Metallurgy of Quasicrystals: Alloys and Preparation
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Quasicrystalline and Related Crystalline Structures Metastable Quasicrystalline Phases Quasicrystalline phases were first discovered in aluminum-rich transitionmetal alloys in the compositional range of 10-14-at.% Mn.1 These alloys were prepared by rapidly quenching molten metal alloys on a rotating copper roller (melt spinning); typical quench rates in melt spinning are —W9 K/s. Quenching these alloys from a melt yielded a mixture of aluminum and quasicrystalline phases, which were a few microns in size. Soon afterward another class of quasicrystalline phases was discovered in Mg-Al-Zn alloys.2 As several alloy systems that form quasicrystalline phases were discovered, a common feature emerged. That is, almost all the quasicrystalline phases can be associated with crystalline phases containing icosahedrally packed groups of atoms. Generally it appears that the icosahedralquasicrystalline phases form mostly at compositions close to their related crys-
MRS BULLETIN/NOVEMBER 1997
talline phases. This correlation has been the basis for the discovery of many new icosahedral-quasicrystalline phases. Great progress on understanding the structure of icosahedral-quasicrystalline phases was made by recognizing34 that two complicated compounds known long ago, a-Mn12(Al, Si)575 and Mg32(Al, Zn)49,6 were approximant structures, respectively, of icosahedral quasicrystals found in the Al-Mn and Mg-Al-Zn classes of alloys. Each of these structures is a bcc packing of clusters that have two concentric atomic shells with full icosahedral symmetry around their center. In these approximant structures, the network is a body-centered-cubic (bcc) lattice. The atomic arrangements in the clusters are different in Al-transition-metal (TM) alloys (54 atoms)5 and in Mg-Al-Zn alloys (44 atoms),6 which separate these classes of alloys into two different structural families. For better visualization, Fig-
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Figure 1. The crystal structure of AI12W. The figure was redrawn from Reference 7.
ure 1 shows a lower order approximant crystal structure for describing the relationship between icosahedral quasicrystals and their related crystalline structures. The crystal structure of A1)2W is bcc with a = 7.580 A, as shown in Figure I.7 This structure is also exhibited by A1]2M where M = Mo and Re. The melt-quenched state of these alloys possesses a quasicrystalline phase. The Ali2W icosahedral cluster, which is a regular icosahedron, is centered at the origin and is linked to the icosahedral cluster at the body center by an octahedron; the octahedron shares opposite faces with both clusters. If the origin and the body center were occupied by a larger icosahedral cluster—for example a Mackay icosahedron—which exists in a-Mni 2 (Al, Si)s7 then a, which is equal to the distance between two neighboring icosahedral clusters at their vertices, would increase to 12,565 A (roughly T times the a of MnAl12 where T = (1 + V5)/2 -1.618). In other words, the larger the icosahedral cluster is, the larger the lattice parameter is and the closer the approxi
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