| The microstructures and defect structures for quasicrystals and approximant phases in the Al-Ni-Rh alloy system have been systematically investigated by means of a combination of conventional transmission electron microscopy (TEM) (including diffraction-contrast imaging, selected area electron diffraction (SAED)), high resolution electron microscopy (HREM), high angle annular dark field transmission electron microscopy(HAADF-STEM), X-ray energy dispersive spectrum (EDS). HREM simulation and energy calculation have been used to define the structures of special boundaries found inξ'-andξ-Al-Ni-Rh phases. In addition, the structural changes induced by electron-beam irradiation for structurally complex phases (ξ'-andξ-Al-Ni-Rh phases) are also presented and discussed. The main conclusions obtained from the present study are summarized as following:1) An Al-Ni-Rh decagonal quasicrystal with 1.6 nm periodicity, which can be formed as main phase in the rapidly solidified Al75Ni15Rh10 alloy was found and its quasiperiodic and periodic features viewed respectively along the five- and two-fold axes were revealed by HREM. It is clearly shown that the Al-Ni-Rh decagonal quasicrystal with 1.6 nm periodicity contains a kind of atom columnar cluster with about 1.6 nm periodicity as the most fundamental structural building block. Structurally complex alloy phases,ξ'- andξ- phases, have been found, for the first time, in the Al-Ni-Rh system. Bothξ'- andξ- Al-Ni-Rh phases contains a kind of flatten hexagon, constructed with atom columnar clusters, as basic structural units. Their structures are characterized with the different arrangements of flatten hexagons, which can form a kind of mixed structure easily due to intergrowthing. Decagonal quasicrystals with different periodicity can also intergrowth with a definite local connection manner between them, in which orientational ordering for the arrangements of atom clusters in quasiperiodic plane is along the same fivefold directions.2) Special planar defects in structurally complexξ'-Al-Ni-Rh andξ-Al-Ni-Rh phases have been studied by TEM, HREM and HAADF-STEM. Structural defects of Phason type, which are similar to those formed in quasicrystals were found to be formed extensively with various variants inξ'- andξ-Al-Ni-Rh phases. The ordered arrangement of cross-shaped defects of high density can form a very unique defect-modulated structure.Three types of special domain boundaries have also been observed in bothξ'- andξ- Al-Ni-Rh phases. The structural features of these boundaries revealed by HREM can be interpreted as different overlapping with different translational vectors for two adjoined domains. The three domain boundaries with their normals parallel to the [001] direction are actually phason planes characterized by a translation vector of , respectively. A two-layer covering model for each special domain boundary with a definite translational vector was proposed. The good matching between each calculated image and the observed HREM image suggests that the model is reasonable. In addition, cleavage energies for the different stacking planes in an atom columnar cluster along its tenfold axis have been calculated and the possible cleavage planes discussed.Furthermore, we believe that the general rules for understanding the superstructures formed within boundary areas are correlated directly with different overlapping or connecting operation with respect to building units of flattened hexagons and P-B polygons. These rules are as following:I) The adjoined domains at least have their basic structural units a side-shared (that is, sharing two column clusters);П) The basic structural units in both domains have overlapping area as large as possible.3) For theξ-Al-Ni-Rh phase formed in the as-cast Al75Ni15Rh10 alloy, phase transition processes induced by electron beam irradiation with different amount of dose were studied by in-situ observations in an electron microscope. The results show that a crystalline approximant-to-amorphous phase transition occurred under low-dose irradiation, during which defects aggregation plays an important role, while a pore with amorphous phase in its rim formed when subjected to the high-dose irradiation. Higher density electron beam induce hollow holes of the hole in the sample and A kind of transition phase with a face-centered structure and with a parameter of 2.2 ? was observed to be formed between the newly formed amorphous phase and the originalξ-phase. This clearly indicts that it is through a transitional structure that structurally complex phases (crystalline approximant and quasicrystalline phases) can transform into an amorphous structure.
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