A Stable Decagonal Quasicrystal in the Al–Cu–Co System

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Abstract
The structure and growth morphology of Al65Cu15Fe20−xCox and Al65Cu20Fe15−xCox alloys have been examined in rapidly solidified, conventionally solidified and fully annealed states. A conventionally solidified Al65Cu15Co20 alloy, which was prepared by the whole substitution of Fe by Co for a stable icosahedral Al65Cu20Fe15 quasicrystal, was found to consist of a stable decagonal quasicrystal with a columnar morphology. The length of the decagonal column is as large as about 0.15 mm and the growth is thought to take place along a preferential direction parallel to the tenfold periodic axis because the plane perpendicular to the longitudinal direction of the column has the morphology of five dendrite arms radiating from the dendrite centers. The finding of the stable decagonal quasicrystal is important in the clarification of the formation mechanism and fundamental properties of the quasicrystals with an icosahedral or a decagonal structure.