Decagonal quasicrystals in the Al(Si)-Cu-Co alloy system

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He, Lun Xiong
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We have found four different decagonal quasicrystals in the Al65Cu20Co 15 alloy system. They have different periodicities along the tenfold axis, namely, 0.4, 0.8, 1.2 and 1.6nm. This suggests that the decagonal quasicrystal might have a fundamental structure with periodicity of 0.4nm, the other decagonal quasicrystals are the superstructures of it. A one-dimensional quasicrystal derived from the decagonalquasicrystal was also found. The lD quasicrystal could be obtained by introducing a linear phason strain field into the decagonal quasicrystal. This produces a link between the decagonal quasicrystal and crystalline phases. If the one-dimensional quasicrystal can be well understood, it will be very helpful for further insight into the structure of the 2-D decagonal quasicrystal. Unfortunately, the one-dimensional quasicrystal can not be obtained in a single grain suitable for the x-ray study. The alloy Al62Si3Cu20Co15 was used to grow single crystals. The resulting grains have deca-prismatic morphology, and the grain size is 0.5mm in diameter and several millimeters in length. The TEM study (CBED, LACBED and HREM) shows that it has high degree of perfection. The CBED and LACBED patterns taken along the tenfold axis, 2-fold axes have very sharp Kikuchi bands and HOLZ lines which denotes that the quasicrystal is perfect in the region of around 1µm. The quantitative study of the HREM image also shows that the atomic clusters form pattern similar to 2-D Penrose tiling (perfect decagonal quasicrystal), and the ⟨X2⟂⟩ is almost constant (which is a property 'of the perfect decagonal quasicrystal). In the Bridgman-method grown ingot, the 1D quasicrystals form some grams having morphology similar to these of the decagonal quasicrystals. The diffraction patterns from both the TEM and x-ray precession camera are very close to the patterns of the decagonal quasicrystal. The x-ray and the TEM study reveal, however, that they are in the form of tenfold twins of 1D quasicrystals with some minor decagonal quasicrystal approximants. The three approximants found by HREM can also be derived from the decagonal quaiscrystal by introducing a linear phason strain.