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

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_71.5Co16Ni_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.