The Structure of an Al–Co–Ni Decagonal Quasicrystal in an Al₇₂Co₈Ni₂₀ Alloy Studied by Cs-Corrected Scanning Transmission Electron Microscopy

Abstract

The structure of an AlCoNi decagonal quasicrystal in an Al 72 Co 8 Ni 20 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.

Structure of an Al-Fe-Ni Decagonal Quasicrystal Studied by Cs-Corrected STEM

“…2a. From the previous interpretations [1,3,5], it can be concluded that all bright dots connected by lines in Fig. 2a and in Fig.…”

“…3a, with an edge-length of 0.25 nm, and consequently the TM atoms and MSs are arranged with BOO. The pentagonal arrangements of MSs in pentagonal frames with denite directions can be seen in the B plane of the structure of W(AlCoNi) [1,10] and τ 2 -Al 3 Co [2] crystalline phases, which are considered to be important approximants for the interpretation of Al-TM decagonal quasicrystals and in quasiperiodic planes of several types of Al-Co-Ni decagonal quasicrystals [1,4,5]. is generated by the projection of a ve-dimensional (5D) hyper-cubic lattice through a window of Fig.…”

Structure of a crystalline approximant related to Al–Co–Ni decagonal quasicrystals studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy and atomic-resolution energy dispersive X-ray spectroscopy

“…Introduction Six types of decagonal quasicrystals (DQCs) and some crystalline approximants have been found in Al-Co-Ni alloys with various compositions from 8 to 25.5 at % Co and from 20 to 5 at % Ni, respectively, with a nearly constant Al content from 70 to 72 at % [1][2][3][4][5], and so the stability of these DQCs formed in alloys with different Ni/Co ratios is considered to be affected by chemical ordering of Co and Ni. However, the study of the chemical ordering of Co and Ni in the Al-Co-Ni DQCs and their approximants has never been performed, because it has been difficult, if not impossible, to distinguish between neighbouring elements Co and Ni, which show very similar contrast to both electron and X-ray probes, although transition-metal (TM) atom arrangements have been exactly determined by recent Cs-corrected scanning transmission electron microscopy (STEM) observations [6][7][8][9][10][11].…”

“…It should be noticed that two types of the 1.2 nm clusters with opposite orientations of the pentagonal arrangements at their centres are arranged in an ordered manner in such a way that two clusters connected by a bond are always different. Figure 2b is derived from the arrangement of bright dots in Figure 2a on the basis of structures of the W-(AlCoNi) phase [6] and several Al-TM DQCs and crystalline approximants [6,7,[9][10][11]. The 1.2 nm clusters arranged with an interval of 2 nm are connected by two types of pentagonal tilings with a bondlength of 0.76 nm, as indicated by solid and broken lines in Figure 2a.…”

“…The cluster is a common structure unit in most of the Al-TM DQCs and crystalline approximants [6][7][8][9][10][11], and is called a 1.2 nm cluster with a diameter of 1.2 nm. The 1.2 nm clusters are located at vertices of a rhombic lattice formed by the periodic array of obtuse and acute rhombuses, which form the Penrose rhombic tiling, with a bond length of 2 nm.…”