Y. Calvayrac

About the AlCuFe icosahedral phase formation

Abstract In the present work, results obtained by differential thermal analysis (DTA) and X-ray diffraction (XRD) confirm [5, 8] that the icosahedral phase (i-phase) is formed by a peritectic reaction. A single i-phase can be obtained after annealing around the compositions Al64Cu24Fe12Al61.75Cu25.5Fe12.75. For the composition Al62Cu25.5Fe12.5, the XRD lines of the i-phase are narrow whatever the annealing temperature (800 °C or 600 °C). Thus, this result establishes that there exists a single phase domain where the i-phase is perfect (without phasons) and remains stable at 600 °C. A provisional vertical section along the line Al70Cu20Fe10Al64Cu24Fe12Al58CuPin28Fe14 is proposed and shows the phases that exist equilibrium at various temperatures in this range of compositions.

The phase diagram and structures of the ternary AlCuFe system in the vicinity of the icosahedral region

Abstract The phase diagram of the ternary system (Al, Cu, Fe) has been systematically studied around the icosahedral region. At 680°C, the i-region extends approximately over a triangle with vertices of Al, Cu and Fe composition of respectively 62.4-24.4-13.2, 65-23-12 and 61-28.4-10.6. This region splits schematically into 3 fields: (i) the perfect icosahedral phase which is stable down to the lowest possible annealing temperature where atomic diffusion is active in a tiny region of composition close to Al62.3Cu24.9Fe12.8; (ii) a well defined periodic phase with rhombohedral lattice which transforms reversibly into ico near 710°C in the lower part of the triangle; (iii) a complex region characterized by additional diffraction effects (peak broadening, lineshapes, etc.) which may correspond to various approximant structures closely related to the i-phase.

Antiphase domains in icosahedral Al-Cu-Fe alloy

Abstract Electron microscopy observations of the newly discovered icosahedral phase in the Al‐Cu‐Fe ternary system show a domain structure as expected from a group‐subgroup formal decomposition from a primitive six-dimensional (6D) hypercubic unit cell to a face‐centred one with twice the lattice parameter, as suggested by Ebalard and Spaepen. The structure of Al65Cu20Fe15 can be seen as an ordered F superstructure of the usual primitive quasilattice with, at least, two motifs in the 6D representation. The smoothness of the observed antiphase boundaries and the relative weak intensities of the superstructure reflections suggest that the ordered motifs should share many geometrical features. A possibility could be that at least one of the orbits of a given atomic species arranges in space according to a 6D primitive skeleton and chemical ordering occurs on the remaining orbits.

A cubic approximant of the icosahedral phase in the (Al - Si) - Cu - Fe system

We present a stable simple cubic structure in the quaternary system (Al - Si) - Cu - Fe with the composition which corresponds to the p/q = 1/1 cubic approximant of the icosahedral Al - Cu - Fe phase. This cubic structure, with lattice parameter a = 12.330 A and density , contains roughly 135 atoms per unit cell. Preliminary studies of NMR and ME spectra as well as electric conductivity properties strongly suggest that this alloy shares most of the basic structural and physical properties of the parent icosahedral phase I-(Al - Cu - Fe).

Tunnelling and point contact spectroscopy of the density of states in quasicrystalline alloys

Point contact and tunnelling experiments performed at low temperatures were used to study the electronic behaviour of the icosahedral quasicrystalline alloys AlPdRe, AlCuFe, and AlPdMn. With samples of high quality we observed at low temperatures a zero-bias anomaly that we related to the decrease of the electronic density of states (DOS) due to the electron-electron interaction. This interaction tends to diminish the DOS at the Fermi level and can be seen as the energy pseudogap of the alloy. Our experiments indicate that the DOS is strongly modified near and consists of a spiky feature in a broad pseudogap, with the width of the feature of the order of 100 meV or even larger for the AlPdRe, whereas it is as small as 20-22 meV for Al-Cu-Fe and 17-20 meV for Al-Pd-Mn. The broad pseudogap has widths larger than 400 meV for AlPdRe, whereas for AlCuFe it is about 80-90 meV and for AlPdMn it is of the order of 110-122 meV. The studies were performed on three samples of the compositions , , and . The junctions were of the types alloy-Au(In, Al) and alloy-insulator-Au(In, Al), and were studied at different temperatures between that of liquid nitrogen and 2 K, and even to 400 mK for the AlCuFe alloy.

Electronic distributions of states in crystalline and quasicrystalline Al-Cu-Fe and Al-Cu-Fe-Cr alloys

Valence and conduction state distributions in a series of Al-Cu-Fe and Al-Cu-Fe-Cr alloys of different structural states and nominal compositions have been investigated by means of soft X-ray emission and photoabsorption spectroscopies. A complete description of the valence band is obtained. The DOS at EF is observed to be very low which is consistent with the high resistivity values found for the icosahedral quasicrystalline phase. The existence of a wide pseudo-gap at EF is evidenced, which is higher in the quasicrystal than in be related crystalline counterparts.

A neutron diffraction determination of short-range order in a Ni63.7Zr36.3 glass

The total structure factors for amorphous Ni63.7Zr36.3 have been obtained by neutron diffraction for three different isotropic compositions of nickel. The use of a zero alloy leads to a direct determination of the Bhatia-Thornton structure factor SCC. The high value of the determinant of the system produces accurate partial structure factors. From which partial pair correlation functions have been calculated. The behaviour of the partial functions and the deduced partial coordination numbers show the existence of strong chemical short-range order, the nature of which is discussed.

On the stability and structure of Cu—Zr based glasses

Abstract Glassy Cu—Zr alloys with compositions ranging from Cu33Zr67 to Cu66Zr34 have been prepared. Their thermal behaviour, structure factor and density have been measured. These results are discussed in terms of an ideal substitutional solid solution and briefly compared to the predictions of the nearly free electron-like model for the stability of metallic glasses.

X-ray absorption and diffraction spectroscopy of icosahedral Al-Cu-Fe quasicrystals under high pressure

Abstract We report in-situ extended X-ray absorption fine structure (EXAFS) and X-ray diffraction studies of quasicrystals under high pressure quasihydrostatic conditions. The EXAFS and diffraction experiments are combined for the first time to give a joint evolution of the short-range and long-range atomic order with pressure. EXAFS experiments performed at the Fe and Cu K absorption edges have been carried out for an icosahedral Al62Cu25.5Fe12.5 alloy from ambient pressure up to about 8 GPa (Fe edge) or 35 GPa (Cu edge). The local order around Fe and Cu atoms has thus been obtained as a function of pressure. X-ray diffraction patterns have been obtained for the same quasicrystal up to 35 GPa and these have given a variation in the 6D lattice parameter with pressure. A remarkable result is that the quasicrystal remains icosahedral up to the highest pressure. The evolution of the 6D lattice parameter and of the first-neighbour distances has provided values of the zero-pressure bulk modulus and of its firs...

Correlations in the electronic properties of AlCuFe quasicrystals and high-order approximants: 57Fe Mössbauer, and 27Al and 65Cu nuclear magnetic resonance studies

We have investigated the local electronic properties and the atomic order in the Al100-x-yCuxFey phases using 57Fe Mossbauer as well as 27Al and 65Cu nuclear magnetic resonance (NMR) spectroscopies. We have studied samples along the existence domains of the icosahedral quasicrystalline (i-) and the high-order approximant phases, given by two close-lying parallel lines in the concentration diagram. In addition, we have also studied a series of intermediate concentrations situated between these lines and retained in a metastable i-phase by quenching. It is found that the 57Fe Mossbauer centre shift (isomer shift) and quadrupole splitting are linearly correlated with each other over the range of compositions and structures for all samples, which reveals systematic changes in the orbital occupations on Fe atoms with composition. In addition, it is found that the 27Al NMR average electronic shift follows this correlation. The results are discussed in terms of the hybridization of iron d states with the p and induced d states of Al. We have studied also a low-order cubic approximant phase containing Si as well as several non-approximant crystalline phases.