No abstract

This paper concentrates on a comparison of Al 3p occupied densities of states in crystalline and quasicrystalline Al-Cu-Fe intermetallics. This comparison is made quantitative by computing the two first moments of this partial distribution, which correspond to the partial contribution of Al atoms to the cohesive energy of the material and to the orbital overlap modulated by the coordination number, respectively. From these data, we conclude that the icosahedral quasicrystal and its approximants represent a region of specifically enhanced stability in the Al-Cu-Fe phase diagram. We assign this effect to the specific role played by Al 3p states in the Hume-Rothery mechanism. We confirm the occurrence of a stronger hybridization between Al 3p and Fe 3d states in the icosahedral structure whereas we point out a weaker interaction with Cu 3d states in the icosahedral compounds as compared to stable approximants.

We have investigated the electronic structure of hexagonal through theoretical as well as experimental means. Two different calculations have been carried out within the LMTO and tight-binding - LMTO methods. The experimental procedure involved soft-x-ray spectroscopy complemented by photoemission measurements. Co states have been found near the Fermi level, in interaction with Al states of hybridized p, s - d character. As a result, a pseudo-gap is generated at the Fermi level. Occupied Al states beyond 5 eV form the Fermi level are found to be almost pure s in character. The comparison between the experimental results and calculations indicates a rather good agreement with results from LMTO, consistent with the fact that this is a more elaborate band structure calculation. Agreement is only acceptable with results from the TB - LMTO calculations but these can be improved with a more careful treatment of the TB structure constants.

The stability of quasicrystals is related to the Hume-Rothery mechanism known to take place in many intermetallics, especially Al-Cu alloys. In order to assess to what extent the electronic density of states in aperiodic Al-Cu-Fe compounds is sensitive to Hume-Rothery stabilization, we present experimental measurements by soft x-ray emission and absorption spectroscopies of the aluminium electronic states in a series of Al-Cu crystalline compounds. Some of them, like icosahedral Al-Cu-Fe, are characterized by nearly spherical Brillouin zones. We observe the opening of a narrow but definite pseudo-gap in such Al-Cu compounds. We point out a strong interaction between Cu d states and the Al states in the middle of the valence band which is well described by a Fano-like effect. Some other characteristic features of our spectra are interpreted on the basis of band structure calculations. A comparison to previous data published for quasicrystalline and approximant phases stresses the importance for the stability of these compounds of sp-d hybridization at the Fermi level. It also leads to the conclusion that the effect of Hume-Rothery interaction is dramatically enhanced by the hierarchical structure of quasicrystals rather than by the almost spherical shape of the Brillouin zone.

The discovery in 1987 of stable quasicrystals in the Al–Cu–Fe system was soon exploited to patent specific coatings that showed reduced friction in ambient air against hard antagonists. Henceforth, it was possible to develop a number of applications, potential or commercially exploited to date, that will be alluded to in this topical review. A deeper understanding of the characteristics of complex metallic alloys (CMAs) may explain why material made of metals like Al, Cu and Fe offers reduced friction; low solid–solid adhesion came later. It is linked to the surface energy being significantly lower on those materials, in which translational symmetry has become a weak property, that is determined by the depth of the pseudo-gap at the Fermi energy. As a result, friction is anisotropic in CMAs that builds up according to the translation symmetry along one direction, but is aperiodic along the other two directions. A review is given in this article of the most salient data found along these lines during the past two decades or so.