Decagonal phase in rapidly solidified Al6Co alloy

The structure of quasicrystals

The aim of the present article is to give a review of the state-of-the-art on quasicrystal structure analysis. After a short discussion of the term “crystal” in chapter 1, the geometrical generation of quasilattices is touched in chapter 2. In the following the higher-dimensional description of 1d, 2d and 3d quasi-crystals is demonstrated in detail as well as the derivation of structure factor equations and symmetry relationships in the higher-dimensional space. Chapter 4 shows the experimental techniques and structure determination methods for the study of quasicrystals. The experimental results of structural studies performed with different tech-niques are critically reviewed in chapter 5. Some of the results of the literature research are that five years after the detection of the first quasicrystal not a single quantitative (in terms of a regular structure determination) analysis of its structure has been carried out, and that the famous Mackay-icosahedra do not play the important role as the basic structural building elements as one supposed before.

X-ray and electron diffraction investigations on the stable decagonal phase in Co–Ni–Al alloys

The stable decagonal phase in alloys with compositions of Co15Ni15Al70, Co11.5Ni16Al72.5, Co16.5Ni11Al72.5 and Co20Ni7.5Al72.5 was investigated by X-ray and electron diffraction. All important projections of the reciprocal lattice of the decagonal phase were obtained by electron diffraction. The tilt angles between the individual zone axes were measured and compared with calculated angles. From the diffraction patterns the reciprocal lattice could be reconstructed. A simple method for the indexing of the reciprocal lattice is introduced. The indexing scheme is based on five linearly independent vectors. The indices for the reflections in the electron diffraction as well as the X-ray powder diffraction patterns are tabulated. The indices of the zone axes studied by electron diffraction could also be determined. In the X-ray diffraction studies a substantial variation of the number and of the position of the diffraction lines with alloy composition was found. The results are compared with results from the stable decagonal phase in the system Co–Cu–Al.

Twenty years of structure research on quasicrystals. Part I. Pentagonal, octagonal, decagonal and dodecagonal quasicrystals

Is quasicrystal structure analysis a never-ending story? Why is still not a single quasicrystal structure known with the same precision and reliability as structures of regular periodic crystals? What is the state-of-the-art of structure analysis of axial quasicrystals? The present comprehensive review summarizes the results of almost twenty years of structure analysis of axial quasicrystals and tries to answer these questions as far as possible. More than 2000 references have been screened for the most reliable structural models of pentagonal, octagonal, decagonal and dodecagonal quasicrystals. These models, mainly based on diffraction data and/or on bulk and surface microscopic images are critically discussed together with the limits and potentialities of the respective methods employed.

Electron crystallography of aluminum alloy phases

A series of structure analyses during 1994-2001 by electron crystallographic techniques applied to phases in aluminum alloys are reviewed. Methods for structure solution employ electron diffraction intensity data collected by the precession technique, by selected area micro-diffraction and by the convergent-beam technique. High-resolution electron microscope images (HRTEM) are treated by different kind of processing, including exit wave reconstruction. Crystallographic calculations are performed either by direct method or Patterson and Fourier procedures, assuming kinematical scattering, or by refinement from models derived from HRTEM images. Dynamical scattering calculations can be introduced in the refinement stage or as a correction procedure applied to part of the intensity data. The phases studied include primary Al-Fe-(Si) particles, Al-Mn-Si dispersoids, Al-Co quasicrystals and two types of precipitate phases in age-hardening Al-Mg-Si and Al-Zn-Mg alloys.

Quasicrystals and noncrystallographic symmetry

New findings on quasicrystals with icosahedral, octagonal, decagonal, and dodecagonal symmetries obtained recently in the Beijing Laboratory of Electron Microscopy, Chinese Academy of Sciences, are presented. Special emphasis is put on the relation between quasicrystalline and crystalline structures. The important role played by electron diffraction and high-resolution electron microscopy in revealing these quasiperiodic structures is pointed out.