Ordered and disordered phases in Al-Ni-Co decagonal quasicrystals

The composition ranges of Al-Ni-Co decagonal quasicrystals in superlattice-ordered and disordered states have been investigated. The single-decagonal-phase region is limited to a very small area around Al₇₁{Ni}₁₆Co₁₃ at 650 ^∘{}C, which expands along an almost Al-constant line at 850 ^∘{}C. The single-decagonal-phase area at 650 ^∘{}C entirely yields the completely ordered state, while that at 850 ^∘{}C yields either a partially ordered state or the completely disordered state. The real-space structural change between the ordered and disordered states has been investigated by high-resolution transmission electron microscopy (HRTEM). The HRTEM images for both states comprise the arrangements of ringlike contrasts, forming tiling patterns with the edge length of about 20 {}. These tiling patterns have been analyzed on the basis of a high-dimensional description of the quasicrystalline structure. The analysis has revealed that the two tiling structures have characteristic features of the ordered and disordered quasicrystalline lattices, respectively. This leads to the conclusion that the rearrangement of the atomic clusters giving the ring contrasts in the HRTEM image is responsible for the present order-disorder structural transformation, which is a new type of order-disorder transformation characteristic of quasicrystals.