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2.11. Materials Science in SpaceP.R. Sahm | Giesserei-Institut RWTH Aachen, 52072 Aachen, GermanyG. Zimmermann | ACCESS e.V., 52072 Aachen, Germany2.11.1. IntroductionMaterials science in space is a small but challenging sectorin the field of materials science. The environment ofreduced gravity existing in space puts fundamental researchin the field of materials processing within our reach.Under microgravity, the buoyancy convection in a melt issignificantly reduced and sedimentation effects are suppressed.This enables the investigation of crystallizationand solidification mechanisms with no interference fromconvective heat and mass transport in the liquid.In this way, the reduced gravity level existing in space providesan important tool for fundamental research projectsin materials science. Performing demanding experimentsin a microgravity environment can therefore be regardedas a revolutionary approach in materials science.fewer defects (Fig. 2.26). Directional dendritic solidificationunder conditions of purely diffusive heat andmass transport conditions in space show significantlyregular patterns consisting of larger dendrites in thespace experiments (Fig. 2.27).500 m1gMATERIALS: SCIENCE AND APPLICATION109500 mg2.11.2. State of the ArtMaterials science experiments have been carried out inspace for more than two decades. Solidification or crystallizationprocesses are sensitive to melt flow or sedimentationeffects. This implies that the experiments need at leastsome minutes of low gravity and therefore can only be carriedout during sounding rocket flights, during space shuttlemissions or at a space station. As a consequence, opportunitiesfor materials science in space are very few and farbetween. Each experiment needs years of intensive preparationand can be regarded as a ‘single shot’ experimentwith a high risk of failure. In this sense, the kind of ongoingexperimental programme, familiar in materials science onearth, does not exist for materials science in space.Nevertheless, previous microgravity experiments haveshown a series of important scientific results. In the followingsome relevant examples are mentioned:• In relation to microstructure formation during columnaralloy growth no comprehensive and systematic study existsand the data available is only limited. Using a binarytransparent alloy acting as a model substance for nonfacetingsolidification, earth experiments show deformedinterfaces and do not allow quantitative pattern evaluation.In contrast, the space growth sample shows anundisturbed and rather regular hexagonal pattern withFig. 2.26. Top view of a cellular solid-liquid interface in a transparent succinonitrile-0.075wt%acetonealloy, directionally solidified at a temperaturegradient of 2.2 K/mm and solidification velocity of 2.5 µm/s. The interfacestructure of the earth-grown sample (upper picture) is greatly interferedwith by convective buoyancy flow in the melt. The cellular morphology inthe space-grown sample (lower picture) is much more regular and allowsfundamental investigation into morphology evolution (ACCESS).1ggFig. 2.27. Cross-sections of directionally solidified Cu-29.5wt%Mn samplesat a temperature gradient of 2.75 K/mm and solidification velocity of5.8µm/s. The dendritic morphology of the space-grown sample (picturedright) is more regular and shows dendrites approx. 30% larger in size.These findings are attributed to the conditions of purely diffusive massand heat transfer in the melt in the microgravity environment (ACCESS).