COST 507 - Repositório Aberto da Universidade do Porto

The three Laves phases were described by the "compound­energy­formrlism" with Cu­
Zn exchange, Mg(Cui_ I Zn r ) 2 and slight antistructure atom formation (Cu and Zn on
the Mg sublattices, Mg on the Cu­Zn sublattices). The Gibbs energy descriptions of
the three quasibinary Laves phases were optimized using published liquidus, solidus
and enthalpy of mixing data of the quasibinary system MgCu2­MgZn 2 [48Koe, 52Lie,
64Kin, 79Pre]. The homogeneity ranges with respect to excess or deficient Mg were
interpolated between the binary end members, since no reliable experimental data for
the range of deviation from the stoichiometric MgX2 are available in the ternary.
The solubilities of Mg in the binary Cu­Zn phases were interpolated from the binary
Cu­Mg fee and Zn­Mg hep parameters respectively, as no experimental data for these
solubilities were found in literature. To satisfy the Bragg­Williams description of ordering
in the ternary range of the /3CuZn phase, its formulation in the compound energy
formalism was extended into ternary and quaternary systems.
The liquid phase was described as substitutional solid solution after the Redlich­Kister
Muggianu formalism. No ternary parameters were introduced.
Using the optimized quasibinary parameters and the estimated ternary parameters, together
with the parameters of the binary subsystems, the ternary Cu­Mg­Zn system was
calculated. The results are shown in Fig. 4 to 9 and compared with the experimental
values.
This optimization was presented at the conference " Thermodynamics of Alloys" at
Marseille, Sep. 1996 and is in preparation for publication in Calphad.
2.7 The Al­Mg­Zn System
The Al­Mg­Zn ternary system is a relatively complex system which includes two ternary
phases, τ and φ. The τ­phase has a large homogeneity region. Its ideal formula is
Mg3 2 (Zn,Al) 4 9. It is cubic, space group Im3, 162 atoms to the unit cell [52Ber]. The
ternary τ­phase was modelled according to its crystal structure with cubic symmetry
as (Mg)26(Mg,Al) 6 (Al,Zn,Mg)4 8 (Al)i in the compound energy formalism. The unit
cell of the ternary (¿­phase was at first time determined in a collaboration between
CNRS/ONERA in Châtillon, CNRS in Vitry­sur­Seine and MPI in Stuttgart using transmission
electron microscopy [97Don]. The unit cell of the (¿­phase is orthorhombic,
space group Pbc2, or Pbcm with large lattice parameters (a=897.9, b=1698.8, c=1934
pm). The (¿­phase was approximated by the sublattice formula Mge(Al,Zn)s.
The optimization of the Al­Mg­Zn ternary system incorporates, in addition to the available
published data, the results of experimental measurements carried out in a collaboration
between CNRS at Vitry­sur­Seine, UMIST at Manchester and MPI at Stuttgart.
The experiments on ternary Al­Mg­Zn alloys were specifically performed to provide
missing data of the ternary solubilities of the ΑΙ­Mg and Mg­Zn phases as well as to
improve the knowledge of the extensions of the homogeneity ranges of the ternary r­
and (¿­phases. The alloys were investigated using X­ray diffraction, differential scanning
calorimetry and DTA in the composition range around the τ- and (¿­phases. The
compositions of the constituent phases were determined by Electron Probe Micro Analysis
(ΕΡΜΑ) and Energy Dispersive X­ray Spectroscopy (EDX).
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