Formation of Icosahedral Quasicrystals in Mg-Al-X (X=Transition Metal) Ternary Systems
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Formation of Icosahedral Quasicrystals in Mg-Al-X (X=Transition Metal) Ternary Systems Naokiyo Koshikawa, Shinichi Yoda, Keiichi Edagawa1 and Shin Takeuchi2 National Space Development Agency of Japan / Tokyo Institute of Technology Sengen 2-1-1, Tsukuba, Ibaraki 305-8505, Japan/ 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan 1 Institute of Industrial Science, University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8904, Japan, 2 Science University of Tokyo, 641 Yamazaki, Noda-shi, Chiba 278-8510, Japan ABSTRACT Formation of icosahedral quasicrystalline phase (i-phase) was investigated for Mg-Al-X (X=Co, Ni, Cu, Rh, Ag, Pt and Au) ternary systems. A Mackay-icosahedron (MI) type i-phase was found to form in the Mg-Al-Rh system at the composition Mg8Al68Rh24. The i-phase has the F-type ordered structure and the quasilattice constant is 0.447nm. Differential thermal analysis (DTA) studies showed that the i-phase has a high stability. On the other hand, the formation of a Frank-Kasper (FK) type i-phase was found in the Mg-Al-Pt system at the composition Mg45Al42Pt13. The conditions necessary for the formation of the i-phases are discussed in terms of the atomic radius ratio and the electronegativity as well as the electron-per-atom ratio. INTRODUCTION The formation of an icosahedral quasicrystalline phase (i-phase) was first reported in an Al-Mn alloy as a metastable phase in 1984[1]. Since then, the i-phases have been found to form in many alloy systems[2]. The i-phases reported so far can be classified into two groups by the difference in the intensity ratio of their X-ray diffraction peaks[3]. The difference in the intensity ratio is considered to reflect the difference in the atomic arrangements of the icosahedral cluster constituting the structure of the i-phase[4,5]. The i-phases of one group are called the Mackay icosahedron (MI) type and those of the other the Frank-Kasper (FK) type. Another classification can be made in view of the Bravais type of the six-dimensional lattice: P-type and F-type[6]. The F-type i-phase is considered to have an ordered structure of the P-type one. It is noteworthy that many thermodynamically stable i-phases have been found for MI-type F-type structures[7-9], for FK-type P-type structures[10,11] and for FK-type F-type structures[12,13]. In a previous paper[14], we reported that both MI-type and FK-type i-phases are formed in the same Mg-Al-Pd system. In that study, the composition ranges and stability of the phases were investigated in detail. In relation to the Mg-Al-Pd i-phases, some preliminary results of the investigation of a MI-type i-phase in the Mg-Al-Rh system were also reported. In the present paper, we report recent progress in our investigation of the formation and stability of the i-phase in the Mg-Al-Rh system, together with those in other Mg-Al-X ternary systems. K9.3.1
EXPERIMENTAL PROCEDURES Mg-Al-X (X=Co, Ni, Cu, Rh, Ag, Pt and Au) ternary alloys were prepared by arc-melting in an argon atmosphere from elemental constituents with purity better than 99.8%
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