Approximants for the Al-Co-Si decagonal phase

Department of earth and Planetary Science, The University of Tokyo
â—‹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 [3].
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[4]. The structure of f-AlCoSi phase serves an unique columnar structural unit similar to those found in the approximants of Al3Mn[5] and Al3Pd.[6].
[1]K.W.Richter & D.T.Gutierrez, Intermetallics 12(2005)848. [2]K.W.Richter, Y.Prots & Y.Grin, Inorg. Chem. 44(2005)4576. [3]J.Menon & C.Suryanarayana, Mater. Trans. JIM 30(1989)878. [4]K.Sugiyama, S.Nishimura & K.Hiraga, J. Alloy Comp., 342(2002)65. [5]K.Hiraga, M.Kaneko, Y.Matsuo & S.Hashimoto, Philos. Mag. B67(1993)193. [6]Y.Matsuo & K.Hiraga, Philos. Mag. Lett., 70(1994)155.