Researchers at the U.S. Department of Energy's Ames Laboratory at Iowa State University have discovered a number of rare earth intermetallic compounds that are ductile at room temperature. The discovery, announced in an article in the September issue of the journal Nature Materials, 2, PP 587-590, has the potential to make these promising materials more useful.
"Over the last several decades, tens of thousands of intermetallics have been identified," Ames Laboratory materials scientist Alan Russell said. "But in order to make them even somewhat ductile, a whole menu of 'tricks' have been developed, such as testing them at high temperatures, or in zero-humidity, or shifting them off stoichiometry. The materials we're studying are the first ones that don't need these contrivances."
So far, the Ames Laboratory research team, led by senior metallurgist Karl Gschneidner, Jr. and Russell, has identified 12 fully ordered, completely stoichiometric intermetallic compounds. These compounds are formed by combining a rare earth element with certain main group or transition metals . The resulting binary compounds have a B2 crystal structure, like that found in cesium-chloride (CsCl), in which an atom of one element is surrounded by a cubic arrangement of eight atoms of the other element.
The study has focused on yttrium-silver (YAg), yttrium-copper (YCu), and dysprosium-copper (DyCu), but a preliminary examination of other rare earth compounds showed that cerium-silver (CeAg), erbium-silver (ErAg), erbium-gold (ErAu), erbium-copper (ErCu), erbium-iridium (ErIr), holmium-copper (HoCu), neodymium-silver (NdAg), yttrium-indium (YIn), and yttrium-rhodium (YRh) are also ductile.
Samples were prepared by arc-melting high-purity elements to form compounds with a 50-50 atomic mix of Y or Dy and Ag or Cu. X-ray diffraction, optical metallography, and electron microscopy confirmed the specimens were single-phase with the fully ordered B2 structure.
In tensile testing, these materials showed remarkable ductility. The YAg stretched nearly 25 percent before it fractured, compared to 2 percent or less for many other intermetallics. In other measurements, the materials showed ASTM fracture toughness values (KIC) comparable with commercial aircraft aluminum alloys.
Why these materials deform while other intermetallics shatter isn't quite clear, but theoretical calculations by Ames Lab physicist James Morris show that the ductile materials possess much lower unstable stacking-fault energies. Because these energies are lower in the ductile materials, it is easier for them to plastically deform instead of fracturing at the grain boundaries.
"There are particular planes (within the B2 structure) that tend to slip most easily," Russell said, "and particular directions on those planes where deformation slip occurs most easily. However, our transmission electron micrographs identify slippage in more than one direction, so there are probably other factors at work as well."
While there may be applications for these ductile materials because of their other characteristics like high-temperature strength or corrosion resistance, Gschneidner and Russell hope that studying these materials will actually lead to a better understanding of the brittle intermetallics.
"The most exciting thing about this is finding a material that breaks all the rules. It provides a great opportunity to figure out fundamentally why the others are brittle," Russell said. "To see one that's the exception gives you a new perspective on all the others."
Gschneidner added, "The exceptions are the ones you want to concentrate on because they can tell you a heck of a lot more than all the ones that obey the rules. It can steer you in a whole new direction."
The research is supported through funding from the DOE's Office of Basic Energy Science. The Ames Laboratory is operated for the Department of Energy by ISU. The Laboratory conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials. More information about the Ames Laboratory can be found at www.ameslab.gov.
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