STRUCTURE AND STRUCTURAL-CHANGE OF AL-NI-CO DECAGONAL QUASI-CRYSTAL BY HIGH-RESOLUTION ELECTRON-MICROSCOPY

An Al70Ni15Co15 alloy, known to form a stable decagonal quasicrystal, was subjected to various heat treatments and studied with electron diffraction and high-resolution electron microscopy. The alloy quenched from 800-degrees-C forms a decagonal quasicrystal, which shows diffraction patterns with a large number of sharp spots located at perfect decagonal symmetry positions and also diffuse scatterings on the background. High-resolution electron microscopic observations of this decagonal quasicrystal show that the structure is formed by a pentagonal tiling of some atom clusters. This tiling may be interpreted by the random tiling model, which is characterized by the existence of phason fluctuations on the randomization of tiling. The quasicrystal was found to undergo a structural change to a rhombic tiling, with partially translational arrangements, of the same atom clusters, by slowly cooling from 800-degrees-C or annealing at 550-degrees-C. These results seem to be first experimental observations to confirm the random tiling model. A structure model for the atomic cluster is proposed.