In various shuttle experiments over the past 3 years, including several with disappointing results, researchers have begun to sort out previously unexplored factors that determine how metals mix and crystallize. Some of the tests use organic substances as surrogates for metals. At two meetings held at NASA's Marshall Space Flight Center in Huntsville, Ala., this summer, researchers discussed their recent Shuttle experiments. What they've learned may one day help improve the processing of metals on Earth. The researchers can develop better models for dendritic growth by first understanding what happens without gravity-driven, or convective, currents to churn up the sample.
Scientists try to use models of dendritic growth to predict a metal's strength and malleability.
Earth's gravity, by causing convection currents, affects the shape and rate of dendritic growth of succinonitrile and pivalic acid crystals. The researchers hoped to see a clearer picture than ever before of crystal growth on the shuttle. The experiments consisted of placing 100 milliliters of each substance in a sealed chamber, keeping it at a precisely controlled temperature just below its melting point, then turning on two video cameras and stinging the liquid with a cool needle to start dendrite growth. The Rensselaer team, controlling the apparatus from the ground, did hundreds of such experiments during three flights on the space shuttle Columbia. The most recent experiment, on pivalic acid, flew in late 1997.