COST 507 - Repositório Aberto da Universidade do Porto

In the Cu­Si system there are six compounds (Cu&Siis­ß, CuigSiô­ri, Cu 33 Si7­8,
CujSi­ε, CusóSiii­y, Q^SÍB­K), all of them were modelled as stoichiometric
phases. The binary phase CugsSiis­ß which has the crystallographic structure
bcc­A2, extends into the ternary Al­Cu­Si system, and the. phase Q^Si^­tc
with the structure hcp­A3 exists in the Cu­Mg­Si system. Although there is a
lack of experimental data necessary for a reliable definition of the ranges of
stability of these binary phases, the phases were nevertheless now modelled as
non­stoichiometric in order to describe the solution of other elements in them.
Table 1 summarizes the invariants which were used for the optimisation of the
bcc­A2 and hcp­A3 phases.
Table 1. Invariant equilibria used for the optimisation of the Cu­Si system
Phase equilbria
liquid + fcc-Al = bcc­A2
liquid + bcc­A2 = Cu33S¡7­8
liquid + bcc­A2 = hcp­A3
fcc­Al + CuseSin­Y = hcp­A3
Cu33SÍ7­ô + hcp­A3 = bcc­A2
Cu33Sirô + hcp­A3 = CuseSiii­y
í40Smil
1125
1097
1115
825
1058
1002
present work
1125
1093
1116
826
1059
1008
The calculated phase diagram is presented in Fig. 1. Above 673 K it shows
good agreement with the known experimental data [73Hul].
2.1 The Al-Cu-Mg system
The phase diagram of the Al­Cu­Mg system is very complicated and contains
three ternary, near­stochiometric compounds Q (AF/C^Mgö), V (AlnCunMgi)
and S(Al2CuMg), one non­stoichiometric phase T with the Mg 32 (Al,Cu)49
prototype structure, and three Laves phases. The Laves­C15 phase, extending
from the Cu­Mg binary to higher Al concentrations, transforms first into the C­
36 and then the C­14 Laves type structures, with slight deviation of
stoichiometry from Cu and Mg ideal compositions. The phase diagram data
[83Bel], [81Mel], [55Zam], [49Ura], [59Phi] and experimental thermodynamic
data [86Rod], [72Pre], [86Not], [46Pet] provide basic information for the
optimisation of this system. New investigations of Al­Cu­Mg alloys were
carried out by [95Kim] and [95Soa] in the COST Action and used in the
assessment.
Figure 2 presents the calculated isothermal section at 785 K and shows the
solubility ranges of the Laves phases. The calculated solvus curves and solvus
surface in the Al­comer are in satisfactory agreement with the experimental
data (Figures 3­4).
The invariant reactions in the Al­comer, which are important for commercial Al
alloys, are given in Table 2 and compared with experimental data.
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