Modulated states in Mg-Al alloys and classical Frank-Kasper phases: a highresolution X-ray diffraction study

The Mg-Al system shows for composition in the 50-60 at.% Al range several s tates, either periodic or quasiperiodic, related to tetrahedral and icosahe dral motif structures. The Mg-Al (61 at.%) rapidly solidified alloy shows a cubic quasiperiodic state with inflation symmetry characterized by the alg ebric number 2 - root3. The main question is to know if this state is an ex ample of a new kind of quasicrystal or if this inflation property is only d ue to a continuous variation of the wavenumber. We report high resolution X -ray diffraction studies of this Mg-Al (61 at.%) metastable state and the r elated stable Frank-Kasper phase (beta -phase). These experiments clearly s how fluctuations of the wavenumber. The quasiperiodic Mg-Al (61 at.%) state appears thus to have typical features of incommensurately modulated phases , but with a persisting connection with Frank-Kasper phases. (C) 2000 Elsev ier Science B.V. All rights reserved.