Formation of icosahedral quasicrystalline phase (i-phase) was investigated for Mg-Al-X (X=Co, Ni, Cu, Rh, Ag, Pt and Au) ternary systems. A Mackay-icosahedron (MI) type i-phase was found to form in the Mg-Al-Rh system at the composition Mg8Al68Rh24. The i-phase has the F-type ordered structure and the quasilattice constant is 0.447nm. Differential thermal analysis (DTA) studies showed that the i-phase has a high stability. On the other hand, the formation of a Frank-Kasper (FK) type i-phase was found in the Mg-Al-Pt system at the composition Mg45Al42Pt13. The conditions necessary for the formation of the i-phases are discussed in terms of the atomic radius ratio and the electronegativity as well as the electron-per-atom ratio.

The origin of the pseudogap across the Fermi level was investigated by analyzing the electronic structure calculated in the framework of the LMTO-ASA method for the RT-type Al-Mg-Zn and MI-type Al-Cu-Ru-Si 1/1-approximants. The pseudogap in the former is proved to originate from the interaction of electronic states with the Brillouin zone planes associated with reciprocal lattice vectors matching with the Fermi sphere in the extended zone scheme. In the latter, the Fermi surface-Brillouin zone interaction coupled with the hybridization effect between the Al-3p and transition metal d-states produces a deep pseudogap at the Fermi level. The real-space charge distribution for electrons at the Fermi level is calculated for the Al-Cu-Ru-Si 1/1-approximant. The charge distribution thus obtained could explain not only the possession of a large resistivity of this approximant but also evidenced that the icosahedral clusters play a key role in reducing the electronic energy in favor of quasicrystalline and approximant phases.

We report our first results on the electronic structure of icosahedral Zn55Mg35Y10 as compared to hexagonal Zn65Mg28Y7 and to the conventional Zn2Mg alloy. The occupied Mg 3s,d, and Mg 3p on the one hand and empty Mg p, Zn d-s and Y d-s states on the other hand have been investigated using soft X-ray emission and photoabsorption spectroscopy, respectively. The various sub-bands presented for Mg are adjusted in the binding energy scale whereas for the other elements they are plotted in the X-ray transition energy scale. We observe that the metallic character of the samples progressively decreases when going from the pure metals to the icosahedral compound.

The quasicrystalline phase forming ability, solidification structure, thermal stability, and phase relationships of a series of conventionally-cast Al62-xCu25.5Fe12.5Bex (x=0,1,3,5,7) alloys were investigated. Single icosahedral quasicrystalline phase was successfully obtained by partial replacement of Al with Be (7 at.%) in an Al62Cu25.5Fe12.5 alloy during crucible cooling. The I-phase in an Al55Cu25.5Fe12.5Be7 alloy has the same face-centered icosahedral structure as that in an Al62Cu25.5Fe12.5 alloy, with less phason strain. Hardness of the I-phase increases with x, i.e. from Hv 710 for x=1 to Hv 809 for x=7.

We report room temperature thermopower values and the temperature dependence for several AlPdMn based quasicrystals. In an effort to further understand the complexities of electrical transport in quasicrystalline systems, thermopower data for icosahedral Al71Pd21Mn8-XReX will be presented and discussed. A relation of room temperature thermopower to the curvature of the thermopower is demonstrated. We propose an empirical fit to the thermopower data, utilizing three free variables. The physical significance of the fit parameters is discussed. These results are discussed in brief concerning the relation to the application of quasicrystals for use as thermoelectric materials.

Steam reforming of methanol (CH3OH + H2O -> 3H2 + CO2) has been performed on a number of Al-Cu-based stable quasicrystals and related alloys. Alloys leached in NaOH aqueous solution exhibit the highest activity for steam-reforming of methanol. Among the alloys, AlCuFe quasicrystal, after leaching treatment, reveals excellent activity. The production rate of H2 reaches 235 l/kgmin at 573K for steam reforming of methanol. The activity is due to Cu nanoparticles at the surfaces of quasicrystalline grains which are generated by leaching treatment. The quasicrystals have two advantages: one is their brittle nature, which allows them to be crushed efficiently; the other is the involvement of Fe, which suppresses the sintering of Cu particles.

The glide resistance of edge dislocations gliding along a two-dimensional quasiperiodic lattice (Burkov II model of the decagonal quasicrystal) has been calculated. The glide resistance consists of τphason and τPeierls components and the τPeierls component depends strongly on the orientation of the dislocation. For the orientation of large τPeierls component, the τphason component is about half of the τPeierls component for individual dislocation glide but becomes negligibly small for glide of a pair of dislocations. The largest τPeierls component is about 0.1G (G: the shear modulus).

The high number of potential interstitial sites suitable for hydrogen and the favorable hydrogen-metal chemistry make quasicrystalline alloys candidates for hydrogen storage applications. Hydrogen charging was performed electrochemically in a 2:1 glycerin-phosphoric acid electrolyte. The microstructure of quasicrystals in Zr69.5Cu12Ni11Al7.5 was investigated by x-ray diffraction and TEM, the kinetics of transformations by DSC and TDA.

Hydrogen was observed to exhibit a significant effect not only on the formation of Zr-based quasicrystalsfrom the glassy precursor material, but also on their stability. Ribbons with an icosahedral microstructure were found to absorb hydrogen up to H/M = 2.0. With increasing hydrogen content an expansion of the quasilattice constant was observed, followed by the formation of approximants and finally amorphization.

Only partial desorption of hydrogen can occur prior to the decomposition of the quasicrystalline phase. Desorption at lower temperatures is hindered by the formation of a thin ZrO2 barrier. Pd plating, often used to overcome such a barrier, improves the absorption kinetics as well as the desorption behavior significantly. DSC of uncoated Zr-based quasicrystals reveals that at low hydrogen contents the decomposition ofthe icosahedral quasicrystals is shifted to lower temperatures; at high hydrogen contents the formation of ZrH2-x was revealed. In the Pd-coated ribbons fully desorption is observed even below 300°C, butonly after hydrogenation up to H/M = 0.5; higher hydrogen contents lead to irreversible changes.

The micromechanism of transformations during hydrogenation as well as the decomposition during annealing of hydrogenated uncoated as well as Pd coated Zr-based quasicrystals are discussed in some detail.

Methods for the ab initio structure analysis of periodic approximant phases from single- crystal X-ray diffraction data are presented. These methods are particularly suited to complex approximant structures with large unit cells and strong pseudosymmetry (where routine X-ray structure solution tools fail) and are based on the “brute-force” deconvolution of the experimentally measured autocorrelation function. This function is obtained directly by a simple Fourier transform of the measured X-ray diffraction intensities. Sub-optimal diffraction data from twinned, nanodomain and polycrystalline specimens can be processed despite the inevitable lack of information due to reflection overlap and limited resolution. The deconvolution process allows complex approximant structures to be unraveled without prior knowledge about the structure-building motifs. Examples are presented for the systems Al-Co-Ni and Al-Co-(Ta).

Solidification microstructures of Mg rich Mg-Zn-Y alloys were studied by using optical microscopy, scanning electron microscopy and transmission electron microscopy. Pseudo eutectic reaction (Liquid→α-Mg + icosahedral phase) takes place during solidification. Alloys containing high Mg content solidifies by primary crystallization ofα-Mg followed by the eutectic reaction at interdendritic region. Mg68Zn28Y4 alloy solidifies by primary crystallization of I-phase followed by theeutectic reaction into a mixture of α-Mg and I-phase. Occasionally D-phase and Mg4Zn7 phases were observed to form with orientation relationships with previously formed I-phase and D-phase, respectively. Thestrength of the alloys increased with increasing the volume fraction of I-phase.

Five fold i-AlPdMn surface prepared under UHV by ion bombardment and annealing was so far considered to be bulk terminated. This result was substantially based on a quantitative LEED analyses [1]. Analysis of the specular rod in a X ray diffraction experiment at grazing incidence supported this result [2]. We present a new study of this surface by high resolution X ray diffraction at normal incidence. In this Bragg configuration the diffraction peak 18 – 29 for instance is at a photon energy of 2.873keV, the 72 – 116 reflection at 5.725keV. This results in an analyzed thickness of the sample surface of a few micrometers.

The surface was cleaned by ion bombardment. During annealing (T≅880K), we clearly observed the progressive disappearance of the initial Bragg peak characteristic of the as cast bulk sample. Conversely a new Bragg peak grows at an energy position shifted by 1eV compared to the position of the original Bragg peak. This is a clear signature for an irreversible structural transformation which takes place on at least the micron thickness. On the transformed surface, both, a LEED pattern and a RHEED pattern, characteristic for a five fold surface were easily obtained.

This high resolution experiment (the relative Bragg peak shift is 3ׁ10−4) was reproduced on samples from different initial compositions. This shows that five fold i-AlPdMn surface changes after preparation by ion bombardment and annealing at 900K on a micrometer thickness. This is not consistent with the conclusion that the surface is simply terminated by a cut of the original bulk. We conclude that a reorganization process of the quasicrystalline structure during annealing proceeds in the surface vicinity (probed depth is close to a few microns).

Binary and ternary aluminum based amorphous precursor films have been prepared by the sequential-flash evaporation technique in the case of Al-Mn, Al-Fe, Al-Cu-Fe and Al-Pd-Mn and by co-sputtering in the case of Al-Pd-Re. For well adjusted compositions and an appropriate heat treatment films can be transformed into polycrystalline single phased quasicrystals for all of the systems mentioned above. The transition can be monitored by in situ measurements of electronic transport properties. The structure factors as a function of composition and annealing state has been measured by electron diffraction. They show a unique behavior if the composition is chosen to that of the related quasicrystal. For high annealing states of the amorphous phase the structure factor becomes more intense at the peak positions of the following i-phase. This is interpreted as a unique radial near- and medium-range order of both the i-phase and its amorphous precursor. The final transformation into the quasicrystal is sharp for Al-Cu-Fe and is broad in the case of Al-Pd-Mn and Al-Pd-Re. The transport properties of the resulting single phased films are similar to that of the related bulk samples and allow systematic studies as a function of composition. Some of the i-Al-Pd-Re films are on the insulating side of the metal-insulator transition.

We attempted to calculate the breakaway stress σb of dislocation from attractive junction made by reaction of dislocations. Assuming that the force f acting on the unit length of dislocation with the Burgers vector B under a shear stress τa is f τ∣b˝∣ where b˝ is the phonon component of B, and that the elastic energy per unit length of dislocation W is approximated by W = G(∣b˝∣2 + c2 ∣b˔∣2) where G is the shear modulus, b˔ the phason component of B and c2 a coefficient of about 3.1 × 10−3. Using the values G = 48.4 GPa at 1070 K, the Taylor factor M = 3 and the measured dislocation density of 1.8 × 1013 m−2, we calculated σb for 21 possible dislocation reactions. Picking up the most possible dislocation reactions, σb distributed between 50 and 80 MPa, and the average of them was 64 MPa. This result strongly suggested the possibility that the main part of the internal stress of the high-temperature deformation of icosahedral Al-Pd-Mn is explained by σb.

High pressure X-ray diffraction studies of quasicrystals are presented and discussed for icosahedral Al-Cu-Ru and Ti-Zr-Ni alloys. The results in hydrogenated Ti-Zr-Ni show that Ti- Zr-Ni may be more compressible after hydrogenation.

Ion implantation experiments were carried out on a single grain Al-Pd-Mn icosahedral quasicrystal. The influence of Re or Au dopant on the thermopower and the electrical resistivity has been measured. Comparison with ternary and quaternary alloys produced by usual metallurgical techniques is made. The results indicate that the efficiency of this quasicrystalline alloy as a thermoelectric material can be improved by adjusting composition and structural quality, as well as by the addition of a small amount of Re, whereas Au implantation is inefficient in that respect.

For more than a decade, quasicrystals were considered to exhibit only fundamental interest in so far that they have raised basic questions regarding the organization and stability of condensed matter. As well, they addressed the mechanisms of wave propagation in aperiodic solids. In more recent years, they turned to become useful materials in various mechanical devices, in solar absorbers or in heat insulation coatings. Nowadays, the science of quasicrystals, and especially the relevant high-dim crystallography techniques, have issued quite useful concepts and methods that prove useful in neighboring domains of condensed matter physics, of mechanics, of low temperature physics, etc. Hence, this article puts more emphasis on a few of the new subjects that quasicrystals science has helped us to study rather than on the few possible breakthroughs of quasicrystalline materials identified yet on the market.

The electronic transport in Fibonacci lattices at zero temperature is studied by means of the Kubo-Greenwood formula within the tight-binding scheme, where a renormalization process capable to address the electrical conductivity in macroscopic quasiperiodic systems is used. The effects of the Fermi-energy location on the ac conductivity are analyzed in detail for a wide range of the system sizes. Special attention is paid to the transparent states, whose transmission coefficient is unity. The results show a rapid decay of their ac conductivity as the frequency increases in comparison with that of periodic systems, and the spectra scale with the inverse of the system size as occur in periodic ones, where analytical results are obtained. Furthermore, a new low-frequency minimum appears when the inhomogeneity of the Fibonacci lattice grows.

Highly faceted microholes (voids) in icosahedral Al-Mn-Pd quasicrystals form during annealing at temperatures between 750 and 830°C via classical Ostwald ripening. The specimens were single-phase icosahedral with a composition of Al71Mn9Pd20. If such a specimen containing voids is cooled to room temperature with a constant cooling rate of typically 0.6 to 5 K/min, the facets of the voids are frequently found to be decorated by a different material. The decorations have typically a morphology similar to fractals. Specimens rapidly quenched after isothermal heat treatments around 830°C never showed similar decorations. By X-ray microanalysis in a scanning electron microscope it has been found that this decoration material is considerably poorer in Al-content that the icosahedral phase, namely: Al60Mn10Pd30. Accordingly, these decorations are precipitations at the surfaces of voids inside a single-phase icosahedral matrix. This implies that these decorations are formed at the void surfaces bya diffusion induced local equilibrium. The local equilibrium can be understood on the basis of the equilbrium results in a single icosahedral phase.

Composite coatings containing quasicrystalline (QC) phases in Al-Cu-Fe alloys were prepared by laser cladding using a mixture of the elemental powders. Two substrates, namely pure aluminum and an Al-Si alloy were used. The clad layers were remelted at different scanning velocities to alter the growth conditions of different phases. The process parameters were optimized to produce quasicrystalline phases. The evolution of the microstructure in the coating layer was characterized by detailed microstructural investigation. The results indicate presence of quasicrystals in the aluminum substrate. However, only approximant phase could be observed in the substrate of Al-Si alloys. It is shown that there is a significant transport of Si atoms from the substrate to the clad layer during the cladding and remelting process. The hardness profiles of coatings on aluminum substrate indicate a very high hardness. The coating on Al-Si alloy, on the other hand, is ductile and soft. The fracture toughness of the hard coating on aluminum was obtained by nano-indentation technique. The K1C value was found to be 1.33 MPa m1/2 which is typical of brittle materials.

Icosahedral quasicrystals Al71.5Pd20.3Mn8.2, Al70.7Pd21.34Re7.96, Al62.5Cu25.5Fe12.5, and α-Al68.31Mn21.21Si10.48 1/1- approximant were investigated by using a monoenergetic slow positron beam. The structural vacancy densities in the first three samples were determined to be 5.0×1020, 7.7×1020, and 4.7×1020 cm−3, respectively, by analyzing the measured S-parameter.