COST 507 - Repositório Aberto da Universidade do Porto

deviation from Kopp­Neumann' s rule derived by [92Aga] from the temperature dependence
of the enthalpy of mixing of the liquid, but the numerical value is refined to about
70% of that assessed by [92Aga].
Kowalski and Spencer [93Kow] in the Cu­Zn system assumed different lattice stabilities
for pure Zn in its stable state (hep with c/a = 1.856) and as hep phase with axial ratio
c/a = 1.556 (c­Cu­Zn phase), which lies close to the ideal axial ratio c/a = 1.633. To
get compatibility, in Mg­Zn the Mg(hcp) solid solution had to be reformulated to show
the same lattice stability for the end member pure Zn as [93Kow] used for the t­Cu­Zn
phase.
Furthermore the MgZn2 Laves phase should be modelled similarly as MgCu 2 with antistructure
atom formation on both sublattices, although the only information for that is
the vague statement: "The homogeneity range of MgZn 2 is about 1 at.%" [90Mas].
The Mg­Zn system was re­optimized using the experimental­data file of [92Aga] and
the new C p^u' d and
data of [95Som] (Fig.2). The Laves phase parameters °¿Mgfzn,M g
^Mg.ziîrZn were estimated by trial and error to reproduce a maximum homogeneity range
of 1 at.% for MgZn 2 .
2.4 The Mg­Si System
The enthalpy of formation and melting as well as heat capacity of Mg2Si were measured
calorimetrically by [97Feu]. The Mg­Si system was re­optimized using the experimentaldata
file of [92Cha] together with the new data of Mg 2 Si [97Feu].
2.5 The Al­Mg­Si System
New measurements on this system were carried out by F. Sommer's group at MPI
Stuttgart. The Al corner of the Al­Mg­Si ternary system was investigated by DTA
and optical micrography. A few points of the solvus of the (Al) solid solution were
precisely determined by dilatometry. The results of these experiments and the updated
Mg­Si system together with literature data were used to redetermine a complete set of
analytical descriptions of the Gibbs energies of all stable phases of the Al­Mg­Si system.
The technically most important part, the solvus surface of the (Al) solid solution
could be improved (Fig. 3). A publication of this re­optimization is in press [97Feu] .
2.6 The Cu-Mg-Zn System
The assessment of the Cu­Mg­Zn system is mainly based on the critical review of experimental
literature provided by COST 507, Coordination group B [94Gho]. The Cu­
Mg­Zn phase diagram is characterised by the formation of three Laves phases MgCu2,
MgZn 2 and the MgNi 2 ­type phase Mg 2 CuZn,3 (τ), which have large solubility ranges.
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