Coarsening in solid-liquid mixtures: A reflight

In any two-phase mixture that contains particles of different sizes, the large particles tend to grow while the smaller particles shrink in a process called coarsening. One illustration is tiny oil droplets embedded in water coalescing into larger droplets but the process occurs in solids as well. Coarsening occurs on Earth during the processing of any metal alloy thus affecting products from dental fillings to turbine blades. Since the properties of an alloy are linked to the size of the particles within the solid, coarsening can be used to strengthen materials. This is the case with the majority of aluminum alloys used commercially today. Conversely, if the coarsening process proceeds too long the material can weaken. This occurs in jet turbine blades and is one of the reasons why they must be replaced after a certain number of hours of service. Thus developing accurate models of the coarsening process is central to creating a wide range of new materials from those used in automobiles to those used in space applications. The results of previous experiments performed on the International Space Station have done just that. These models have been incorporated into a computer code that is being used to design many new materials, including materials of importance to NASAs spaceflight program. Solid-liquid systems are ideal to study this coarsening process. However, gravity can induce particle sedimentation and thus hamper the studies of coarsening in these mixtures on Earth. Using the microgravity environment of the International Space Station (ISS), it is possible to study the coarsening process in solid-liquid mixtures with reduced interference from the sedimentation that occurs on Earth. Specifically we will study the rate at which particles of tin suspended in a liquid comprised of a molten tin-lead alloy coarsen. The results of our experiments, results of the engineering tests of our furnaces aboard the International Space Station and future plans for experiments will be discussed.