ORNL's Iron Aluminides: An Emerging Success Story

Modified iron aluminides of industrial value were first developed at ORNL in the late 1980s. ORNL researchers were seeking a good substitute for stainless steel, which tends to corrode. They turned to iron aluminide alloys, which are resistant to air oxidation and corrosion. But they had to overcome this alloy's annoying tendency to break too easily at room temperature. They found that this brittleness is due to the formation of hydrogen and its diffusion into the alloy when aluminum reacts with moisture in the air. To decrease brittleness, they tried adding various alloying elements until they found the key ones that prevent or minimize the deleterious effects of reactions between aluminum and water vapor.

Claudette McKamey inserts an iron aluminide specimen into a furnace at ORNL. She is heating the specimen to determine the high-temperature strength and ductility of ORNL's modified iron aluminide alloy.

The resulting ductile iron aluminide earned an R&D 100 award in 1990 for Vinod Sikka, C. T. Liu, and Claudette McKamey. McKamey, who started at ORNL as a secretary and then became a metallurgist, made a key contribution: she discovered that small additions of chromium make iron aluminide more ductile. For her work, McKamey was named Inventor of the Year in 1989 by Martin Marietta Energy Systems, which then managed ORNL for the Department of Energy.

ORNL's iron aluminides (Fe3Al) do not corrode readily in sulfur-containing environments because they form an aluminum oxide coating. "The aluminum oxide (Al2O3) film resists attack by sulfur," says Sikka, "but if the sulfur should break through, Al2O3 forms rapidly again to prevent the propagation of any sulfur attack. Our iron aluminide alloy provides higher resistance to sulfidation when exposed to hydrogen sulfide and sulfur dioxide gases than any other iron-based or nickel-based alloy."

For this reason, ORNL's iron aluminides are good candidates for filters in coal gasification plants. These plants produce a low-to-medium BTU fuel gas from coal. By-product ash must be removed from the gas by filters. As gas passes through the filter, sulfur compounds in the gas react with commercial metal filters, causing them to deteriorate. Because ORNL's iron aluminides resist attack by sulfur, they are being evaluated as coal gasification filters.

In 1990 Ametek obtained a license from Energy Systems to produce Fe3Al powder to make filters. These filters, manufactured by the Pall Corporation in Cortland, New York, are being tested for coal gasification applications.

Other companies may find ORNL's iron aluminide desirable because of another selling point: it has a lower density and a higher strength-to-weight ratio than many stainless steels. These properties make iron aluminide an attractive candidate as a structrual material for some automotive components.

ORNL's iron aluminides possess three properties that also make them ideal as heating elements. They are highly resistant to oxidation and corrosion, and they have electrical resistivities that are higher than many commercially available heating-element materials. Evaluation by a private vendor showed that iron aluminide heating elements do not burn up as fast as commercial heating elements. In one experiment, an ORNL-modified iron aluminide element lasted almost three times as long as the commercial one.

Commercial use of iron aluminides as heating elements has been slow because of their limited ductility. But Sikka thinks ORNL research has found a solution to this problem.

"After iron aluminide wire is produced, one vendor found that the wire can be stretched only about 2 to 3% of its original length before breaking," Sikka says. "This problem limited its use in drawing it through a die to fabricate a heating element. But our laboratory research shows that, if the wire is run through hot water so that it's heated to about 100°C, its ductility improves ten times—from 2 to 20%."

Sikka says that, in a cooperative research and development agreement with Hoskins Manufacturing Company, he hopes it will be shown that heating elements can be made from iron aluminides by using both the Exo-Melt (see main story) and hot-water processes. If he's right, the iron aluminide saga might someday become as big a success story as that of nickel aluminide.

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