A two step model for the transformation of decagonal quasicrystals (DQC) to their crystalline approximants is presented. First, a partially disordered approximant domain structure results rapidly from small atomic displacements. Subsequently, atomic diffusion leads very slowly to an ordered approximant crystal (AC) structure. The model is discussed on atomic scale. Its basic idea is to describe the structure of a quasicrystal (QC) as incommensurately modulated structure (IMS). In that description, the average structure (AS) of a QC is at the same time the AS of all its irrational and rational approximants. In this approach, the positional quasicrystal- to-approximant-crystal (QC-AC) transformation can be performed by atomic displacements smaller than any interatomic distance. This mechanism, however, leads inherently to to a certain amount of chemical disorder. In most cases also some positional disorder will result. The tools developed for the description of phase transitions of IMS can analogously be used for QC. Examples for one- and two-dimensional quasiperiodic structures are discussed.

Besides the six established decagonal states of the Al-Co-Ni quasicrystal two more modifications have been discovered by means of transmission electron microscopy. One is a pentagonal quasicrystal with a superstructure found in specimens with a very high Co-content and quenched from the highest possible temperature lying within the stability field of decagonal Al-Co-Ni. Its electron diffraction patterns are characterized by a 5-fold rotation axis as a unique symmetry element as well as superstructure reflections similar to those of a related decagonal phase. The other is a one-dimensional quasicrystal closely related to decagonal Al-Co-Ni. The modulation length of 61 Å along the periodic direction in its pseudo 10-fold diffraction patterns can be assumed to be caused by a strong linear, uniform, phason strain in the material.

High-angle annular dark-field (HAADF) images of an A172Ni20Co8 decagonal quasicrystal and monoclinic Al13Fe4 and τ2-inflated Al13C04 approximants have been presented. We found that a 2 nm atom cluster of Al72Ni20Co8, which have been believed to be tenfold so far, has no tenfold symmetry. A new structural model of Al72Ni20Co8 has been constructed on the basis of the HAADF images and the structure of monoclinic Al13Fe4 approximant. We have found that the monoclinic Al13Fe4 and τ-inflated Al13Co4 approximants are composed of the same basic structure of a pentagonal atom cluster with an edge length of 0.47 nm. We have also found that a fivefold atom-cluster with a 2 nm diameter exists in τ-Al13Co4, which was observed in decagonal quasicrystals of Al-Ni-Fe, Al-Ni-Co, Al-Cu-Co and Al-Co by highresolution electron microscopy. A structural model of τ-inflated Al13Co4 is constructed on the basis of the convergent-beam electron diffraction symmetry, HAADF images and the structure of monoclinic Al13Fe4.

The peak broadening and peak positions of Bragg reflections in a single-phase decagonal Al70Ni10Co20 quasicrystal has been studied by the synchrotron X-ray diffraction technique at SPring-8. Full-widths at half-maximum (FWHM) of the Bragg reflections along the longitudinal direction (L) and transverse direction (TI) which is perpendicular to L-direction in an aperiodic plane have linear Q⊥, dependence. This means that the Al70Ni10Co20 decagonal phase has random phason strains. On the other hand, another transverse direction (T2), which is perpendicular to the aperiodic plane, have linear Q, dependence. So the single crystal has some distortion of mosaics to the periodic layer stacking. Notable peak shifts from ideal Bragg peak positions were not observed.

Atomic-resolution Z-contrast images provide conclusive evidence for chemical disorder in an ideal Al-Ni-Co quasicrystal. The so-called basic Ni-rich, high-temperature, quasiperiodic structure exhibits chemical disorder at the center of the 2 nm diameter clusters that involves both the Al and Ni(Co) atoms. The atomic structure of the 2 nm clusters is derived directly from the images and differs from previous models. The presence of closely-spaced, half-occupied atomic columns (or columnar vacancies) in specific rings within the clusters is directly observed. We propose that these columnar vacancies mediate the phason flips necessary for transformation to random tiling.

Both icosahedral and decagonal quasicrystalline particles of a few tens of nanometers in size have been found in Mn-ion implanted GaAs single crystals with a dose of 8×1016 Mn+/cm2 by high resolution electron microscopy (HREM). The identity of these quasicrystals was confirmed by the Fourier transforms made from these HREM images and their composition measured by micro-probe X-ray analysis and electron energy loss spectroscopy was 38–45 at.%Mn, the rest being Ga. The orientation relations among the icosahedral quasicrystal (i), the decagonal quasicrystal (d), and the GaAs matrix found by both HREM images and their Fourier transforms are: i- 5 // d-10 // GaAs[110] or [114]. In addition, nano-particles of a cubic, a hexagonal, and an orthorhombic Ga-Mn crystalline approximant have also been found in the GaAs matrix.

Structural relations between quasicrystalline and related crystalline rational approximant phases have been of interest for some time now. Such relations are now being used to understand interface structures. Interfaces between structural motif - wise related, but dissimilarly periodic phases are expected to show a degree of lattice match in certain directions. Our earlier studies in the Al-Cu-Fe system using the HREM technique has shown this to be true. The structural difference leads to well defined structural ledges in the interface between the icosahedral Al-Cu-Fe phase and the monoclinic Al13Fe4 type phase. In the present paper we report our results on the HREM study of interfaces in Al-Cu-Fe and Al-Pd-Mn systems. The emphasis will be on heterophase interfaces between quasiperiodic and periodic phases, where the two are structurally related. An attempt will be made to correlate the results with calculated lattice projections of the two structures on the grain boundary plane.

Composites of nanoparticles of lead were prepared by embedding into Al-Cu-V and Al- Cu-Fe icosahedral, and Al-Cu-Co decagonal phase alloys by means of rapid solidification. The microstructure of these composites, the orientation relationships and the interfaces formed between the particles and the matrix has been studied by transmission electron microscopy. The melting and solidification behaviour of the lead particles in these matrices has been studied by differential scanning calorimetry. The microstructure and the nature of the particle-matrix interface has been related to the melting/solidification behaviour of the particles in the matrix.

Over the last few years, work in our laboratories in Ames has focussed on elucidating the surface characteristics of Al-based quasicrystalline materials, namely icosahedral (i-) Al-Cu-Fe and i-Al-Pd-Mn. Our work involves the study of the clean surfaces of these materials under ultrahigh vacuum conditions. We find that surfaces cleaned by Ar+ sputtering are depleted in Al relative to the bulk composition. Single grains, after sputtering, undergo a two-stage regrowth process as they are annealed. After heating to about 600 K, a crystalline overlayer is formed. This is rather abruptly replaced at about 750 K by a surface that appears quasicrystalline within the resolution of the experimental techniques used. Calculations based on low-energy electron diffraction (LEED) measurements of this higher temperature state indicate that the Al-rich layers in the bulk model of these materials are the favored surface terminations. Results of low-energy ion scattering (LEIS) corroborate this finding. Consistent with this, we find that the oxidation behavior and general reactivity of these materials are analogous to pure Al.

The Katz-Gratias-de Boissieu-Elser (KGdeBE) model for the icosahedral quasicrystals i-AlFeCu and i-AlPdMn is studied and applied to the surface structure of i-AlPdMn.

The five-fold surface of the Al70 Pd21 Mn9 quasicrystal has been studied using STM, LEED and AES. STM images from surfaces which have been sputtered and annealed to 875 K reveal 20-30 Å protrusions that have been identified by others as Mackay-type clusters. Higher-resolution images reveal substructures in these clusters having dimensions 2-3 Å. Longer annealing times at 875 K produced large areas having flat terraces which were imaged with atomic resolution. The LEED pattern from this surface has sharp spots on a low background, and AES indicates that the surface is deficient in Mn relative to the bulk. For surfaces annealed to 1050 K for less than 2 hours, STM images indicate that cluster and terrace phases coexist, and a third phase having aligned arrays of clusters is identified which appears to be intermediate between the cluster and terrace phases.

The surface of the AlPdMn quasicrystal perpendicular to a fivefold axis has been probed by LEED, XPS, Auger spectroscopy and surface EXAFS.

An Al-Pd-Mn cubic alloy having a bulk chemical composition somewhat analogous to that of the icosahedral Al-Pd-Mn quasicrystal is studied. Our goal is to compare the surface structure and properties of the cubic alloy with those of the quasicrystalline alloy. In this paper, we report the first observations for the (110) surface of the cubic alloy using (primarily) low energy electron diffraction (LEED). The surface is prepared by sputtering and annealing in ultrahigh vacuum (UHV). In addition to the substrate LEED pattern, at least three superstructures evolve sequentially with annealing temperature.

Sputtering with Ar+ ions induces structural phase transitions at the pentagonal surface of the icosahedral quasicrystal Al70Pd20Mn10. Sputtering at different temperatures changes the surface composition, thereby stabilizing different structures. At room temperature, the structure changes to body-centered cubic but, at elevated temperatures, it displays decagonal symmetry. In both cases, annealing the sample restores both the bulk composition and the icosahedral symmetry of the original surface.

We have investigated formation of oxides on quasicrystalline and crystalline alloy surfaces of similar composition, in different oxidizing environments. This includes a comparison between a quaternary orthorhombic approximate of Al-Cu-Fe-Cr quasicrystal and the ternary Al-Cu-Fe quasicrystalline and crystalline phases. We noted that each sample showed the following common trends: preferential oxidation of the Al, enrichment in the concentration of Al present at the surface upon oxidation, water concentration is directly related to oxide thickness, and the oxide thickness displays a strong correlation with the bulk concentration of Al in the sample.

The oxidation behavior of i-A163Cu25Fe12 at 800°C in air was investigated by means of TGA, XRD, SEM and TEM. In the beginning a homogeneous oxide layer is formed by the subsequent growth of metastable γ-Al2O3 and Θ-Al2O3. Nucleation of the thermodynamical stable α-Al2O3 occurs at the interface oxide/quasicrystal. The following growth of α-Al2O3 through the oxide layer leads to the formation of oxide nodules. The high growth rate of the α-Al2O3 can be explained by the incorporation of copper ions. The oxidation resistance of the quasicrystal is insufficient at high temperatures, because no protective oxide layer is formed. The high temperature oxidation behavior of Al-Cu-Fe quasicrystal and the aluminides β-FeAl and β-NiAl is compared regarding the oxidation rate, the oxide phases and the concentration changes in the material due to selective oxidation of aluminum.

We report the electrochemical potentials at which localized pitting and repassivation occur on icosahedral Al-Cu-Fe, and on a series of related alloys and elemental metals. The electrochemistry occurs in a buffered NaCI solution, pH 8.4. Under these conditions, pitting and repassivation appear to be controlled mainly by the chemical composition of the alloy, although the quasicrystalline phase displays an anomalous resistance to repassivation. Corrosion of this phase proceeds by dissolution of Al and Fe, leaving behind pits which are Cu-enriched.

Elastic moduli, anharmonicity, hardness and high temperature plastic behavior of icosahedral and decagonal quasicrystals are reviewed. Elastic properties are discussed in relation to the bonding nature of the quasicrystals. The plastic deformation behavior at high temperatures is discussed based on the characteristics of the dislocation mechanism in the quasiperiodic lattice. A Peierls potential, a phason relaxation and a dragging stress due to production of a defect field are considered as the controlling factors for the thermally activated dislocation glide; a dislocation in quasiperiodic lattice is accompanied not only by the elastic (phonon) strain field but also by the phason strain field which is difficult to be relaxed completely with the dislocation glide.

Poly-quasicrystalline specimens of the Al-Cu-Fe system have been pre-strained and further deformed at various temperatures, above and below the brittle-to-ductile transition temperature, under constant strain-rate conditions. The experiments have been specially designed to examine the respective contributions of the thermally reversible and structural dependent parts to the total flow stress. We show that the so-called internal stress, which results from the deformation builtin microstructure, has a rather low value, if any. In addition, the flow stress is found to be fully temperature reversible, if one takes into account previous plastic strains. We suggest that these results can be explained by a weakening of the friction stress, due to the formation of soft slip bands.

TEM observations of dislocations and faults in an as-grown Al-Pd-Mn single crystal show that dislocation movements have occurred by pure climb in mirror planes. A comparison with creep properties of h.c.p. metals along their c-axis indicates that pure climb is a realistic deformation mode when glide is inhibited at high temperature.