COST 507 - Repositório Aberto da Universidade do Porto

Thermodynamic Assessment in the Al­Cu­Mg­Zn System
Dll
P. Liang, H. L. Lukas, H. J. Seifert and F. Aldinger
Max­Planck­Institut für Metallforschung, Pulvermetallurgisches Laboratorium,
Heisenbergstr. 5, D­70569 Stuttgart, Germany
Abstract
The main part of this research is the assessment of the Al­Cu­Mg­Zn system, which
is one of the key systems of the COST 507 project. Our contribution to it includes
full optimization of the Cu­Mg­Zn and Al­Mg­Zn ternary systems and revision of the
thermodynamic descriptions of the Ai­Mg, Mg­Si, Mg­Zn and Al­Mg­Si systems, using
experimental results planned after the assessments of round I and carried out by
our partners of the COST 507 project. 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 Manchester and MPI 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.
In the Al­Mg­Si system the technically most important part, the solvus surface of the
(Al) solid solution could be improved, on one hand due to a better description of the
Gibbs energy of the Mg2Si binary phase, derived from enthalpy of formation and melting
as well as from heat capacity measurements, on the other hand by DTA, dilatometrie
and metallographic investigation of Al­rich alloys, which were carried out by F. Sommer'
s group at MPI Stuttgart.
1 Introduction
The phase relationships in the quaternary Al­Cu­Mg­Zn system are very complex and
experimentally not well established. Only a partial phase diagram in the Al­rich corner
was investigated [47Str]. No quaternary phase has been found. I. e. the phases encountered
are the same as in the ternary sub­systems Al­Cu­Mg, Al­Cu­Zn, Al­Mg­Zn and
Cu­Mg­Zn or quaternary extensions of them.
In the Al­Cu­Mg­Zn system, several non­stoichiometric phases with the same crystal
structures exist in different ternary subsystems with quaternary ranges of homogeneity
(T­Phase in the Al­Cu­Mg system and r­ Phase in the Al­Mg­Zn system, Mg^Znn and
Mg 2 Cu 5 Al6, the Laves phases in the Cu­Mg­Zn and Al­Cu­Mg systems). In modelling
these phases, the model descriptions have to be compatible with regard to possible mixing
in the quaternary system. Therefore, the T­phase, Laves phases and Mg2Cur,AlG in
the Al­Cu­Mg system have been modelled so as to allow combination with the r­phase,
­ 114 ­