Formation of a Superlattice Order from a Fundamental-Lattice Decagonal Quasicrystal of Al₇₂Ni₂₀Co₈

Abstract

A structural change of an Al 72 Ni 20 Co 8 decagonal quasicrystal into the S1-type superlattice ordered phase has been observed at about 450°C by in-situ high-resolution electron microscopy. The Al 72 Ni 20 Co 8 alloy, prepared by water quenching from 900°C, is composed of mirror-symmetric atom-clusters ( m -cluster) with a 2-nm diameter, which form the Penrose pentagon tiling with an edge length of 2.0 nm. It has been found that above 400°C, most of the m -clusters transformed into fivefold-symmetric cluster ( f -cluster) and the arrangement of the f -cluster formed the S1-type superlattice. It is considered that the formation of the S1 superlattice order is attributed to the disappearance of the 1.2-nm intercluster bond and to the preferential order of the sense of the f -cluster, which takes opposite senses between the neighboring clusters with a 2-nm intercluster distance. The phason flip of the positions of the 2-nm atom-clusters, which was first reported by Edagawa et al. [Phys. Rev. Lett. 85 (2000) 1674], has been observed with an atomic scale resolution. It has been found that the phason flip is caused by a flip between the fivefold-symmetry cluster and the mirror symmetric cluster. Structural models of the m - and f -clusters were reexamined.