of parts (Latrobe et al., 1988~. If a solid-liquid mixture is held isothermally, the solid particles will evolve in time to decrease the total solid-liquid interracial area, and so the average size of the solid particles will increase in time. This process, called Ostwald ripening, is observed in nearly all two-phase mixtures, from raindrops in clouds near the equator to solids composed of mixtures of different crystals. As a result, the ripening process influences the properties of many commercially important materials, among them a wide variety of aluminum alloys and high-temperature materials used in jet turbines. NASA-funded work has provided many new insights into this important phase transformation process. Akaiwa and Voorhees predicted theoretically the interparticle spatial correlations and the kinetics of ripening in systems with the high- volume fractions of ripening phase that are typically employed experimentally (Akaiwa and Voorhees, 1994~. In agreement with theory, terrestrial experiments showed that the ripening process in model solid-liquid mixtures was a function of the volume fraction; however, the mixtures were found to ripen faster than predicted by theory (Latrobe et al., 1988~. To avoid the sedimentation of the solid particles that accompanies terrestrial experiments, ripening experiments were performed in space. As a result of the ideal conditions afforded by the microgravity environment, it was possible to show that the classical theory for ripening, first postulated in the 1960s, did not describe the experimental results (Alkemper et al., 1999~. The experiments have produced a set of benchmark data that has been used by other investigators in their research and teaching. The finding that systems undergoing Ostwald ripening may not be described by the classical theory influenced the development of software that is used to simulate the nucleation, growth, and ripening process in commercial alloys. In particular, the Precipicalc soft- ware that has been developed by the materials design company Questek was designed to take advantage of the spaceflight results. General Electric, and Pratt and Whitney, two jet engine manufacturers, are currently using this new software. Measurement of Thermophysical Data With the advent of fast computers and powerful algorithms, it is now possible to simulate many important processes that are used to produce materials. However, for such models to be truly predictive, it is necessary to employ accurate values for the thermophysical properties for the material being modeled. Many of these thermophysical parameters, such as diffusion coefficients of elements in high- temperature semiconductor and metallic liquids, are extremely difficult to measure, and when measured -0

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MATERIALS SCIENCE RESEARCH PROGRAM 55 on Earth their values are frequently perturbed by buoyancy-driven convection. Thus, spaceflight experi- ments are required to produce benchmark sets of thermophysical data. These experiments, however, will involve only a limited number of systems due to the constraints imposed by space experimentation. While these experiments have yet to be performed in space, the importance of performing this work is well recognized by industry, as evidenced by letters of support for this research from the staff of the Brimrose Corporation (Murphy, 2001) and the chairman and chief executive officer of the II-VI Corpo- ration (Johnson, 2001~. Both companies are involved in the production of semiconductor materials for sensor and electro-optic applications. In addition, certain II-VI materials have been and will be used in NASA space-based telescopes, indicating that this work may have a direct impact on NASA if allowed to proceed. Crystal Growth and Defect Control Nearly all materials used for structural applications are composed of more than one chemical component. As a result, a mushy zone, a partially solidified region of significant spatial extent, forms during solidification. Convection within the mushy zone can occur due to the variation of the liquid density with temperature and composition. This leads to the formation of vertical plumes of liquid, or freckles, within the mushy zone. The resulting strong microstructural inhomogeneity in the solidified material has a considerable adverse impact on the properties of many cast materials. Major insights into the physics underlying freckle formation were made by Hellawell and coworkers early in the NASA materials science program (Sample and Hellawell, 1984; Sarazin and Hellawell, 1988~. They showed by direct observation that convection was responsible for the formation of freckles and that rotating the sample can damp the formation of freckles. Porier and coworkers were among the first to propose physics-based models of convection and solute segregation within mushy zones (Felicelli et al.. 1991: Ganesan and Poirier, 1990~. ", ", Since crystals are grown from a melt or solution, buoyancy-driven convection of the fluid can play an important role in the resulting distribution of chemical components and defects in the crystal. A nonuniform distribution of chemical components can be quite deleterious to certain electronic and mechanical properties. Before the infusion of NASA funding, understanding of the effects of convec- tion on crystal growth was limited to a few simple models of the convection process, many of them ad hoc. While providing some insight into the role of convection on the crystal growth process, these theories did not address the coupling between fluid flow and mass transfer in a self-consistent manner. A solution to the Navier-Stokes equations during crystal growth is required wherein the solid-liquid interface is allowed to deform in concert with the diffusive and convective processes in the liquid. Coriell et al. were the first to employ such an approach to examine the effects of solutal and thermal convection on the stability of a solid-liquid interface during directional solidification (Coriell et al., 1980~. Chang and Brown (1983) performed numerical simulations that allowed for large deformations of the solid-liquid interface and both thermal and solutal convection in the liquid. They examined the dopant distribution in the resulting crystal and correlated the distribution with the levels of convection, the shape of the solid-liquid interface, and the design of the Bridgman furnace. They found that the segregation can be as large as 60 percent of the mean concentration and that the design of the furnace can change the degree of segregation. Further work by Adornato and Brown found that solute segrega- tion was minimized for near-diffusion controlled growth or for situations where there is intense laminar mixing (Adornato and Brown, 1987~. The implication of this result is that intermediate levels of convection, such as those found at certain levels of reduced gravity or in weak magnetic fields, can actually produce larger segregation and thus more undesirable material. In addition to these results, the

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56 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA support provided by NASA had an impact on the theory and understanding of Czochralski growth (Derby et al., 1985; Derby and Brown, 1986) and float zone growth (Coriell et al., 1977~. NASA-funded research aided in the development of a community of scientists working on the mathematical modeling of crystal growth and solidification. FUTURE DIRECTIONS IN MATERIALS RESEARCH Materials science has played a central role in many of the discoveries that have shaped our world, from integrated circuits to low-loss optical fibers and high-performance composite materials. The recommended research areas for NASA's materials science program, given below, will continue this tradition of science-driven discoveries of great importance to the nation and to NASA. Many of these promising areas build on research currently in the program, while others are new and unrelated to the existing program. Furthermore, the committee expects that many other new and exciting areas will emerge from the nation's research community. Nucleation Within and Properties of Undercooled Liquids The properties of many materials are strongly linked to the phases that nucleate during crystalliza- tion. Hence, to tailor the properties of a material for a given application conditions must be chosen such that only desirable phases nucleate during the processing of the material. Thus the nucleation process plays a prominent role in setting materials properties. Unfortunately, the conditions governing the nucleation of new phases are not well understood. Experiments in microgravity can play an important role in providing a basic understanding of the nucleation process, since they eliminate a potent site for nucleation the walls of a container while still permitting large volumes of liquid to be employed in the experiment. In systems where nucleation on foreign particles in the bulk is not a problem, it is possible to study carefully the nucleation process in bulk samples using a wide range of sample vol- umes. An example is the formation of a glass from a liquid. Glasses typically form when a liquid is cooled sufficiently fast to prevent the atoms from arranging themselves in a crystalline lattice. A new class of metallic alloys has been discovered that can form glasses under very slow cooling rates, making it easy to cool significant volumes of the liquid alloys far below their melting points. The ability of the bulk metallic glass alloys to undercool allows the properties of the undercooled liquid state to be examined. For example, recent studies appear to indicate that the classical theory for crystal nucleation from a liquid is inapplicable in these materials and that the liquid alloys exhibit atomic transport and theological characteristics that are very different from standard liquid metallic alloys. These materials also exhibit novel properties: They are twice as hard as stainless steel with similar toughness. For example, bulk metallic glass composites have recently been found to have significant ductility in tension, opening a wide range of applications, from aircraft materials to medical implants. Fundamental studies of the undercooled liquid state of these bulk metallic glasses can be performed in microgravity since it is possible to undercool large volumes of liquid using containerless methods and thus avoid heterogeneous nucleation due to the container. For example, if the properties of the melt that lead to bulk metallic glass formation are better understood, this might allow aluminum-based bulk metallic glass alloys to be formulated. Such bulk metallic glasses could have a huge impact on both industry and NASA since they would possess high specific strength and tensile ductility and thus would be ideal for a number of aerospace applications. Another example of nucleation from a liquid is quasicrystal formation. Quasicrystals are crystals with an unusual fivefold crystallographic symmetry. Here the issue is whether there are structural precursors to the quasicrystal in the undercooled liquid, a liquid that

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MATERIALS SCIENCE RESEARCH PROGRAM 57 is cooled below its melting point. Establishing such a link between the structure of the liquid and quasicrystal formation will shed new light on why certain alloys form quasicrystals and others do not. Dynamics of Microstructural Development During Solidification The microstructures of solidifying materials provide beautiful examples of spontaneous pattern formation. The development of dendritic and cellular microstructures is governed by relatively simple partial differential equations whose solutions are complex, possibly chaotic, and extremely sensitive to small perturbations. Moreover, since the majority of metallic materials are cast, the solidification process is of enormous industrial importance. The ability to directly link processing conditions to the resulting materials properties is still not at hand as the mechanisms governing the development of dendrite and cell morphology are not well understood. Outstanding questions include the effects of interactions between individual dendrites or cells on their spatial distribution and morphology, the evolution of dendrite morphology during transient heating or cooling, and the effects of noise and initial conditions on the resulting patterns. The interactions between dendrites are particularly important in the development of the mushy zone. A National Research Council study on the future of condensed matter and materials physics identified the mushy zone as "perhaps the most important theoretical challenge" in metallurgical pattern formation and also chose the study of the mushy zone as one of the research priorities in nonequilibrium physics (NRC, 1999~. A major impediment to the study of these solidifica- tion processes, however, is convection of the liquid phase, since convection makes it nearly impossible to compare results with theoretical predications and greatly complicates the interpretation of experimen- tal data. Performing experiments in a microgravity environment where convection is much reduced is thus crucial to understanding these complex pattern-forming systems that are of great commercial importance. Morphological Evolution of Multiphase Systems Materials used commercially are usually composed of more than one phase for example, the strength of a jet turbine blade is linked to the size, shape, and spatial distribution of the precipitates that are embedded in the matrix of the blade. These multiphase systems are created by a nucleation, growth, and Ostwald ripening process through which a single phase decomposes into two or more phases. Examples of such phase transformation processes abound. They occur in systems as diverse as poly- mers, wherein a second phase of different composition can form by either spinodal decomposition or nucleation with a liquid matrix, and in metallic alloys, wherein the morphology of dendrites in mushy zones evolves in time by Ostwald ripening. In other cases, a two-phase mixture is created by physically mixing two phases, such as the solid-liquid mixture found during liquid-phase sintering. During thermal processing, the morphology of the mixture evolves and the volume fraction of vapor bubbles decreases. Despite the clear commercial relevance and scientific importance, an understanding of the dynamics of phase transformation processes is not at hand. Phase separation and the processing of materials where one of the phases is liquid inevitably leads to sedimentation due to the density difference between the component phases. Performing experiments in a microgravity environment greatly reduces the rate of sedimentation and allows the dynamics of the transformation process to be investigated carefully. As a result, microgravity experiments can provide new and important insights into the dynamics of the evolution of multiphase materials. Research into Ostwald ripening and liquid-phase sintering has affected industrial practice in the past, and the committee expects that there is a good probability that it will in the future.

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58 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA Computational Materials Science Materials are typically designed using an extremely tedious and expensive trial-and-error approach. However, with the advent of high-speed computers and modern algorithms, it is possible to simulate the behavior of materials using a computer. Since the properties of materials are a function of processes that occur over an enormous range of length scales, from the nanometer to the meter scale, a wide variety of methods is required. On the smallest scales, quantum mechanical methods are used to simulate proper- ties that depend on the electronic structure of a material. In the tens of nanometers to micrometer scales, mesoscopic methods (such as the phase field method described earlier) are employed to describe the evolution of the morphology of the constituent phases. Finally, the methods of computational mechan- ics are used on the largest scales. As a result, numerical simulation is being viewed as an equal partner with experiments in determining the properties of materials. A recent example of the power of this approach is the work of Johannesson et al., wherein density functional theory was used to find the most stable phases in alloys with four chemical components that can be constructed using 32 different metals or 192,016 possible alloys (Johannesson et al., 2002~. On the mesoscale, three-dimensional calculations of morphological development in alloys, while still challenging, are becoming commonplace. The practical ramifications of such an approach are profound; it is now possible to design a material using simulations to yield a desired set of properties. No longer are parametric models required, since the calculations are predictive, efficient, and accurate. This will yield a new paradigm for designing industrially relevant materials, since the materials will be created with a minimum of costly, time-consuming experiments. This approach can have a significant impact on NASA because it ensures that the materials properties of interest to NASA will be attained, in much less time and at a lower cost. Such computational efforts are a natural outgrowth of the strong effort in process modeling research currently supported by the program. Computational materials science promises to yield a revolutionary new approach to designing new materials processes as well as tailoring materials properties for a given application. Furthermore, the integration of process modeling and diagnostics promises to one day be the key to rapid implementation of new materials-processing technologies. These approaches are just beginning to be embraced by industry. Thermophysical Data of the Liquid State in Microgravity Computational modeling of materials processing requires accurate thermophysical data of the liquid state. Obtaining such data on the ground that are not affected by convection is very difficult in low- melting-point systems and nearly impossible for high-melting-point materials. For example, there are very few accurate measurements of the solute diffusivities in liquid metallic or semiconductor alloys as a function of temperature. Performing such experiments in microgravity will provide insights into the physics of the liquid diffusion process as well as much needed thermophysical data for industry. Accurate thermophysical data along with computational models will yield realistic predictions of quan- tities such as the degree of microsegregation following solidification. The magnitude of the microsegregation in turn can have a significant deleterious effect on a wide array of materials properties Nanomaterials and Biomimetic Materials There are many new avenues for materials research at the nanoscale and at the interface between the biological and materials sciences. These new directions are discussed in Chapter 7.

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60 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA Ungar, L.H., and Brown, R.A. 1985. Cellular interface morphologies in directional solidification: The formation of deep cells. Phys. Rev. B 31: 5931-5940. Wang, S.L., Sekerka, R.F., Wheeler, A.A., Murray, B.T., Coriell, S.R., Braun, R.J., and McFadden, G.B. 1993. Thermody- namically-consistent phase-field models for solidification. Physica D 69: 189-200. Wheeler, A.A., Boettinger, W.J., and McFadden, G.B. 1992. Phase-field model for isothermal phase-transitions in binary- alloys. Phys. Rev. A 45: 7424-7439.