Structure of an approximant crystal in Ni-rich Al₇₁Ni₂₂Co₇

Abstract

A novel approximant crystalline structure formed in an Al₇₁Ni₂₂Co₇ alloy annealed at 973 K is reported, for which the S1-type superlattice decagonal quasicrystal forms as a major phase. Electron diffraction and atomic-resolution Z-contrast scanning transmission electron microscope investigations successfully identified the approximant as having a monoclinic unit cell with a = c = 3.2 nm (b = 0.4 nm), β = 108°; the structure is constructed by the five-fold symmetric decagonal clusters 2 nm across, which are arrayed by inverting their pentagonal directions between the neighbours (antiferro order of the cluster sense). A plausible atomic model of the approximant is proposed by referring to a model structure of the S1-type decagonal quasicrystal, which is composed of the similar local linkages of the five-fold symmetry clusters. The present approximant is shown to share some structural characteristics with the PD_3c-approximant reported for a Co-rich Al_71.5Ni_12.5Co₁₆ alloy.

Keywords:

• electron microscopy
• decagonal quasicrystal
• approximant crystal
• atomic structure

Acknowledgement

This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas ‘‘Atomic Scale Modification’’ from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).