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5. Materials Science Research Program
Pages 50-60

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From page 50...
... However, understanding the fundamental relationship between process history, 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 experimentation the absence of buoyant fluid effects and sedimentation could provide insights that would be difficult, or impossible, to gain by any other approach.
From page 51...
... 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 semiconductors; · Control of coarsening, liquid-phase sintering, and grain structure in industrial alloys; and · Measurement of accurate thermophysical properties, such as surface tension, viscosity, and diffusion, 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.
From page 52...
... 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.
From page 53...
... The phase field model thus enables numerical simulation of the myriad topologically complex solidification 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)
From page 54...
... 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 software that has been developed by the materials design company Questek was designed to take advantage of the spaceflight results.
From page 55...
... 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.
From page 56...
... 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.
From page 57...
... 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 solidification 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 experimental data.
From page 58...
... Accurate thermophysical data along with computational models will yield realistic predictions of quantities 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.
From page 59...
... 1983. Radial segregation induced by natural-convection and melt solid interface shape in vertical Bridgman growth.
From page 60...
... 1985. Cellular interface morphologies in directional solidification: The formation of deep cells.


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