Seeing The Forest For The Trees: USMP-4 Science Research On Flight Day 1

November 21, 1997

If your car engine hasn't overheated, you can give some thanks to a team of scientists who are growing tree-like crystals aboard the Space Shuttle this week.

Knowledge from the first two missions with the Isothermal Dendritic Growth Experiment, now on board Columbia, is being used in designing automobile engine coatings and other industries that use metals.

"In the last year or so," said Dr. Martin Glicksman, "there's been a huge amount of improvement in the foundry codes, the metals casting computer models, which are being used to predict details like dendritic microstructures of metals."

IDGE was started Wednesday evening, soon after launch aboard Columbia. Initial operations indicate that Glicksman and his investigators can look forward to another collection of exceptional data from the mission

The first two missions already are helping American industry.

The casting of metals to make sophisticated materials has been one of mankind's most important important skills. While we have been doing it for a few millennia, only in the last few years have we tried to peek at the inner details of how metals are formed.

One of those details is the growth of tree-like crystals - dendrites - that interlock like a massive jigsaw puzzle that can't be untangled. Dendrites can rain out of the solution and form a rich mush at the bottom of a casting and leave the top poor in an important alloying element. Or, they can shrink and leave voids that weaken the casting and become a crack after a product has been used a few times.

To understand the growth of metallic dendrites better, Glicksman designed the IDGE to use transparent chemicals that grow dendrite crystals as stand-ins for metals to reveal what we can't see inside molten metal as it solidifies.

The crystals are grown very slowly with the solution cooled below the normal freezing point. A cold stinger extends into the chamber to encourage crystal growth at one point where cameras can record what happens. When the crystal touches the walls of its tiny growth cell, it is remelted and the process repeated.

On its first two missions, IDGE used succinonitrile, or SCN, which is similar to iron and other iron-like metals. You won't find SCN in the drug store, but we all benefit from it. It's a key chemical in making nylon and is used in some antistatic agents. This third mission uses pivalic acid, which freezes similarly to nonferrous metals such as aluminum.

"The two materials mimic the solidification of these two major classes of useful metals," Glicksman explained. In space, SCN crystals (right) grew with sharper dendrites than those grown on Earth where convection would start.

"The first flight gave us the initial data set on growth speed and tip radius," Glicksman said. A key measurement he took was the shape of the tip - the parabolic model that had been used was found to be an oversimplification .

"We found a much closer correspondence between microgravity data and theory. We also proved that all prior terrestrial growth experiments were corrupted by convection."

With experience from the first flight, Glicksman and his team prepared an improved series of experiments for the second flight in which 120 growth cycles (three times what they planned) were achieved. In science, one experiment may lead to a discovery, but you need dozens of test runs to show that it was not a fluke and then to gather data so you can write a law that other scientists and engineers can apply with confidence.

"From the statistical information we collected, we did some computerized tomography [computerized cross-section maps]," Glicksman said. "That three-dimensional information is now being used to test certain computer codes for microstructures. They are now available to challenge our assumptions and to act as new benchmarks."

Already, IDGE results are being applied on the factory floor.

"We're pleased to say that some of the scientific data are being adapted into sophisticated computer casting models," he continued. "These models cut the casting design process down from a couple of weeks to about a day." Further, new castings can be designed so they use just enough metal to do the job and so the engine performs as planned."This has a huge impact on virtually any casting model in terms of improving quality and predicting material performance," he added.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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