COST 507 - Repositório Aberto da Universidade do Porto

order to facilitate the evaluation of the data in the ternary Cu­Mg­Y system, an investigation
of the partial thermodynamic properties of binary liquid Mg­Y alloys was
added.
In all cases, magnesium vapor pressures were measured by an isopiesic method which
had been developed during Round I of the COST 507 Action [94Gna]. From these,
thermodynamic activities of magnesium were derived in the usual way [89Ips]. A
calorimetrie method [80Som, 82Soml] was employed to determine enthalpies of
mixing for liquid alloys. The results were combined to provide a complete description
of the thermodynamic mixing behavior for the liquid alloy systems.
2 The Binary Mg-Y System
For the isopiestic vapor pressure measurements in the binary Mg­Y system, tantalum
crucibles were originally used, since graphite had been ruled out due to the high
stability of the respective yttrium carbides. However, it was found that liquid
magnesium­yttrium alloys had a strong tendency to creep out of these crucibles and to
react with the graphite crucible holders. Therefore it was attempted to use alumina as
container material, which created a new problem since gaseous magnesium reacts
rather rapidly with aluminum oxide. Finally, magnesia crucibles were used to contain
the samples which prevented any side reactions, but which still did not completely
prevent creeeping of the liquid alloys.
Activités of magnesium were determined at 1173 K for liquid alloys with magnesium
contents beyond 50 at%. A complete description of the experiments and the corresponding
results can be found in Appendix 1; they have been published as [97Ganl].
3 The Ternary Cu-Mg-Y System
Magnesium vapor pressures were determined using the described isopiestic method
[93Gna] along an isopleth with a constant concentration ratio of χα/ χ γ = 1/2. This
composition was selected because of the eutectic around 65 at% Y in the Cu­Y binary
system [920ka] which was to guarantee a maximum stability range of the liquid phase
in the corresponding ternary section. As in the Mg­Y binary system we had to use
magnesia crucibles to contain the samples due to the reactivity of yttrium with
graphite. Although some of the liquid alloys showed some creeping tendency it was a
much less severe problem than in the binary Mg­Y system. From the magnesium vapor
pressures thermodynamic activities of magnesium were derived and converted to a
common temperature of 1173 K using partial enthalpy data from the calorimetrie
measurements. The integral Gibbs energy of mixing for liquid alloys in this section
was calculated by means of a Gibbs­Duhem integration following the method outlined
by Peiton and Flengas [69Pel].
The entalpy of mixing for ternary liquid alloys was measured along five different
isopleths, starting in all cases with liquid binary alloys: one section starting from a
binary liquid Cu­Y alloy with 67 at% Y (as in the isopiestic experiments, see above),
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