Results of plastic deformation experiments on two quasicrystalline phases of different structure, icosahedral Al-Pd-Mn and decagonal Al-Ni-Co, and of an approximant phase, ξ'-Al-Pd-Mn, are described. They are interpreted on the basis of a model for the deformation process, the clusterfriction model, which is discussed in a number of modifications.

In situ straining experiments on Al-Pd-Mn single quasicrystals in a high-voltage electron microscope between 675 °C and 750°C showed viscously moving dislocations with segments oriented in preferred directions. First measurements from video recordings indicate that dislocations on parallel traces move at two different velocities. For the first time, macroscopic samples were deformed in compression below 680 °C. Above a transition temperature of about 635°C for specimens of a fivefold compression axis and a strain rate of 10−5 s−1, the specimens show a yield point followed by a range of steady state deformation. Below that temperature, they are brittle. However, plastic deformation was achieved down to 555°C by using a lower strain rate and performing stress relaxation tests before the fracture stress was reached. The samples then show very strong work-hardening. The rapid decrease of the activation volume with decreasing temperature, which is characteristic of the high-temperature range, does not continue at low temperatures.The results are interpreted by dislocation generation and recovery and by the cluster friction mechanism controlling the dislocation mobility.

Deformation experiments were performed on single crystals of the ξ-AIPdMn approximant in bending geometry at high temperature. Two different mechanisms of plastic deformation are shown to exist in this phase: one based on dislocations and another novel mechanism based on the motion of phason lines. Burgers vector and line directions of dislocations were determined. Phason lines are shown to build a periodic lattice. The interaction of a dislocation with the phason line lattice results in dislocations on another length scale. This meta-dislocation in the periodic phason line lattice has a Burgers vector of magnitude 165 Å. The relative importance of phason lines and dislocations for the plastic deformation is discussed as a function of the orientation of the sample with respect to the bending geometry.

Fine decagonal phase lamellae-bearing icosahedral Al-Pd-Mn quasicrystals were tested compressionally at temperatures of 997 to 1073 K and initial strain rate of 3 × 10−5. to I × 10−4 S−1, and stress relaxation tests were performed at various stages of deformation. The results confirmed the thermally activated nature of deformation, and the stress exponent of strain rate was around 4. Internal stress for deformation estimated by Kikuchi's method reached 50 to 90 % of applied stress. The effective stress exponent of strain rate was revealed to be about 1.3. At least a part of high internal stress was attributable to complicated dislocation microstructure.

Dislocation motion and crack propagation are studied in model quasicrystals both by geometrical considerations and large scale molecular dynamics simulations. It turns out that two characteristic features of quasicrystals govern plasticity and the rough appearence of cleavage planes: the phason degree of freedom and the cluster structure.

The study of the electronic structure of quasicrystals is still a matter of investigations to date. The paper will report several significant experimental results obtained through spectroscopic techniques which involve the bulk material as well as the surface. Comparison is made with data from related crystalline alloys, densities of states calculations and structural models available for icosahedral quasicrystals.

We have measured in this experiment the XPS (x-ray photoemission spectroscopy) and SXES (soft x-ray emission spectroscopy) valence spectra, the electrical resistivity, the Hall coefficient and the Rietveld structural analysis for a series of the RT-type (Rhombic Triacontahedron) Al60-xMg40Xx (X = Zn, Cu, Ag and Pd; x = 5 - 40) 1/1-approximants. The electrical resistivity at 300 K is successfully analyzed in terms of the carrier concentration per atom e/a and the ratio of the lattice constant over the “ideal” lattice constant defined as LCR. The latter is chosen as a quantitative measure of the degree of the hybridization between Al and transition metal elements Cu, Ag and Pd. We show that a sharp resistivity increase with increasing the concentration of the transition metal element can be attributed to the deepening of the pseudogap at the Fermi level, which is brought about by the combination of the Fermi surface-Brillouin zone interaction and the growth of the hybridization effect.

The highly non-metallic electron transport properties in icosahedral Al70.5Pd21(Re,Mn)8.5 phases and their dependence on alloy composition and sample preparation are reviewed and fuirther studied. The study has led to the highest resistivity ratios ρ(4.2K)/ρ(295K)˜280 reported so far on i-AIPdRe alloys. Similar investigation has also resulted in enhanced resistivity ratios in i-AlCuOs. Evidences for the metal-insulator transition in these i-alloys are documented and discussed in light of the new results. Electron diffraction studies (article by Beeli, Poon, Guo, and Haussler in this Conference Proceedings) have provided evidence for electron localization induced by phason disorder. These findings may provide the basis for understanding the nature of electronic states and effects of disorder on electron localization in quasiperiodic systems.

X-ray measurements on samples of Al-Pd-(Fe,Ru,Os) systems with various annealing temperatures and times have been performed in order to gain insights into formations of icosahedral (I) phases and related approximants and to produce single phases of them. As a result, well ordered I phases have been obtained in Al-Pd-(Ru,Os) systems when annealed at 1000 °C for several hours. Formations of the (1/0-1/0-1/0) and the (2/1-2/1-2/1) approximants have been observed in wide compositional ranges in Al-Pd-(Fe,Ru,Os) and Al-Pd-(Ru,Os) systems, respectively. Electrical resistivity measurements of I phases and related approximants are performed as a function of temperature. The resistivity of I phases at room temperature is as high as that of highly resistive I phases such as Al-Pd-Mn and AI-Cu-(Fe,Ru) systems and the resistivity ratio (P12K/P300K) reaches about 1.5 in Al-Pd-Ru I phases. A negative temperature coefficient of the resistivity is observed in (1/0-1/0-1/0) approximants of Al-Pd-(Fe,Ru) systems, which indicates the electronic transport is quite anomalous even in the lowest approximant of the I phase.

Mechanism of the structure formation of the I-phases, which is required for complete understanding of the origin of their high electrical resistivity and preparation of the high quality samples, is proposed. During annealing within 30 minutes, atomic difiusion of Al and Pd atoms from Al3Pd phase to Al07Re0.3 phase occurs and porosity is formed in the Al3Pd phase. It is supposed that the porosity formation results from Kirkendall effect.

Modulated photocurrent method originally developed for analyzing the electronic states of semiconductor has been applied to an Al-Pd-Re quasicrystal having high electrical resistivity. The measured DC component of the photocurrent has indicated that the mobility of the photocarrier is approximately the same as that of the dark carrier at the Fermi level. The data of the amplitude and phase shift of the AC component can be explained well by a simple model in which only the two processes, carrier generation and recombination, are involved. The recombination time obtained by fitting is by about six orders larger than those reported for semiconductors. The long recombination time as well as the energy independent mobility of carriers are discussed in view of a spiky structure in electron density of states expected for the quasicrystals.

We report an optical investigation of single phase icosahedral Al59B3Cu25.5Fe12.5. The optical conductivity presents a striking depression at low frequencies and a prominent peak at 3 104 cm−1.A phenomenological analysis of the conductivity allows us to correlate the high energy excitations to a mechanism of electron hopping.

Stable quasicrystals exhibit specific and unusual physical properties, such as, diamagnetism, low electrical conductivity, low thermal conductivity, and large themoelectric power at room temperature. These properties can be understood with a Bragg's reflexions scheme due to their dense filled reciprocal space.This leads to small gaps on the Fermi surface (some tenths of eV), much narrower than the usual Hume-Rothery ones (of order of 0.5 eV) which explain their stability. These gaps lead to the existence of quasi Umklapp processes, crucial for the interpretation of thermoelectric power. In some cases, the positive phonon drag contribution due to Umklapp processes, add with the electronic one's and dominates at room temperature with a large positive thermoelectric power. A crude estimate of the figure of merit gives some hope for applications of some quasicrystals and high approximants as new thermoelectric materials.

To investigate anharmonic phonon properties, thermal expansion measurements have been performed for icosahedral (i-) Al-Pd-Mn and decagonal (d-) Al-Cu-Co quasicrystals by X-ray diffractometry in the temperature range between 100 and 800K. The linear thermal expansion coefficient α(T) of i-Al-Pd-Mn is 1.1 × 10−5K−1 at 300K, which is about a half of that of pure Al phase. The measured α(T) of d-Al-Cu-Co coincides almost completely with that of i-Al-Pd- Mn and no appreciable anisotropy was detected between the tenfold periodic direction and a quasiperiodic direction perpendicular to it. The Grfneisen parameter γ(T), has been evaluated from the measured α(T) using the data of lattice specific heat and elastic moduli previously reported. The γ(T) values for the two phases are comparable to those for conventional metallic crystals and almost constant over the temperature range studied.

We have studied the dependence versus nominal composition and temperature of the thermal conductivity in the AlCoFeCr system close to the region where an approximant of the decagonal phase forms. This compound of the Al5Co2 type shows a melting temperature above 1100°C and is of interest because it dispays low thermal conductivity whereas its expansion coefficient is comparable to those of metallic substrates. Used as a coating, it is of potential value for the sake of producing thermal barriers. From a study of the occupied band by X-ray emission spectroscopy, we show that its poor thermal conductivity above room temperature fits with the recurrent localization scheme of electronic states.

Magnetism in the Zn-Mg-Ho icosahedral quasicrystal has been studied by neutron scattering. Powder samples of the icosahedral and related crystalline phases were reexamined to clarify the origin of the previously-reported long-range magnetic order [Charrier et al., Phys. Rev. Lett. 78 (1997) 4637]. The long range order was found to originate from the related crystalline phase, which is a contaminant in the previously-used samples. Whereas for high-quality icosahedral phase, we could detect only magnetic diffuse scattering. This apparently shows the absence of the long range order in the icosahedral phase. The diffuse scattering was studied in detail by using a single quasicrystalline sample. It was found that the diffuse scattering appears as satellites from intense nuclear Bragg reflections. This indicates that corresponding spin correlations can be regarded as developed between spins on the six-dimensional virtual hypercubic lattice. A magnetic modulation vector for the correlations is proposed.

Simulated annealing calculations were performed for Ising spins on Ho sites under Ruderman-Kittel-Kasuya-Yosida-like alternating exchange interactions for an icosahedral Zn-Mg-Ho structure model with intent to investigate the magnetic structures at low temperatures. Magnetic structures were analyzed by means of a Patterson function and diffraction patterns arising from spin-dependent scattering amplitudes. The diffraction patterns from the magnetic structure consist of spots at low temperatures in the case of an antiferromagnetic interaction for the shortest distance (3.37 Å) and a ferromagnetic interaction for the second shortest distance (5.46 Å). The Patterson maps indicate a long-range antiferromagnetic correlation. These facts mean that a long-range ordered magnetic structure is realized at low temperatures. The contrast of the diffraction patterns is different from the patterns arising from the spin-independent scattering amplitudes from the Ho sites. A reversed sign interaction, ferromagnetic for the shortest distance and antiferromagnetic for the second shortest distance, results in broad intensity minima in the magnetic diffraction at the positions of strong spots in the spin-independent diffraction by lattice sites. It corresponds to the degradation of the long-range correlation.

Solar selective absorbers for thermal energy applications can be realized with films of quasicrystal forming alloys. Absorbers were produced by sputtering thin films of AlCuFe and AlCuFeCr. Alumina and floatglass were used as dielectric antireflective coatings, which are necessary for the required optical properties. For applications, several aspects are of importance: high solar absorptance and low thermal emittance have to be achieved. Moreover, the coatings have to show good adhesion and stability with respect to oxidation in air at elevated temperatures and exposure to humidity and condensation of water. The present study emphasizes these aspects, especially degradation stability, for coatings deposited on technical copper foils with large roughness. Oxidation stability is achieved easily, while humidity stability depends strongly on the choice of materials. The combination AlCuFeCr/floatglass turned out to be stable, while the combination AlCuFe/alumina is destroyed rapidly.

Quasicrystalline materials (QCs) have very interesting properties which make them good candidates to be used as coatings resistant to wear, high temperature oxidation, corrosion, etc. The potential application of AI71Co13Fe8Co8 (QC1), a quasicrystalline approximant alloy, as a thermal barrier coating (TBC) for components of aeronautical and power generation turbines was explored in our laboratories and the results are presented in this paper. QC1 is stable at I 1000°C and has a thermal conductivity similar to that of yttria stabilized zirconia (YSZ), the most commonly used material for TBCs. QC1 powders were low pressure plasma sprayed on IN100 superalloy and coatings of excellent quality were obtained after a series of optimization experiments. The “as deposited” coatings were characterized by XRD, SEM and TEM and the results indicated that the coatings retained the same microstructure and phase composition as the bulk material. However, when coated specimens were heat treated at 950°C, interdifussion between coating and substrate took place resulting in the transformation of the QC approximant phase. To avoid or retard this process, the effect of several diffussion barrier layers, deposited between substrate and coating, was studied and the results showed that some of these barriers were quite efficient in slowing the diffusion process. Extensive cyclic oxidation and hot corrosion studies were carried out, showing promising results for this new type of TBC coatings. A short term ground engine test was carried out with coated blades.

Dry sliding friction and wear experiments were performed with pin-on-disc techniques using WC and brass pins in contact with quasicrystalline (Al65Cu23Fe12) and crystalline (Al50Cu35Fe15 and Al70Cu20Fe10) discs, which were prepared by powder metallurgy routes. The contact loads (1, 5 and 10 N) used in this study produced similar coefficients of friction 0.3-0.4 for the Al65Cu23Fe12 and Al50Cu35Fe15 phases. These values are higher than previously reported for quasicrystalline and related crystalline phases. Possible reasons for these differences are presented. The contrasting wear behavior between different pins and discs is discussed in terms of contact area, oxidation, material transfer mechanisms and fracture characteristics.