Aluminides: From Powders to Products Using Reaction Synthesis

Nickel and iron aluminides, which are lighter and stronger than stainless steel, are also valued because they resist heat. However, these alloys in the form of a sheet, tube, or block also resist being shaped into a final product. The reason: aluminides of nickel, iron, or titanium are extremely brittle at room temperature and, therefore, are nearly impossible to machine.

To solve this problem, Chain T. Liu of ORNL's Metals and Ceramics Division, who first developed ductile nickel aluminide alloys at ORNL, has spearheaded research in reaction synthesis using elemental metallic powders. Reaction synthesis is a chemical change in which an intermetallic alloy is formed from metallic powders using heat generated during the reaction. The method is considered attractive for producing aluminide products in desired shapes with little or no machining.

Reaction synthesis also can be used to produce aluminide products of high density for structural uses. Aluminides have many uses because of their low density and high strength at elevated temperatures. However, for structural applications requiring full density to bear heavier weights—for example, in cars, aircraft, and the space station—aluminides would not be of interest unless they were close to full density with minimum defects such as pores.

Working in collaboration with visiting scientists from Japan at different times, Liu has used reaction synthesis to produce a high-density, defect-free nickel-aluminide alloy. Traditional processes have produced nickel-aluminide alloys with a 97% density; such a porous material is too weak and brittle for many structural applications.

"By using reaction synthesis," Liu says, "we formed a ductile nickel-aluminide alloy product with a density as high as 99.9%."

Reaction synthesis for preparing alloys and forming them into products has other advantages. The self-generation of heat while making the alloy reduces the need for energy for material processing. Also, the cost of alloy and product preparation is lowered because of reduced energy requirements and the ability to form aluminide products directly from elemental powders.

In reaction synthesis of intermetallic alloys such as aluminides, fine powders of aluminum are mixed with the other metal of interest (iron, nickel, titanium) in the proper proportion for the desired compound (say, FeAl or Ni3Al). After the powders are mixed in a rotating container called a ball-milling machine, the "green" unreacted samples are consolidated by pressing the powders together in a die, similar to forcing dough into a cookie cutter, to make a product of a desired shape. The resulting "green" disks are placed in a reaction chamber under vacuum and heated to 660°C, a temperature close to the melting point of aluminum powders.

At this point, reaction synthesis kicks in. Because the liberated heat raises the temperature of the neighboring layers of atoms of aluminum and the other metal to as high as 1400°C, the reaction is self-sustaining like that in a burning cigarette (except the alloy-forming reactions move faster—3 to 6 inches per second depending on the compound). Liu and his colleagues found that applying a few megapascals of pressure by a compression machine during reaction synthesis produces a material of near-theoretical density. "The idea," Liu says, "is to squeeze the pores out of the material so it's more like cheddar cheese than Swiss cheese."

More recently, Debra Joslin, Dewey Easton, Liu, and Stan David, all of ORNL's Metals and Ceramics Division, have been studying reaction synthesis of iron aluminides (FeAl and Fe3Al). Using high-speed videotaping equipment, they studied the reaction behavior of iron aluminides during synthesis in air. They found that the reaction rate depends on compact composition and powder particle size; it increases with greater aluminum content, and it decreases with increasing powder size. They also found that pores formed in the reaction-synthesized products can be reduced by applying a compressive force.

Vinod Sikka and S. C. Deevi have shown that reaction synthesis, not under vacuum or pressure, can be used to produce ingots of nickel, iron, and titanium aluminides. Through forging, rolling, hot extrusion, and applied pressure, products can be cast from the ingots.

The Exo-Melt process described in the main article was developed by Sikka and Deevi by extending principles of reaction synthesis to the melting and casting of iron aluminide (Fe3Al) and nickel aluminide (Ni3Al). This process, which uses half as much energy as traditional processes and addresses safety concerns of the alloy preparation industry, was used at ORNL to melt and cast more than 100 ingots of Ni3Al- and Fe3Al-based alloys.

As a result of these successes, a unique facility has been developed at ORNL to study reaction synthesis of intermetallic alloys. The facility will be used to study processing parameters and the feasibility of forming near-net-shape products. Someday industry may find this emerging method of making products from powders too good to resist.

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