Electron microscopy analysis of decagonal quasicrystals in the Al-Cu-Co-Sisystem

Large needle-shapoed Grains of a decagonal quasicrystal in the Al-Cu-Co-Si quaternary system were prepared by slow cooling From the melt. Scanning ele ctron microscopy, X-ray diffraction and convergent-beam electron diffractio n patterns confirmed that the needles have a decagonal quasicrystal structu re. Thin slices normal to the tenfold directions were prepared for transmis sion electron microscopy. Diffuse streaks along symmetric directions around the fundamental spots and in between weaker diffraction spots as well as s atellite spots were observed in the tenfold diffraction patterns. Bright-fi eld and dark-field images showed discontinuous lines or striations lying pe rpendicular to the direction of diffuse streaking. Dark contrast at the poi nt of discontinuity of the fringes are observed along a particular directio n. High-resolution electron microscopy images also indicated features corre sponding to the striation contrast. The striation contrast is shown to be a ssociated with a phason disorder boundary in the partially ordered decagona l phase. Satellite spots are attributed as a characteristic feature of the partially ordered decagonal superlattice structure, which is an intermediat e state between a fully disordered and fully ordered structure of the decag onal phase. The atomic rearrangement or phason displacements in certain sym metric directions (along the pentagrids or Ammann lines in the structure) g ive rise to the striation contrast observed in the images. The evolution of domains consisting of periodic and quasiperiodic parallel boundaries in lo calized regions can be interpreted as one of the signatures of an intermedi ate structural state prior to the formation of a fully ordered decagonal la ttice.