A new orthorhombic phase in Al–Cu–Co representing a rational approximant to the decagonal quasicrystalline phase

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Abstract
A new orthorhombic phase has been found by transmission electron microscopy in Al65Cu15Co20. Its crystal structure can be considered as a rational approximant to the structure of the decagonal quasicrystal forming in the same alloy. The lattice parameters are a=1.97nm, b = 0.40nm and c= 2.33nm. The new phase was observed in the immediate local vicinity of the decagonal phase. In between the two phases a transition region was observed which consists of twin-related micro-domains. The observations were analysed in terms of a quasicrystal-to-crystal transition. Application of linear phason strain theory yielded an evolution of the diffraction spot intensities in good agreement with the experimental observations.