Department of earth and Planetary Science, The University of Tokyo
○Kazumasa Sugiyama Hironori Sato
Since the addition of transition metals and silicon into an Al matrix involves the formation of a variety of intermetallic phases which contribute to special mechanical strength and high thermal and electrical conductivity, the ternary phase diagram of Co-Al-Si system was extensively studied [1,2]. On the other hand, much attention has been paid to the Al-Co alloy system because a large number of quasi-crystalline phases together with their approximants have been found in their Al-rich region. The ternary system of Al-Co-Si is also classified in this category .
The crystal structures of approximant structures; f-AlCoSi (Pnma; a=1.3859(1)nm, b=2.3019(1)nm, c=0.7322(1)nm,) and W-AlCoSi (Cm;a=3.9658(4)nm, b=0.8139(1)nm, c=2.3658(2)nm, b=90.02(1)o) grown in the Al-rich region of the Al-Co-Si system, were analyzed by the single crystal X-ray diffraction. Both compounds exhibit surprisingly large unit cells and complex structures. In particular, the structure of W-AlCoSi phase is isostructural with W-AlCoNi phase by providing a fundamental atomic arrangement for the columnar atom cluster observed commonly in the Al-Co-Ni decagonal quasicrystals. The structure of f-AlCoSi phase serves an unique columnar structural unit similar to those found in the approximants of Al3Mn and Al3Pd..
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