Jul 13, 1997

Follow the Bouncing Ball to Improved Metals


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Science on Flight Day 14 of MSL-1


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July 14, 1997 11 a.m. CDT


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What sometimes looks like a sci-fi robot peering out of the TV screen is actually a window that scientists are using to look into the basic nature of the metals with which we build modern society.

"We're looking at how steels solidify in order to optimize processes and produce the right structures," said Dr. Doug Matson of the Massachusetts Institute of Technology. "It's very important to the guy in the steel mill running a strip casting unit" and other steel processes.

Matson is co-investigator on TEMPUS, a German-built furnace being used by German and American scientists to investigate several basic properties of metals. TEMPUS is a German acronym for containerless electromagnetic processing in weightlessness.

Human technical progress has been marked by increased control over the formation of metals. This has been paced by our understanding of the basic properties of each metal: the melting and freezing point, how it flows, when it will break, and so on.

In many areas, laboratory studies have hit a limit in defining these basic physical properties. Contact with the melting pot can alter measurements. Even levitating samples on Earth can produce questionable results as materials flow within the sample.

One answer is to go to space where the sample is weightless and most effects can be eliminated.

To that end, the German Space Research Agency (DLR) developed TEMPUS. The furnace is making its second space mission; it flew on the second International Microgravity Laboratory (IML-2) in 1994.

"The reason you go into space is to levitate the sample without a strong electromagnetic field that causes a lot of convection," Matson explained. "You have these weaker positioning forces and we can select how much power goes into the sample, and measure when its motions go from laminar (smooth) to turbulent flow."

magnetic field lines drawing
TEMPUS uses about the size and shape of a little heater coil for warming coffee. One coil generates an electromagnetic field to position the metal samples in the center of the furnace, and the other coil transmits radio frequency energy to heat the sample as if it were in a microwave oven. (At right are diagrams of the magnetic field lines produced by the two sets of coils, which are used to position and heat the sample.)


silvery TEMPUS sample
Researchers on the ground can watch the samples through a camera that peers through a slit in the side of the furnace. As the metal balls melts and it forms a mirror-like surface, and the ball looks like a silver-headed, slit-eye robot staring at the viewer. Actually, the dark band is the image of the slit through which the TV camera is viewing the sample (image at left).

Soon, the sample heats even more and becomes incandescent, glowing like a hot light bulb filament, or steel in a casting pot.

As the sample bounces and spins (mpeg movie 962KB) within the center of the furnace, other instruments measure the heat radiated by the sample.


TEMPUS sample
Each of the 22 samples are 7 to 8 mm (almost 1/3 inch) in diameter, about the size of a small gumdrop. Some are single elements - such as gold or zirconium - while others are alloys - blends of two or more metals. (A sample from the IML-2 mission is shown at left, still in the cage.)


Steel, the alloy familiar to most people, is not a single formulation but a family of metals building on the basic strength of iron and adding new qualities like workability, resistance to corrosion, or reduced brittleness.

Matson is working with principal investigator Dr. Merton Flemings, also of MIT's Department of Materials Science and Engineering. They are experimenting with two basic samples, iron with 16 percent chromium and 12 percent nickel, and iron with 12 percent chromium and 16 percent nickel.

Matson and Flemings also are working with Dr. Gerardo Trapaga, also of MIT, who is continuing the work of the late Dr. Julian Szekely on measurements of surface tension and viscosity.

While metals appears to be homogeneous to the naked eye, they are composed of dense crystalline structures that form as the metals cool and solidify.

"We're looking at phase selection," Matson said. "Two possible crystalline structures can form" as the steel cools and solidifies. One aspect of Matson and Flemings' studies is deep undercooling. It seems contradictory, but most materials can be cooled below their freezing points and remain liquid if the material is pure and undisturbed. Measuring how viscosity ("You squeeze the metal and watch how the waves damp out") and surface tension (the same effect that lets bugs walk on water) change with temperature will help steel mills refine processes for casting steel strips.

"We're looking at the properties when it gets undercooled and those flows dampen out," Matson explained. The rapid spinning of the metal samples helps ensure that the iron-chromium-nickel blend is well mixed, but it dampens out as the sample is undercooled. Matson said the TEMPUS team has routinely undercooled samples by as much as 280 degrees C below its normal freezing point of 1,470 degrees C.

An important feature of the undercooling phenomenon is how quickly the sample freezes. Matson said the researchers need to see how the freezing front spreads across the sample.

"It's so fast that you can't analyze it if you don't know where it started. So we apply a nucleation stimulus needle," he said. "We move the cage down and a zirconia-coated needle serves as a nucleation point."

Although it had some operating problems (now resolved) early in the mission, TEMPUS has given its science team all the data they sought. More than 20 experiments have been run in over 150 hour of operations, with most exceeding expectations. But Matson does not think that is the end of the story.

While the TEMPUS team has received excellent data from the instruments aboard TEMPUS, they will also spend several months dissecting their samples in labs at their home institutions.

"We'll cut them up and look at how the crystal phases form and how they differ from what we've seen on Earth," he said. "When you do experiments, such as you do here, you're answering some questions, but you're raising more than you thought you could ask," he said. "And that opens more research areas both on the ground and in flight."

Some of that flight work may be done aboard International Space Station which is expected to carry a facility similar to TEMPUS.


For the next 3 days, you can follow along and learn about the science being performed on the mission through activities on this WWW site, as well as the "Liftoff" Mission Home Page, and the Shuttle Web Site.



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Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack