Ductile intermetallic compounds discovered

Ductile intermetallic compounds discovered

Keywords: Compounds, Materials, Metals

Scientists have known for over 100 years that intermetallic materials –compounds consisting of two or more metals bonded together – possess chemical, physical, electrical, magnetic, and mechanical properties that are often superior to ordinary metals. The problem with these promising materials is that they are quite brittle.

Researchers at the US 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, Vol. 2, pp. 587-590, has the potential to make these promising materials more useful.

“Many intermetallic materials are too brittle to handle,” said Ames Laboratory Senior Metallurgist Karl Gschneidner, Jr “If you drop them, they shatter. But you can beat on these new materials with a hammer, and they won't shatter or fracture ... they're that ductile.”

So far, the Ames Laboratory research team, led by Gschneidner and Materials Scientist Alan Russell, has identified 12 fully ordered, completely stoichiometric intermetallic compounds. Such materials could be used to produce practical materials from coatings that are highly resistant to corrosion or that maintain strength at high temperatures to flexible superconducting wires and extremely powerful magnets.

“Tens of thousands of intermetallics have been identified,” said Russell, “But in order to make them even somewhat ductile, a whole menu of“tricks” has 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.”

By combining a rare earth element with certain main group or transition metals, the resulting binary compound has a B2 crystal structure. Alpha-numeric designation, developed by crystallographers, means that the compound has a 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) were also ductile (Plate 1).

 Plate 1A button of yttrium-silver shows dents and deformations from repeated hammer blows. The gadolinium-silicon-germanium material was shattered with a light tap

In tensile testing, these materials showed remarkable ductility. The YAg stretched nearly 25 per cent before it fractured, compared to 2 per cent or less for many other intermetallics. In other measurements, the materials showed American Society for Testing and Materials fracture toughness values (KIC)comparable with commercial aircraft aluminum alloys.

Why these materials deform while other intermetallics shatter is not 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,” said Russell. “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 DOE by Iowa State University. The Laboratory conducts research in various areas of national concern, including energy resources, high- speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials. For more information about the Ames Laboratory, visit the Web site: www.ameslab.gov