Defect-modulated Al-Ni-Rh crystalline approximants, which are structurally related to a decagonal quasicrystal with 1.6 nm periodicity, have been found to form in an as- solidified Al75Ni15Rh10 alloy. Their structural features were studied by a combination of high- resolution electron microscopy (HREM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The results show that the prefect structures of Al-Ni-Rh crystalline approximants are characterized by periodic arrangements of hexogen units (H) constructed from atom columnar clusters. Defect-modulated structures were formed due to ordered arrangement of one- or two-dimensional structural defects of high density. The basic structural defect is associated with incorrect arrangements of atom columnar clusters which appear as polygons different from the H units. These polygons of atom columnar clusters represent a type of linear defect extending along the columnar axis direction, and they can array to form a planar defect. By means of the HAADF-STEM imaging technique, the structural nature of the core area for the linear defect, which usually exhibits dark contrast in HREM observations, has been clearly revealed.
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