Crystal Structure of New Approximant Phase in Al-Fe-V-Si System
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Crystal Structure of New Approximant Phase in Al-Fe-V-Si System Vladimir I. Ezersky, Avi D. Rochman and Michael M. Talianker Department of Materials Engineering, Faculty of Engineering Sciences, Ben-Gurion University of the Negev, Beer-Sheva, 84105, POB 653, Israel. ABSTRACT A model proposed for the atomic structure of the new hexagonal Al-Fe(V)-Si phase which was found in rapidly solidified Al-Fe(V)-Si alloys is discussed in terms of relation this phase to approximants. The structure of this new phase was described as hexagonal packing of double Mackay icosahedral units similar to those of which the cubic α-(AlFeSi) approximant phase is built up. On the basis of this model the experimental high resolution electron microscopic (HREM) images of the hexagonal phase were successfully interpreted. Like the α phase this structure is characterized by the local icosahedral ordering and can be regarded as an approximant.
INTRODUCTION Since the publication of the work by Elser and Henley [1] the study of the special class of structures called approximants has aroused great interest. The approximant phases were defined as periodic structures representing rational approximation of a corresponding quasicrystalline material. It was agreed that the approximants possess structural features, which are very close to those of the associated quasicrystal and, therefore, are of importance for the understanding of local atomic order and constructing atomic structural model of quasicrystals. Theoretically the approximants were described as three-dimensional projections of highdimensional hyper lattice along rational directions, while corresponding quasicrystalline structures could be generated from the projections along irrational directions associated with the gold number τ=(1+¥ Experimentally the approximants were identified on the basis of several criteria. One of them is that both the approximant phase and the related quasicrystal possess the same structural building unit while the atomic arrangement in this unit is characterized by icosahedral local ordering. The classification schemes of approximants are usually directly related to the structural peculiarities of the building unit. Another criterion is the existence of close relationship between electron diffraction patterns of approximants and a corresponding quasicrystal: the distribution of strong spots along specific directions in both patterns must be similar. Also noteworthy is that in many cases approximants exhibit definite orientation relationship with the quasicrystal [2]. In fact, this requirement can be considered as a direct consequence of the structural similarity of their building blocks. An additional structurally independent criterion is that approximants, being Hume-Rothery crystalline phases, possess approximately the same valence electron concentration as that of the associated quasicrystal [3]. Correspondingly, the composition of the approximant phase should be in the vicinity of the quasicrystal phase. Among a very large number of approximants that were
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