Field Ion Microscopy of Quasicrystals
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FIELD ION MICROSCOPY OF QUASICRYSTALS H. B. ELSWIJK and J. Th. M. DE HOSSON Department of Applied Physics, Materials Science Centre, University of Groningen, Nijenborgh 18, 9747 AG Groningen, The Netherlands ABSTRACT A Field Ion Microscope Imaging Atom Probe (FIM-IAP) has been applied to study the atomic structure of the icosahedral phase of the Al-Mn system. A multiple twinning model for icosahedral quasicrystals could be disproved, by calculating its FI image, and comparing it with experimental images. One of the decorations of the three dimensional Penrose packing (3DPP) of which FI images were calculated does compare favourably with experimental images, as far as the relative prominence of the two and fivefold poles, and the interplanar distances along the twofold directions are concerned. In particular an atomic decoration of vertices and two sites in the interior of the thick rhombohedron, which is one of the two building blocks of the 3DPP, turned out to be in agreement with the experimental findings. INTRODUCTION The discovery [1] that rapidly quenched samples of several transition-metal aluminium alloys like Al-Mn produce electron diffraction patterns with 'sharp' spots and icosahedral symmetry has initiated a flurry of theoretical and experimental observations. For these so-called quasi crystalline materials a model was constructed which at the same time was aperiodic and contained perfect orientational order [2]. This model, the so-called Three-Dimensional Penrose Packing (3 DPP), has a simular diffraction pattern as i-(Al-Mn). Besides the quasicrystal model, numerous other models have been proposed to explain the icosahedral phases. The leading competitors have been multiple twin models [3J[4], the large unit cell model and the icosahedral glass model. Multiple twinning is a common phenomenon in crystal growth in which wedges of crystallites join together about a center of symmetry to form a cluster. In general multiple twin clusters are very low energy configurations in which crystals may grow. Field-Ion Microscopy provides an experimental test to an atomic model of quasi crystalline materials by viewing the material directly on an atomic scale. However, in order to draw firm conclusions from FIM observations it turns out to be essential to perform computer simulations of FiM images both of 3 DPT and multiple twinning models and to compare them with experimental images. EXPERIMENTAL FIM RESULTS The quasi crystalline alloys which have been investigated by means of FIM include: Al 6Mn and Al7 Mn 2.Also, for comparison, a crystalline sample of Al4Mn was imaged in order to investigate the imaging characteristics of these alloys. The quasi crystalline alloys were produced by melt spinning. Here we confine ourselves to the results obtained of Al7 Mn2.The FI specimens have been prepared from the thinnest species of melt spun material and were found to be icosahedral without exception by TEM investigation. In general quasicrystalline Al-Mn alloys with Mn-contents of 20 at% and more are known to contain decagon
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