Icosahedral quasicrystals (i-QCs) are long-range ordered solids that show non-crystallographic symmetries such as five-fold rotations. Their detailed atomic structures are still far from completely understood, because most stable i-QCs form as ternary alloys suffering from chemical disorder. Here, we present the first detailed structure solution of i-YbCd 5.7 , one of the very few stable binary i-QCs, by means of X-ray structure determination. Three building units with unique atomic decorations arrange quasiperiodically and fill the space. These also serve as building units in the periodic approximant crystals. The structure is not only chemically feasible, but also provides a seamless structural understanding of the i-YbCd 5.7 phase and its series of related i-QCs and approximant crystals, revealing hierarchic features that are of considerable physical interest.Icosahedral quasicrystals (i-QCs) are the only class to show quasiperiodicity in three dimensions 1,2 . Obviously, structural knowledge is essential for understanding the physical properties, stability and tailoring applications of these exotic materials 3 . However, in contrast to other types of QCs that show periodic order in at least one direction, i-QCs cannot make effective use of two-dimensional (2D) imaging techniques such as high-resolution electron microscopy or high-angle annular dark-field scanning transmission electron microscopy for their structural characterization 4 . The i-QCs' structure determination is best achieved in the context of hyperspace crystallography 5,6 , where the structure can be described as a periodic crystal in higher dimensions. For i-QCs, the periodic space is 6D and decomposes into two orthogonal 3D subspaces: the parallel (physical) space and the perpendicular (complementary) space. The 6D unit cell is decorated by 3D objects known as 'occupation domains' (OD), the 3D QCs being obtained as a section of this decorated 6D lattice. This approach allows modelling and refinement of the structure against experimental diffraction data in a way similar to that achieved for 3D periodic crystals 6 . Although much progress has been achieved recently, for instance in the i-AlPdMn phase 7 , the models proposed so far are still being debated 8 . Indeed, the amount of observed diffraction data is in general rather limited, which precludes a detailed refinement of the chemical order in ternary QCs. Therefore, the atomic order in i-QCs remains a challenging and outstanding question. The recent discovery of the first stable binary icosahedral YbCd 5.7 QCs 9,10 has been a breakthrough and led to discoveries of a whole series of related ternary i-QCs 11 . This i-QC offers a unique opportunity for the structural analysis of i-QCs. Indeed, the i-YbCd 5.7 phase can be obtained as high-quality single grains. Furthermore, it is binary and exhibits very good X-ray contrast between Cd (Z = 48) and Yb (Z = 70) atoms.Finally, there is a series of periodic 'approximant crystals' (ACs) to the QC, having almost the same chemical composition and for...

No abstract

A thermodynamically stable quasicrystalline single phase with an icosahedral structure was found to be formed at an atomic composition of Al65Cu20Fe15 in a fully annealed state as well as in a conventionally solidified state. The stable quasicrystal consisted of large grains with an average size of 0.2 mm after annealing for 48 h at 1118 K (0.98 Tm).

No abstract

New Al-based amorphous alloys with high strength and good ductility were produced in an Al-Y-Ni system by liquid quenching. The tensile fracture strength ( σf) and Vickers hardness reach 1140 MPa and 300 DPN for Al87Y8Ni5. The specific strength defined by the ratio of σf to density is as high as 38, being much higher than that for conventional alloy steels. The high-strength Al-base amorphous alloys are expected to attract strong attention as a new type of high-strength material with low density.

No abstract

No abstract

No abstract