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M: Metallphysik

M 22: Symposium on Structurally Complex Alloy Phases IV (SCAP IV)

M 22.1: Fachvortrag

Thursday, March 14, 2002, 16:00–16:15, H16

Intermetallic Laves phase compounds - cubic vs. hexagonal crystal structure — •Frank Stein, Martin Palm, and Gerhard Sauthoff — Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany

Although the group of Laves phase intermetallics are well known since long, there are still unsolved problems concerning the stability of the respective crystal structures. Laves phases have the general composition AB₂ with the larger A atoms in the centre of a 16-atom Frank-Kasper polyhedron and the smaller B atoms in the centres of icosahedra. The Laves phases crystallize with a cubic MgCu₂-type structure or a hexagonal MgZn₂-type or MgNi₂-type structure which differ only by the particular stacking of the same two-layered structural units. It is still not possible to predict which of the possible structure types is the stable one for a Laves phase compound AB₂. Phase transformations from cubic at low temperatures to hexagonal at high temperatures were observed as well as stress-induced transformations from the hexagonal structure to the cubic one. In addition, deviations from the stoichiometric composition were reported to result in a change of the stable structure for various systems. In the ternary system Fe-Al-Zr the crystal structure of the Laves phase Zr(Fe,Al)₂ changes three times as a function of the Al content from cubic (ZrFe₂) to hexagonal to cubic and back to hexagonal (ZrAl₂). The occurrence and the character of the phase transformations of the different Laves phase polytypes is not yet understood. In order to demonstrate the complexity of the problem, various examples in other alloy systems will be presented.