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

The τ1 and τ2 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 τ2 phase has been studied several times but the reported results are inconsistent. In this work, the τ2 phase has been investigated in order to determine the homogeneity range at 330 °C and 360 °C and to revisit its crystal structure. The τ2 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 °C. Heterogeneous equilibria with the five phases Φ, τ1, β, Mg21Zn25 and (Mg) have been observed. The Zn-rich β phase is a new intermetallic compound with a large cubic unit cell. At 360 °C, the variation of the Zn content is slightly larger (8 at.%) and the two phases Φ and (Mg) are exchanged in heterogeneous equilibria by a liquid and a quasicrystalline phase. The crystal structure of the τ2 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. [Display omitted] •The phase range of the Al–Mg–Zn 2/1 approximant at 360 °C and 330 °C was determined.•The 1/1 and the 2/1 approximant were distinguished optically by etched samples.•The crystal structure has been refined simultaneously with X-ray and neutron data.•The structural disorder is shown to be correlated.•According to total energy calculations, the approximant has two compositions at 0 K.