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

Abstract

Six types of decagonal quasicrystals (DQCs) found in Al-Co-Ni alloys with a wide compositional ratio of Co/Ni are considered to be stabilized by chemical ordering of Co and Ni, but the elucidation of this ordering has never been performed on alloys containing neighbouring elements Co and Ni. In order to examine the chemical ordering, an Al-Co-Ni crystalline phase, the PD 3c phase, which is an important approximant for understanding the structures of ordered Al-Co-Ni DQCs, in an Al 71.5 Co 16 Ni 12.5 alloy has been studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM) and atomic-resolution energy dispersive X-ray spectroscopy (EDXS). From the combination of observed high-angle annular dark-field and annular bright-field STEM observations, an atomic arrangement of the crystalline approximant can be directly derived. The chemical ordering of Co and Ni in the arrangement of transition-metal (TM) atoms and mixed sites (MSs) of TM and Al atoms can be clearly detected on atomic-resolution EDXS maps obtained with the special technique. Co atoms are located at the TM atom positions arranged in pentagonal tiling with a bond length of 0.76 nm, whereas Ni is enriched in the MSs located in pentagonal tiles of Co atoms. It can be concluded that the change of an area ratio of Co-rich clusters to Ni-rich MSs produces various types of DQCs with different compositional Ni/Co ratios.

The structure of an Al–Rh–Cu decagonal quasicrystal studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy

“…However, the structures of all Al-TM DQCs can be characterized as arrangements of TM atoms with BOO in the 0.76 nm pentagonal tiling having τ 2 -inflated 2 nm or τ 3 -inflated 3.2 nm pentagonal tiling. Also, pentagonal arrangements of MSs located in pentagonal tiles with a definite orientation in the 0.76 nm pentagonal tiling are a common structural feature in all Al-TM DQCs, and TM atoms forming the 0.76 nm pentagonal tiling and ones in MSs are surely different kinds of TM atoms, as can be seen in the two Al-Co-Ni approximants [14,15]. The present model of the Al-Rh-Cu DQC is basically different from the structure proposed by single-crystal X-ray diffraction [21], in respect of the presence of the 0.76 nm pentagonal tiling of Rh atoms and MSs of Al and Cu atoms.…”

“…atoms and MSs of Al and Cu atoms on the two quasiperiodic planes can be derived from the observed HAADF-STEM image, on the basis of previous results [3][4][5][6][7][8]14,15]. The Rh atoms are arranged in pentagonal tiling formed by pentagonal and star-shaped pentagonal tiles with an edge-length of 0.76 nm, and also pentagonal arrangements of MSs are located in the pentagonal tiles with definite orientations.…”

“…The ring contrasts are arranged with a constant interval of 0.66 nm, and large clusters formed with ten ring contrasts arranged by tenfold rotational symmetry are observed at some places, as indicated by a large circle. corresponding bright dots in HAADF-STEM images of Al-Co-Ni DQCs [3,5,6], and also from the result of the chemical ordering in the two Al-Co-Ni approximants [14,15], in which Co atoms are located at TM positions arranged in pentagonal tiling with a bond-length of 0.76 nm and Ni occupy MSs of Al and TM atoms, it can be considered that the brighter dots in Figure 1(a) correspond to the projected positions of Rh atoms and weak bright dots on the 2 nm circles are MSs of Al and Cu atoms. On the other hand, in the ABF-STEM image of Figure 1(b) formed by contrast proportional to Z 1/3 [20], Al atom positions as well as Rh atom positions and MSs are represented as dark dots, and ring contrasts formed with ten dark dots are observed, as indicated by a small circle in Figure 2(b).…”

“…On the basis of these structural features, observed HAADF-STEM images of several Al-TM DQCs and approximants can be interpreted, and so new structural models characterized by BOO arrangements of TM atoms have been proposed [3][4][5][6][7][8] in replacement of the cluster-based models that have been widely accepted for a long time [9][10][11][12]. Also, recently, chemical ordering of Co and Ni in two crystalline approximants of PD 3c [13] in an Al 71.5 Co 16 Ni 12.5 and the W-(AlCoNi) in an Al 72.5 Co 20 Ni 7.5 has been found by atomic-resolution energy dispersive X-ray spectrometry (EDXS) attached to the Cs-corrected STEM [14,15]. In the structures of the approximants, Co atoms of a major TM element are located mainly at TM atom positions arranged with pentagonal tilings, and Ni atoms of a minor element occupy MSs.…”

Atomically Localized Ordered Phase and Segregation at Grain Boundaries in Au–Ag–Cu Ternary Alloy Nanoparticles

“…The high-resolution elemental distributions were visualized through EDS using two JEOL large-solid angle SD detectors. Such large-solid angle EDS detectors make it possible to determine the elemental information about the alloys at the atomic scale. − …”