The Structure of Type-II Al-Ni-Co Decagonal Quasicrystal Studied by Atomic-Scale Electron Microscopic Observations

The structure of an Al–Ni–Co decagonal quasicrystal, showing electron diffraction patterns called “type-II”, in an Al71.5Ni12.5Co16 alloy, has been examined by high-angle annular detector dark-field scanning transmission electron microscopy and high-resolution transmission electron microscopy. Its structure has a columnar cluster of atoms with a decagonal section of about 2 nm in diameter as a structural unit, and forms a heterogeneously aperiodic tiling of pentagonal and rhombic tiles in a two-dimensional arrangement of the columnar clusters. Translational arrangements of pentagons and rhombuses in the tiling can be observed locally. Also, the characteristic of a NaCl-type ordered arrangement of two kinds of columnar clusters of atoms, which have different orientations of pentagonal symmetry, is observed in the tiling.