Abstract
Large needle-shaped grains of a decagonal quasicrystal in the Al–Cu–Co–Si quaternary system were prepared by slow cooling from the melt. Scanning electron microscopy, X-ray diffraction and convergent-beam electron diffraction patterns confirmed that the needles have a decagonal quasicrystal structure. Thin slices normal to the tenfold directions were prepared for transmission electron microscopy. Diffuse streaks along symmetric directions around the fundamental spots and in between weaker diffraction spots as well as satellite spots were observed in the tenfold diffraction patterns. Bright-field and dark-field images showed discontinuous lines or striations lying perpendicular to the direction of diffuse streaking. Dark contrast at the point of discontinuity of the fringes are observed along a particular direction. High-resolution electron microscopy images also indicated features corresponding to the striation contrast. The striation contrast is shown to be associated with a phason disorder boundary in the partially ordered decagonal phase. Satellite spots are attributed as a characteristic feature of the partially ordered decagonal superlattice structure, which is an intermediate state between a fully disordered and fully ordered structure of the decagonal phase. The atomic rearrangement or phason displacements in certain symmetric directions (along the pentagrids or Ammann lines in the structure) give rise to the striation contrast observed in the images. The evolution of domains consisting of periodic and quasiperiodic parallel boundaries in localized regions can be interpreted as one of the signatures of an intermediate structural state prior to the formation of a fully ordered decagonal lattice.