Electron microscopy study of striation contrast in Al-Cu-Co-Si decagonal quasicrystals

Detailed transmission electron microscopy study was carried out in single c rystals of a decagonal phase in the Al-Cu-Co-Si quaternary system. X-ray di ffraction and convergent beam electron diffraction patterns of the powder s amples confirmed the structures to be decagonal quasicrystals. No microcrys talline nor crystalline phases could be identified. Thin slices normal to t he 10-fold directions were prepared for transmission electron microscopy. D iffuse streaks along symmetric directions around the fundamental spots were observed in the diffraction patterns. Bright field images and dark field I mages showed discontinuous lines or striations lying perpendicular to the d irection of diffuse streaking. The striation contrast appears to be origina ting from anti-phase boundary (APB) in the decagonal superstructures. The d iffuse streaks seem to be a characteristic feature of a partially ordered d ecagonal superlattice structure. The atomic rearrangement or phasonic movem ent in certain symmetric directions along the pentagrids or Ammann lines in the structure has obviously caused the type of contrast observed in the im ages. The evolution of rhombic domains consisting of APBs in localized regi ons can be understood as one of the signature of an intermediate structural state formed prior to a superstructure formation.