Regensburg 2002 – scientific programme

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M: Metallphysik

M 19: Symposium on Structurally Complex Alloy Phases I (SCAP I)

M 19.1: Talk

Thursday, March 14, 2002, 09:30–10:00, H16

Structurally complex alloy phases, an introduction — •Knut Urban — Institut fuer Festkoerperforschung, Forschungszentrum Juelich

Binary, ternary and higher metallic alloys can form crystal structures based on giant unit cells containing hundreds, even thousands of atoms. Although, from a crystallographic point of view, giant-unit cell intermetallics were studied quite early, very little is known on their physical properties. The electronic and steric reasons which force an atomic system into such structures are essentially unknown, and the mechanisms and kinetics of the diffusion processes governing atomic ordering and crystal growth remain a mystery.

The giant unit cells have a complex substructure based on polyhedral atom arrangements or clusters which, partially overlapping or linked by bridging elements, are making up the cells. Such clusters are, e.g., the 12 or 13 atom icosahedron, the 17 atom Friauf polyhedron, the 52 to 54 atom Mackay-type and the 105 atom Bergman cluster. It is particularly interesting that these clusters define a physical correlation length which is substantially smaller than the unit cell dimension.

In the past studies on the giant-unit cell intermetallic alloy phases was hampered by the lack of suitable large-grain or single crystalline material. Due to recent progress in crystal growth of ternary alloys the situation is about to change. This opens up new possibilities for research on this largely unknown class of materials.