2013 Volume 54 Issue 5 Pages 720-724
The structure of an Al–Co–Ni decagonal quasicrystal in an Al72Co8Ni20 alloy has been examined by Cs-corrected high-angle annular detector dark-field (HAADF)- and annular bright-field (ABF)-scanning transmission electron microscopy (STEM) observations with the incident beam parallel to the periodic axis. Observed ABF- and HAADF-STEM images clearly show the existence of large columnar clusters of a decagonal section with an about 3.2 nm in diameter and their arrangement with bond-orientational order (BOO) of a bond-length of 3.2 nm. Individual transition-metal (TM) atoms and mixed sites (MSs) of Al and TM atoms are represented as separated bright dots in observed HAADF-STEM images, and consequently arrangements of TM atoms and MSs on two quasiperiodic planes can be directly determined. The TM atoms and MSs are located at lattice points of a Penrose lattice with an edge-length of 0.25 nm, and so they are arranged with BOO on the two quasiperiodic planes. An atomic arrangement including Al atoms in the 3.2 nm cluster is speculated from observed HAADF- and ABF-STEM images.