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Abstract:
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.
