THE ROLE OF DEFECTS IN THE CRYSTAL QUASICRYSTAL TRANSFORMATION/

The transformation between quasicrystals and related crystals, the so- called approximant phases, appears as a major point in the understandi ng of quasicrystal stability. Structural defects of approximant phases seem to be involved in the mechanism of crystal/quasicrystal transfor mation. Theoretical works as well as observations are supporting this point of view. We here report observations made by transmission electr on microscopy on two systems which provide relevant examples for two t ypes of mechanisms. In both cases, the approximant defects are identif ied as antiphase boundaries. In the first system (Al-Li-Cu), the trans formation is due to a progressive organization of the approximant phas e defects. In the second system (Fe-Cr-Mo), the vertex of intersecting defects exhibits key structural feature for the transformation in a q uasicrystal.