PERFECT AND MODULATED QUASI-CRYSTALS IN THE SYSTEM AL-FE-CU

The occurrence of a reversible phase transformation from a rhombohedral crystalline structure to a perfect quasicrystalline state of chemical composition Al63.5Fe12.5Cu24 has been studied by transmission electron microscopy. Both direct in situ heating experiments and thermal treatments of bulk dodecahedral particles have been carried out in order to characterize the various intermediate stages of this transformation and its reversibility. The most spectacular intermediate state can be considered as a modulated quasicrystalline structure which, depending on the thermal treatment, appears either at about 550-degrees-C or at about 750-degrees-C. Under fast cooling, it may be retained as a metastable state at room temperature. A modulation mechanism of the perfect icosahedral phase is proposed based on a phason field theory. It has been shown that the observed wavelength of the modulated quasicrystalline structure can be deduced, to first order, from a dynamical phason condition applied to atomic displacements defined within a two-dimensional-one-dimensional cut and projection scheme of the quasicrystalline-to-crystalline transition.