The Structure of an Al-Co-Ni Crystalline Approximant with an Ordered Arrangement of Atomic Clusters with Pentagonal Symmetry

Abstract

The structure of a crystalline approximant (called “PD_3c phase”) in an Al_71.5Co₁₆Ni_12.5 alloy annealed at 900°C for 120 h has been studied by high-angle annular detector scanning transmission electron microscopy (HAADF-STEM) and high-resolution transmission electron microscopy (HREM). The PD_3c phase has an orthorhombic structure with lattice parameters a=5.2 nm, b=0.4 nm and c=3.7 nm, and the space group of Pmn2₁ (No. 31). Its structure is characterized as an ordered arrangement of columnar atom clusters with pentagonal symmetry, and can be interpreted as a crystalline approximant related to an ordered decagonal quasicrystal formed by the projection of a NaCl-type hypercubic lattice.