The phase equilibria of the Al-Cu-Fe quasicrystalline phase (y phase) is complex and conventional crystal growth techniques like the Bridgman and Czochralski methods are not applicable in preparation of large crystals. Large single grains of the y phase been have prepared by either slow cooling or isothermal anneals. In the later technique, arc melted ingots were subjected to either single or multiple heat treatments between 825 and 840°C to encourage grain growth. Following heat treatment, grains of the icosahedral phase are found either as isolated pentagonal-faceted crystals within the ingot or within clusters of intergrown grains. The growth of the large grains is independent of the sample processing history of the sample, is facilitated by the presence of liquid at the growth temperatures and is constrained by the physical dimension of the ingot. The microstructure of both grain types is similar containing a minor quantity (on the order of 5–10 %) of a second phase and a high degree of porosity in the as-grown state. The second phase is usually present as a thin layer between adjacent grains or associated with a pore within a single grain. The grain porosity is distributed throughout the ingot. These defects can be removed through post-growth hot isostatic pressing and anneal treatments.

We discuss the preparation of thin icosahedral films (Al-Cu-Fe and Al-Pd-Re) via the route of the amorphous (a-)phase which in some aspects is a precursor to the icosahedral phase. A direct transition from the a- to the i-phase occurs for Al-Cu-Fe films at 430°C on the time scale of minutes. The resulting films are of good quality as shown by diffraction and electronic transport properties. The surface of the resulting films is very smooth.

We report on the formation by reactive diffusion of a pure decagonal quasicrystalline film in Al/Co bilayer and multilayer structures of overall composition Al13CO4. We show that this method is highly selective since several stable phases which exist around this composition are never observed. The quasicrystal is not the first reaction product but is obtained as a second reaction step by a peritectoid reaction between Al9Co2 and the remaining cobalt. DSC analysis allows the determination of both the enthalpy of formation of the decagonal quasicrystal (ΔHf = -31500 ± 2000 J/g-atom) and its activation energy of growth (2.6 ± 0.5 eV).

An extensive investigation into the At-AICo-AlNi ternary subsystem is presented. Observations have used the techniques of differential thermal analysis, magnetothermal analysis, dilatometry, metallography, X-ray diffraction, transmission electron microscopy, and high-resolution electron microscopy. Representative graphic documentation, as liquidus projection surface, isothermal sections, temperature-concentration section, and reaction scheme are presented. 11 phases from the binaries Al-Co and Al-Ni and the three ternary phases Y2 (Co2NiAl9), X and the decagonal phase D were found at room temperature. The decagonal phase is formed from the melt peritectically via a critical tie line and its primary formation area dominates at the liquidus projection surface. 45 three-phase regions are present according to the reaction scheme.

Several phase variants in the area of the decagonal phase were detected by transmission electron microscopy. Phase fields of the variants were determined from samples quenched from their respective temperatures. In-situ experiments on transformations of variants were performed by dilatometric measurements. The subdivision of the D phase area into the fields of the variants is discussed.

We have grown a decagonal single quasicrystal with a size of centimeter order in an A172Co16Ni12 alloy by floating zone method at a growth rate of 0.5 mm/h. The single decagonal quasicrystal has been inspected by Laue X-ray as well as neutron diffraction, revealing a very high quasicrystalline quality. By quenching the liquid during growth, a flat solid-liquid interface has been directly observed. There exists solute redistribution at the growing solid-liquid interface. At steady state growth Al is enriched and Co, Ni are depleted in front of solid-liquid interface. Solute partition ratios of Al, Co and Ni were determined to be about 0.91, 1.57 and 1.08, respectively.

The nucleation behavior of the 1/1 quasicrystal approximant phase was investigated in Ti-TMSi- O (TM=Mn, Cr, Fe) alloys made near the stoichiometric composition. Containerless solidification studies were performed using electromagnetic (rf) levitation and drop-tube techniques. The solidification microstructures indicate that α-Ti nucleated first in Ti-Cr-Si-O and β-Ti in Ti-Mn-Si-O alloys; the 1/1 phase nucleated and grew on these titanium oxide dendrites. In Ti-Fe-Si-O alloys, the 1/1 phase nucleated and grew directly from the liquid, allowing information on the nucleation rates to be obtained from undercooling data.

The high number of tetrahedrally coordinated sites for interstitial hydrogen and the favorable hydrogen-metal chemistry make quasicrystalline Zr-Cu-Ni-Al alloys a candidate for hydrogen storage applications. The icosahedral phase in Zr69.5Cu12Ni11Al7.5 has been shown to absorb electrochemically hydrogen up to H/M = 1.6. Only partially desorption of hydrogen was observed by TDA at about 500°C. Since hydrogen desorption seems to be hindered by a surface barrier, the hydrogen remains in the quasicrystal thus influencing their thermal stability. Such effects of hydrogen were studied by DSC and microstructural investigations. With increasing hydrogen content the decomposition of icosahedral Zr69.5. Cu12Ni11A17.5 is shifted to lower temperatures. Quasicrystals decompose by a discontinous transformation into the tetragonal Zr2Cu phase, the tetragonal Zr2Ni and the hexagonal Zr6NiAl2. Hydrogenation does not influence the phases formed during decomposition, but leds to the formation of a finer microstructure. We assume that the defects formed during the hydrogenation accelerate the nucleation of the stable crystalline phase.

Single grains of real quasicrystals have been investigated using X-ray imaging techniques at a third-generation synchrotron radiation source (ESRF). Facetted microholes have been observed. The experimental results are discussed with reference to a description of the quasicrystal structure in terms of overlapping atomic clusters (self-similar packing or random covering modifications). Relations between holes and quasicrystal formation are also discussed.

In this paper radiotracer diffusion studies from our laboratory on monocrystals of icosahedral quasicrystalline Al-Pd-Mn (i-AlPdMn) are summarized. Diffusion of 65Zn, 54Mn, and 59Fe has been investigated as function of temperature and in the cases of 5 4Mn and 65Zn also as function of hydrostatic pressure. Diffusion of 65Zn was studied to ‘simulate’ Al diffusion. Over a very wide temperature range no deviation from Arrhenius-behaviour is found. Some preliminary results of 114m In diffusion are also reported. The diffusivities in i-AlPdMn reveal parallels to foreign atom diffusion in Al suggesting a vacancy-type mechanism of diffusion for i-AlPdMn. Signs and magnitudes of the activation volumes of +0.67 Ω (Ω = mean atomic volume) for 54Mn and of +0.74 Ω for 65Zn diffusion in Al-Pd-Mn favour a vacancy mechanism as well.

Our results will be discussed together with diffusion studies on i-AlPdMn performed by other groups. The results of Nakajima and coworkers on Mn, Ge, Co and Cr diffusion and the results of Frank and coworkers on Pd and Au diffusion in the high-temperature regime are consistent with the above mentioned conclusion. For the low temperature regime of Pd and Au diffusion the suggested possibility of a phason-related mechanism will be discussed.

Surface segregation of aluminium was observed during oxidation experiments of icosahedral A162Cu25.5 Fel12.5, performed in-situ and at different temperatures in the ultra-high vacuum chamber of a scanning Auger electron spectrometer. Two regimes, below and above 770K, were observed in relation with severe segregation of Al atoms at the surface for T > 770K. We postulate that this temperature dependent segregation rate is representative of the aluminium transport towards the surface of the quasicrystal. By analogy with classical diffusion experiments, we can thus determine reasonable estimates of the activation energy for Al self-diffusion in this quasicrystal. The results are consistent with the existence of phason flips below 770K and thermal vacancies above this temperature.

The dodecagonal (dd) quasicrystalline tantalum telluride dd Ta1.6Te and the crystalline approximant Ta97Te60 have been modified by partly replacing tantalum by vanadium. The impact of the substitution on the structures has been studied by X-ray and electron diffraction and by high-resolution transmission electron microscopy. The layered-type approximant structure of Ta83V14Te60 was determined by single crystal X-ray means. The partitioning of vanadium on 21 out of 29 crystallographically inequivalent metal sites is referred to, but not controlled by the Dirichlet domain volume available at the sites. A HRTEM projection of dd (Ta, V)1.6Te onto the dodecagonal plane is analysed with respect to the arrangement of (Ta, V)151Te74 clusters on the vertices of an irregular aperiodic square-triangle tiling, the edge length of which corresponds to the distance between the centres of two such clusters. The clusters comprise about 1 nm thick corrugated lamellae which are periodically stacked by weak Te-Te interactions.

The controled growth of large single grain of quasicrystalline phase in metallic alloys is a difficult task that requires a detailled knowledge of the corresponding phase diagram. We discuss the metallurgical specificities of the ternary systems AlCuFe and AlPdMn with respect to single icosahedral grain elaboration. We show that adding a few percents of Silicon in the AlCuFe system leads to stabilize two low order cubic approximants one, α, based on Bergman clusters with a parent F-type icosahedral phase and the other, α', based on Mackay clusters with a parent P-type icosahedral phase. A section of this quaternary phase diagram at constant Si content shows that these cubic phases have no common stability region but are located on opposite sides of an orthorhombic phase that is expected to be a complex synthetic structure mixing both Mackay and Bergman type atomic clusters.

Two types of atom clusters found in the β-(A1PdMnSi) cubic phase, referred to as a 2/1 crystalline approximant, with a composition of approximately Al70Pd23Mn6Si1 which is near to the composition Al72Pd20Mn8 of the icosahedral phase, are discussed in detail for understanding the structure of the Al-Pd-Mn icosahedral phase. A large dodecahedral atom cluster located at the body-centered position can be divided into 19 atom shells with approximately icosahedral symmetry, and a dodecahedron of the 12th shell internally touches the surface of the cubic unit cell with a lattice constant of 2.0211 nm. At each vertex of the dodecahedron, a small icosahedral atom cluster consisting of 12 Al atoms surrounding a central Pd atom is located. The dodecahedron is connected to each other by edge-sharing, namely by sharing two small icosahedral atom clusters, along the twofold rotational direction, and forms a simple-cubic packing of the atom cluster in the β-(AlPdMnSi) cubic phase. Another atom cluster located at the origin fills up gaps of the simple-cubic packing of the large dodecahedral atom cluster. By using the dodecahedral and bridging atom clusters, the structure of the Al-Pd-Mn icosahedral quasicrystal is discussed.

By combining the powdered x-ray Rietveld analysis and the High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM), we have made the detailed structural analysis for the Mackay-type Al55Si7Cu25.5Fe12.5, 1/1-cubic approximant. Electron diffraction measurements revealed its space group to be Pni3. Both position and occupancy in every atomic site are refined by means of the Rietveld analysis. The factors Rwp and Riwhich are taken as measures for the precision of the Rietveld analysis, are reduced to 2.57 and 3.89 %, respectively. The refined structure of this approximant is a simple cubic with the lattice constant of 12.31916(2) Å. Two different icosahedral clusters consisting of triply stacked atomic shells exist at the vertex and body centered positions in the CsCl-type unit cell. In addition, we propose that there exists most likely a “structural disorder” in the first shell of the cluster in one of the sublattices. The HAADF images taken along [100] and [111] directions agree with the projection images calculated from the Rietveld-refined structure, lending a strong support to the validity of the present atomic structure model deduced from the Rietveld analysis.

High-resolution transmission electron microscopy has been applied to study the real atomic structure of the decagonal (d-) quasicrystal in the Zn-Mg-rare-earth (RE) system, which is the first d-phase based on Frank-Kasper phase. We show that the phase has a novel structure in which the atomic arrangement in the tenfold symmetry plane can simply be interpreted as the Penrose tiling decorated by individual atoms - the simplest realization of the Penrose tiling as a real atomic structure. This is supported by the fact that a similar local atomic configuration exists in the Zn7Mg4 crystal structure. This simple structural model is in sharp contras to the idea of atomic clusters, which has been successfully used to describe the structure of quasicrystals in Altransition metal alloys. The present results strongly suggest that the symmetric atomic clusters are not an essential factor for formation of quasicrystals. Instead, a new idea of quasi-unit-cell and its covering is applied for structural description. The atomic structure of the Zn-Mg-RE icosahedral phase is also implied to follow the present concept, based on the fact that its related crystalline phases with hexagonal lattices are not built of giant atomic clusters with icosahedral symmetry.

A relationship between hexagonal phases in the Zn-Mg-RE (RE=rare earth) system and icosahedral quasicrystalline phase is discussed. There are three hexagonal phases in the vicinity of Zn6Mg3RE1 which are related with a lattice parameters by a ratio 3:5:7 but with approximately the same c parameters. A structural model for the phase with medium a lattice parameter is proposed based on the structures of the other two phases recently determined. We also show a preliminary result of X-ray diffraction study of the Zn-Mg-Ho icosahedral phase.

Crystal approximant phases are important because they are believed to have a similar local atomic structure to corresponding quasicrystals. Interstitial hydrogen is used as a probe of the local structures of the Ti-based quasicrystals and crystal approximants. Phase pure samples of ct(TiCrSiO), a Mackay type 1/1 crystal approximant, which were plasma etched and coated with a thin layer of Pd, were loaded with deuterium from the gas phase to a maximum deuterium atom to metal atom ratio (D/M) of 0.26. After deuteration, the sample becomes a fine powder and remains single-phase. A Rietveld structural refinement of powder x-ray and neutron diffraction data was made to determine the location of the interstitial sites. For the fully deuterated sample the deuterium atoms sit at both octahedral and tetrahedral sites. The octahedral sites are formed by six Ti atoms in the first and second shells of the icosahedra. Deuterium atoms are also located at the tetrahedral interstitial sites formed between the clusters along <100> directions. In the partially loaded samples with DiM = 0.11, the deuterium atoms occupy only octahedral sites, demonstrating a preference for these sites.

We present results from the first extended X-ray absorption fine-structure studies in a Ti/Zrbased quasicrystal. The icosahedral Ti53Zr27Ni20 phase and an icosahedral Ti45Zr38Ni17 phase that also contains 1.7 hydrogen atoms for each metal atom were investigated. Experiments performed above the Ti, Ni and Zr K absorption edges have allowed to identify different types of atomic subshells around the Ti, Ni and Zr atoms. Significant differences are observed between these two quasicrystalline alloys, suggesting preferential sites for the hydrogen atoms.

Phason disorder is expected to modify the neutron and x-ray scattering spectra of quasicrystals. However, it is not clear yet if phasons are coherent modes in real space, or if they should be considered as local random defects. In this work, a comparative study of different sorts of phasons is performed, and their effects on the dynamical structure factor of Fibonacci chains are analyzed in detail. The results show that coherent and random phasons can be distinguished for high values of the momentum transfer. Finally, it is observed that a random phason produced in the quasicrystal's hyper-space leads to a coherent phason field in real quasicrystalline lattice.

A single-crystal X-ray analysis revealed that a decagonal Al70Mn17Pd13 has a noncentrosymmetric structure with the 5-dimensional (5D) space group P105mc. The structure refinement based on a 5D cluster model gave Rw= 0.119 (R = 0.167) for 1428 independent reflections and a reasonable chemical composition of Al69.1Mn18.5Pd12 4.