ORDERED AND DISORDERED PHASES IN AL-NI-CO DECAGONAL QUASI-CRYSTALS

The composition ranges of Al-Ni-Co decagonal quasicrystals in superlat tice-ordered and disordered states have been investigated. The single- decagonal-phase region is limited to a very small area around Al71Ni16 CO13 at 650 degrees C, which expands along an almost Al-constant line at 850 degrees C. The single-decagonal-phase area at 650 degrees C ent irely yields the completely ordered state, while that at 850 degrees C yields either a partially ordered state or the completely disordered state. The real-space structural change between the ordered and disord ered states has been investigated by high-resolution transmission elec tron microscopy (HRTEM). The HRTEM images for both states comprise the arrangements of ringlike contrasts, forming tiling patterns with the edge length of about 20 Angstrom. These tiling patterns have been anal yzed on the basis of a high-dimensional description of the quasicrysta lline structure. The analysis has revealed that the two tiling structu res have characteristic features of the ordered and disordered quasicr ystalline lattices, respectively. This leads to the conclusion that th e rearrangement of the atomic clusters giving the ring contrasts in th e HRTEM image is responsible for the present order-disorder structural transformation, which is a new type of order-disorder transformation characteristic of quasicrystals.