The real-space structural change accompanying the order-disorder transformation recently found in an Al-Ni-Co decagonal quasicrystal has been investigated by high resolution transmission electron microscopy (HRTEM). The arrangements of ring-like contrasts observed in the HRTEM images have been analyzed on the basis of a high-dimensional description of the quasicrystalline structure. The analysis has revealed that the rearrangement of the atomic clusters, which gives the ring contrasts in the HRTEM image, is responsible for the present order-disorder structural transformation. This is a new type of order-disorder transformation characteristic of the quasicrystal. A domain structure of variants, which is usually formed in conventional ordered crystalline phases, has been observed also in the present ordered phase.
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