Crystal structure, disorder and composition of the 2/1 approximant in the Al-Mg-Zn system revisited

The tau(1) and tau(2) phases in the Al-Mg-Zn system are famous examples for complex intermetallic alloy phases. They are 1/1 and 2/1 approximants of a quasicrystalline phase. The crystal structure of the tau(2) phase has been studied several times but the reported results are inconsistent. In this work, the tau(2) phase has been investigated in order to determine the homogeneity range at 330 degrees C and 360 degrees C and to revisit its crystal structure. The tau(2) phase is a ternary solid solution phase, which is able to vary its composition by about 5 at.% Al, 2 at.% Mg and 6 at.% Zn at 330 degrees C. Heterogeneous equilibria with the five phases Phi, tau(1), beta, MB21Zn25 and (Mg) have been observed. The Zn-rich beta phase is a new intermetallic compound with a large cubic unit cell. At 360 degrees C, the variation of the Zn content is slightly larger (8 at.%) and the two phases Phi and (Mg) are exchanged in heterogeneous equilibria by a liquid and a quasicrystalline phase. The crystal structure of the tau(2) phase has been studied at the composition Al15.4Mg42.0Zn42.6 by a combined refinement of X-ray and neutron data. Total energy calculations at 0 K reveal two stable variants at the compositions Al14.8Mg40.8Zn44.4 and Al13.0Mg40.8Zn46.2. The chemical and structural disorder can be illustrated with the help of a Henley-Elser decoration of a canonical cell tiling. The crystal data are close to those reported previously reported by Spiekermann, but not in agreement with the results of Sugiyama et al. and Lin et al., which have reported compositions outside the homogeneity range. (C) 2014 Elsevier Ltd. All rights reserved.