Computer Coupling of Phase Diagrams and Thermochemistry 32 (2008) 470–477
Contents lists available at ScienceDirect
Computer Coupling of Phase Diagrams and
Thermochemistry
journal homepage: www.elsevier.com/locate/calphad
Thermodynamic assessment of the Si–Zn, Mn–Si, Mg–Si–Zn and
Mg–Mn–Si systems
Adarsh Shukla ∗ , Youn-Bae Kang, Arthur D. Pelton
Centre de Recherche en Calcul Thermochimique, Département de Génie Chimique, Ecole Polytechnique, Montréal, Québec, Canada
article
info
Article history:
Received 28 April 2008
Received in revised form
1 July 2008
Accepted 2 July 2008
Available online 25 July 2008
Keywords:
Silicon
Zinc
Manganese
Magnesium
Phase diagrams
a b s t r a c t
The binary Si–Zn and Mn–Si systems have been critically evaluated based upon available phase
equilibrium and thermodynamic data, and optimized model parameters have been obtained giving the
Gibbs energies of all phases as functions of temperature and composition. The liquid solution has been
modeled with the Modified Quasichemical Model (MQM) to account for the short-range-ordering. The
results have been combined with those of our previous optimizations of the Mg–Si, Mg–Zn and Mg–Mn
systems to predict the phase diagrams of the Mg–Si–Zn and Mg–Mn–Si systems. The predictions have
been compared with available data.
© 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Although magnesium-based materials have a long history of
important commercial applications, including automotive, there
remains much to be learned about the basic properties of the metal
and its alloys. With the recent renewed interest in lightweight
wrought materials, including both sheet and tube applications,
there has been an increased focus on developing a better understanding of novel magnesium alloys, including those that incorporate additions of such elements as Si, Mn and Zn. These alloy systems, along with other potential candidates, are being actively pursued as possible routes to develop magnesium materials with improved ductility, or even practical room temperature formability.
The properties of cast or wrought material depend first and
foremost upon the phases and microstructural constituents (eutectics, precipitates, solid solutions, etc.) which are present. In an alloy with several alloying elements, the phase relationships are very
complex. In order to effectively investigate and understand these
complex phase relationships, it is very useful to develop thermodynamic databases containing model parameters giving the thermodynamic properties of all phases as functions of temperature
and composition. Using Gibbs free energy minimization software
such as FactSage [1,2], the automotive and aeronautical industries
and their suppliers will be able to access the databases to calculate
the amounts and compositions of all phases at equilibrium at any
∗
Corresponding author.
E-mail address: adarsh.shukla@polymtl.ca (A. Shukla).
0364-5916/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.calphad.2008.07.002
temperature and composition in multicomponent alloys, to follow
the course of equilibrium or non-equilibrium cooling, to calculate
corresponding heat effects, etc.
As part of a broader research project to develop a thermodynamic database for Mg alloys with 25 potential alloying metals,
the present study reports on the evaluation and optimization of
the Si–Zn, Mn–Si, Mg–Si–Zn and Mg–Mn–Si systems.
Previous optimizations of the Mn–Si system in the framework
of COST 507 [3] and by Chevalier et al. [4] were based upon a
Bragg–Williams (BW) random-mixing model for the liquid phase.
The liquid in this binary system is expected to exhibit considerable
short-range-ordering (SRO) as evidenced by the very negative
enthalpy of mixing curve. As has been shown by the present
authors [5], the use of a BW model in liquids with a high degree
of SRO generally results in unsatisfactory results and in poor
predictions of ternary properties from binary model parameters.
In the present work, the Modified Quasichemcial Model (MQM)
has been used to account for the SRO in the liquids. As well,
there are vapor pressure measurements [6,7] of Mn–Si alloys
and measurement of the enthalpy of formation [8] of compounds
which were not taken into account in previous optimizations. The
liquid phase in the Si–Zn system shows slight positive deviations
from ideality. This system was optimized previously [9] using
a BW random-mixing model. The MQM, which takes clustering
into account, was used in the present work in order to obtain a
better description and prediction in the Mg–Si–Zn system, and for
consistency with the fact that the MQM is used for the other binary
subsystems in this ternary system. Hence the Si–Zn and Mn–Si
systems have been re-optimized in the present study.
A. Shukla et al. / Computer Coupling of Phase Diagrams and Thermochemistry 32 (2008) 470–477
Fig. 1. Optimized phase diagram of the Si–Zn system.
471
Fig. 2. Optimized phase diagram of the Mn–Si system.
The binary model parameters have then been combined
with those from our previous optimizations of the Mg–Zn [10],
Mg–Mn [11] and Mg–Si [12] systems in order to predict
thermodynamic properties and phase equilibria in the Mg–Si–Zn
and Mg–Mn–Si ternary systems.
2. Modified quasichemical model (MQM)
The Modified Quasichemical Model (MQM) in the pair approximation [13] was used to model the liquid alloys. A description of
the MQM and its associated notation is given by Pelton et al. [13].
The same notation is used in the present paper. The composition of
maximum SRO is determined by the ratio of the coordination numj
bers Ziji /Zij . The values of the coordination numbers selected in the
present study are listed in Table 1. All the binary subsystems of the
Mg–Si–Zn and Mg–Mn–Si systems exhibit SRO near the equimolar
j
composition; hence Ziji = Zij in all cases.
From the MQM model parameters of the binary liquid phases,
the thermodynamic properties of a ternary liquid phase may be
estimated as discussed previously [14]. If ternary experimental
data are available, additional ternary model parameters may be
added.
3. Binary systems
All calculations and optimizations in the present study were
performed with the FactSage thermochemical software [1,2].
The optimized model parameters of all phases obtained in the
present study are listed in Table 1. The crystallographic data [15] of
all phases in the Si–Zn and Mn–Si systems are listed in Table 2. The
Gibbs energies of all stable and metastable condensed phases of the
elements were taken from Dinsdale [16], while the Gibbs energies
of the gaseous elements were taken from the JANAF Tables [17].
3.1. The Si–Zn system
The optimized phase diagram of the system is shown in Fig. 1. It
may be noted that no temperature dependent terms were required
in the optimized parameters (Table 1) of the liquid Si–Zn solution.
This system was optimized by Jacobs and Spencer [9] who used
a BW random-mixing model for the liquid phase. The present
calculated eutectic temperature and composition are 419.2 ◦ C and
XZn = 0.999.
The only data available are the coordinates of the liquidus. No
thermodynamic property data were found. Girault [18], Thurmond
and Kowalchik [19] and Moissan and Siemens [20] determined the
liquidus in the range 0.85 to 5.3 at.% Zn by a gravimetric method.
John et al. [21] and Schneidner and Krumnacker [22] measured
Fig. 3. Optimized phase diagram of the Mn–Si system for the region XSi = 0.0 to
0.5.
the liquidus by DTA in the range from 1 to 55 at.% Zn. The solid
solubility of Zn in Si was determined by diffusion investigations
in the temperature ranges from 820 to 1076 ◦ C [23], 1040 to
1200 ◦ C [24] and 900 to 1360 ◦ C [25]. All these investigations
reported negligible solubility of Zn in Si. In the absence of data for
the solubility of Si in solid Zn, negligible solubility was assumed.
The experimental liquidus points are fitted equally well in
the present and previous assessment [9]. In the present work
only two fitting parameter have been used, whereas the previous
optimization used three parameters.
3.2. The Mn–Si system
The optimized phase diagram of the Mn–Si system is shown in
Fig. 2 and is compared with the experimental data in Figs. 3 and
4. Various calculated thermodynamic properties of the system are
compared with experimental data and previous optimizations in
Figs. 5–13.
A complete literature review of the Mn–Si system up to 1990
was reported by Gokhale and Abbaschian [26]. COST 507 [3] and
Chevalier et al. [4] performed thermodynamic optimizations of
the system, using a BW random-mixing model for the liquid
phase. Du et al. [27] slightly modified the optimized parameters
of COST 507 [3] for the CUB and Mn6 Si. Chakraborti and Lukas [28]
optimized the phase diagram data without taking account of any
thermodynamic data. Kanibolotskii and Lesnyak [29] fitted the
thermodynamic properties of the system to polynomial equations,
but performed no optimization.
The optimized Mn-rich side of the phase diagram (X ≤ 0.5)
is compared with the experimental data in Fig. 3. Gokhale and
A. Shukla et al. / Computer Coupling of Phase Diagrams and Thermochemistry 32 (2008) 470–477
472
Table 1
Optimized parameters of the Si–Zn and Mn–Si systems from the present work, and of the liquid phase in the Mg–Zn system from Spencer [10]
Liquid: (notations as described previously [13])
Co-ordination numbers:
Si–Zn:
Mn–Si:
Mg–Zn:
Si
Zn
Zn
ZSi
SiSi = ZZnZn = ZSiZn = ZZnSi = 6
Si
Mn
Mn
ZSi
SiSi = ZMnMn = ZSiMn = ZMnSi = 6
Mg
Mg
ZMgMg = ZZn
=
Z
=
ZZn
ZnMg = 6
ZnZn
MgZn
Optimized values for ∆gAB , in Joules:
∆gSi–Zn :
∆gMn–Si :
∆gMg–Zn :
2299 + 1946XZnZn
(−33 054 + 6.694 T) + (−20 920)XMnMn + (1.674 T)XSiSi + (11 715 − 4.184 T)X2MnMn
(−6778 + 3.128 T) + (−1996 + 2.008 T)XMgMg + (−2975 + 1.674)XZnZn
Solid solutions:
Excess Gibbs energy (Joules/mol of atoms):
CUB:
CBCC:
BCC:
FCC:
XSi XMn [(−157 737 + 26.778 T) + (−41 003 + 21.338 T)(XSi –XMn )] + XSi (5.021 T)
XSi XMn [(−151 042 + 26.778 T) + (−32217 − 8.368 T)(XSi –XMn )]
XSi XMn [−83 680 + 26 778(XSi –XMn )]
XSi XMn [−101 253 + 17 991(XSi –XMn )]
Stoichiometric compounds:
Compounds
0 a
∆H298
(J/(mol of atoms))
0 b
S298
(J/[(mol of atoms) K])
0 a
∆S298
(J/[(mol of atoms) K])
Cp (J/[(mol of atoms) K])
Mn6 Si
Mn9 Si2
Mn3 Si
Mn5 Si3
MnSi
Mn11 Si19
−18 094
−23 091
−27 900
−35 250
−38 000
−32 333
29.471
27.745
27.000
25.562
20.800
17.267
−0.834
−2.038
−1.868
−1.630
−4.716
−6.460
0.857 Cp (Mn, CBCC) + 0.143Cp (Si, Dia.)
0.818 Cp (Mn, CBCC) + 0.181Cp (Si, Dia.)
0.375 Cp (Mn, CBCC) + 0.125 Cp (Si, Dia)
0.625 Cp (Mn, CBCC) + 0.375 Cp (Si, Dia.)
0.500 Cp (Mn, CBCC) + 0.500 Cp (Si, Dia.)
0.366 Cp (Mn, CBCC) + 0.633 Cp (Si, Dia.)
a
b
Enthalpy and entropy of formation from the elements at 298.15 K.
Absolute Third-Law entropy at 298.15 K.
Table 2
Crystallographic data [15] of all phases in the Si–Zn, Mn–Si, Mg–Si–Zn and Mg–Mn–Si systems considered in the present optimization
Phase
Struktur-bericht
Prototype
Pearson symbol
Space group
Liquid
FCC
BCC
CUB
CBCC
HCP
Si (Dia.)
Mg51 Zn20
Mg12 Zn13
Mg2 Zn3
MgZn2
Mg2 Zn11
Mg2 Si
Mn6 Si
Mn9 Si2
Mn3 Si
Mn3 Si
Mn5 Si3
MnSi
Mn11 Si19
–
A1
A2
A13
A12
A3
A4
–
–
–
C14
D8c
C1
–
–
D03
–
D88
B20
–
–
Cu
W
Mn
Mn
Mg
C (Dia.)
Mg51 Zn20
–
–
MgZn2
Mg2 Zn11
CaF2
–
Si2 U3
BiF3
–
Mn5 Si3
FeSi
–
–
cF 4
cI2
cP20
cI58
hP2
cF 8
oI158
–
mC 110
hP12
cP39
cF 12
hR53
tP10
cF 16
–
hP16
cP8
tP120
–
Fm3m
Im3m
P41 32
I43m
P63 /mmc
Fd3m
Immm
–
B2/m
P63 /mmc
Pm3
Fm3m
R3
P4/mbm
Fm3m
–
P63 /mcm
P21 3
P4n2
Abbaschian [26] reported speculative phase boundaries between
the FCC, BCC and liquid phases upon which the calculations in Fig. 3
were based.
Wieser and Forgeng [30] reported phase equilibria in the
region from 2 to 24 at.% Si by metallography and XRD. These
authors reported a phase ε with a homogeneity range from 12
to 15 at.% Si and a phase ξ with a homogeneity range from 16
to 18 at.% Si (Mn6 Si and Mn9 Si2 respectively in Fig. 3). Kuz’ma
and Gladyshevskii [31] and Gupta [32] confirmed the existence
of the ε and ξ phases respectively by XRD. Later, Gokhale and
Abbaschian [26] referred to these compounds as R and υ and
suggested their approximate stoichiometries as Mn6 Si and Mn9 Si2
respectively.
Boren [33] from XRD, and Vogel and Bedarff [34] from thermal
and metallography, reported the existence of Mn3 Si. Later, Letun
et al. [35] reported an allotropic transformation of this phase
Comments
Mn is stable phase
Mn is stable phase
Mn is stable phase
Mn is stable phase
Mg and Zn are stable phases
High temperature form
Low temperature form
at 677 ◦ C. The Gibbs energy change of this transformation
was assumed to be zero in the present work. The congruently
melting compounds Mn5 Si3 and MnSi were first reported by
Doerinckel [36] from thermal and metallographic analysis, and
later by Vogel and Bedarff [34] who used the same technique.
The optimized silicon-rich side of the phase diagram is compared with the experimental data in Fig. 4. Different authors [33,
37–41] disagree on the exact designation, the width of the singlephase region (0.3–0.5 at.% Si), and the structure of the highest silicide, Mn11 Si19 . Gokhale and Abbaschian [26] preferred the
designation MnSi1.75−x with a homogeneity width of 0.4 at.% Si.
Morkhovets et al. [42], from microstructural and thermal analysis, reported this compound as MnSi1.72 . Following Chakraborti and
Lukas [28] and Chevalier et al. [4], this phase is modeled in the
present study as stoichiometric Mn11 Si19 .
There are no data on the solubility of Mn in solid Si. The slope of
the liquidus curve [43] at XSi = 1.0 is consistent with the limiting
A. Shukla et al. / Computer Coupling of Phase Diagrams and Thermochemistry 32 (2008) 470–477
473
Fig. 4. Optimized phase diagram of the Mn–Si system for the region XSi = 0.4 to 1.
van’t Hoff equation when the solid solubility is negligible. Hence,
negligible solubility was assumed. All intermediate compounds
have been modeled as stoichiometric, there being no data to the
contrary except for Mn6 Si and Mn9 Si2 . For these compounds, it
proved impossible to reproduce simultaneously all the data points
of Wieser and Forgeng [30] for the boundaries of the single-phase
regions of these phases and of the CUB phase with reasonable
thermodynamic parameters. Since there are no corroborating data
from other authors in this region of the phase diagram, it was
decided to treat these phases as stoichiometric.
Zaitsev et al. [6,44] reported vapor pressure measurements in
two-phase regions using high-temperature mass spectrometry.
The data from their first paper [6] are in good agreement with
the present calculations as can be seen in Fig. 5. The data from
their second paper [44] were presented only in the form of a small
ambiguous figure.
In a different work, Zaitsev et al. [7] reported vapor pressure
measurements of monatomic Si and Mn over liquid alloys by
Knudsen effusion/mass spectrometry. The data for Mn vapor
pressures are well reproduced as seen in Fig. 5(a). The calculated
vapor pressure of monatomic Si is shown in Fig. 6. The composition
dependence of the data in Fig. 6 is well reproduced by the model,
while the calculated curves and measured points differ by a nearly
constant value. In view of the expanded vertical scale of Fig. 6,
and the very low vapor pressure of Si, this difference is within
the experimental error limits. Since the authors [7] did not report
measurements of the vapor pressure of pure liquid Si, one cannot
check for consistency with the data for the vapor pressure of pure
Si [17] used in the present calculations.
Tanaka [45] by a transportation method at 1400 ◦ C, and Ahmad
and Pratt [46] by a torsion-effusion technique, measured Mn
partial pressures over liquid alloys. The present calculations are
compared with these vapor pressure data in Fig. 5. At high Si
contents, these data are inconsistent with the vapor pressure data
of Zaitsev et al. [7] in Fig. 5(a). The data of Tanaka [45] in Fig. 5(c)
are inconsistent with the vapor pressure data of Zaitsev et al. [6,7]
in Fig. 5(a) and with the vapor pressure of pure liquid Mn given by
JANAF [17].
Batalin and Sudavtsova [47] performed EMF measurements
in the range 1247–1427 ◦ C to report the partial excess Gibbs
energy of Mn in the liquid phase at 1400 ◦ C. These Gibbs energies
are compared with the present calculations in Fig. 7. These data
are inconsistent with the Mn vapor pressure data [6,7] in Fig. 5.
Previous optimizations [3,4] did not take into account the vapor
pressure data. In the present work, more weight was given to the
vapor pressure data [6,7] than to the EMF measurements.
Zaitsev et al. [48] derived standard enthalpies of formation
of intermediate compounds from vapor pressure measurements
Fig. 5. Optimized vapor pressure of monatomic Mn over liquid Mn–Si alloys.
using high-temperature mass spectroscopy. Their vapor pressure
data points were presented only in the form an ambiguous figure.
Meschel and Kleppa [8] measured the standard enthalpies of
formation of Mn5 Si3 and MnSi by direct synthesis calorimetery.
Zaitsev et al. [48] also cite data from other sources [49–53]. All
these data are shown on Fig. 8. In the present work, the greatest
weight was given to the results of Meschel and Kleppa [8].
It should be noted that the optimized entropies of formation of
the intermetallic compounds from the elements at 25 ◦ C (Table 1)
are all very small as is generally expected for such compounds. That
is, the optimized entropies are physically reasonable.
Gel’d et al. [54] and Esin et al. [55] measured partial enthalpies
of mixing in Mn–Si melts at 1500 ◦ C by high-temperature
isothermal calorimetry. It is unclear in the article of Esin et al. [55]
whether all their data points are experimental or if some points
were obtained by integration of the Gibbs-Duhem equation. These
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A. Shukla et al. / Computer Coupling of Phase Diagrams and Thermochemistry 32 (2008) 470–477
Fig. 6. Optimized vapor pressure of monatomic Si over liquid Mn–Si alloys.
Fig. 9. Optimized partial enthalpies of mixing at 1500 ◦ C in liquid Mn–Si alloys.
Fig. 10. Optimized integral enthalpy of mixing at 1500 ◦ C in liquid Mn–Si alloys.
Fig. 7. Optimized excess Gibbs energy of Mn in liquid Mn–Si alloys at 1400 ◦ C.
Fig. 11. Optimized entropy of mixing at 1500 ◦ C in liquid Mn–Si alloys.
◦
Fig. 8. Optimized standard enthalpy of formation at 25 C of the intermediate
compounds in the Mn–Si system.
data are compared with the present and previous optimizations
in Fig. 9. Batalin et al. [56] at 1450 ◦ C, and Gorbunov et al. [57]
at 1500 ◦ C, measured the integral enthalpy of mixing in liquid
solutions by high-temperature isothermal calorimetry. These data
are well reproduced by the present optimization as seen in Fig. 10.
The shape of the partial enthalpy curves in Fig. 9, which are
very negative with points of inflection and a point of intersection
near the equimolar composition, are strongly indicative of a high
degree of SRO about this composition. An entropy of mixing with
a strong minimum near the equimolar composition as in Fig. 11 is
thus expected. These shapes of the enthalpy and entropy of mixing
curves are well reproduced by the MQM.
4. Ternary systems
4.1. The Mg–Si–Zn system
The phase diagrams of the Mg–Zn and Mg–Si systems from
previous optimizations [10,12] are shown in Figs. 12 and 13
respectively. Spencer [10] obtained the optimized phase diagram
in Fig. 12 using the MQM for the liquid solution, and taking model
parameters for the solid phases from the optimization of Liang
A. Shukla et al. / Computer Coupling of Phase Diagrams and Thermochemistry 32 (2008) 470–477
475
Fig. 12. Previously optimized phase diagram of the Mg–Zn system [10].
Fig. 14. Predicted liquidus projection of the Mg–Si–Zn system.
Fig. 13. Previously optimized phase diagram of the Mg–Si system [12].
et al. [58]. His optimized model parameters for the liquid phase are
reported in Table 1. These previous optimizations were combined
with the present optimization of the Si–Zn system in order to
predict the thermodynamic properties and phase diagram of the
Mg–Si–Zn system.
Mutual solubilites between binary compounds were assumed
to be negligible, there being no experimental data available. All the
intermetallic compounds have different crystal structures (Table 2)
and different stoichiometries. These factors mitigate against there
being appreciable mutual solubility of the compounds. In the
absence of any evidence for ternary compounds, none were
assumed.
The thermodynamic properties of the ternary liquid phase
were calculated by the MQM from the binary model parameters.
The ‘‘symmetric approximation’’ [14,59] was used. No additional
ternary parameters were included. The resultant calculated
polythermal projection of the liquidus surface is shown in Fig. 14.
Bollenrath [60], using cooling curves, reported a partial
section along the Mg2 Si–MgZn2 join. These data are compared
with the calculated section in Fig. 15. When the ‘‘asymmetric
approximation’’ [14,59] was used with Si and Mg as ‘‘asymmetric
component’’, the calculated liquidus deviated by over 50◦ from the
measurements [60]. Hence the ‘‘symmetric approximation’’ was
preferred.
4.2. The Mg–Mn–Si system
The phase diagrams of the Mg–Mn and Mg–Si systems
from previous optimizations [11,12] are shown in Figs. 16 and
13 respectively. These optimizations were combined with the
Fig. 15. Optimized section of the Mg–Si–Zn phase diagram along the Mg2 Si–MgZn2
join.
Fig. 16. Previously optimized phase diagram of the Mg–Mn system [11].
present optimization of the Mn–Si system in order to predict the
thermodynamic properties and phase diagram of the Mg–Mn–Si
system. The resultant calculated polythermal projection of the
liquidus surface is shown in Fig. 17. No measurements of the
liquidus have been reported.
As in the Mg–Si–Zn system, and for the same reasons, no
mutual solubilities between binary compounds were assumed. In
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A. Shukla et al. / Computer Coupling of Phase Diagrams and Thermochemistry 32 (2008) 470–477
This results in a better representation of the partial properties of
the solutes in dilute solution in magnesium, the activities of solutes
in dilute solution being of much practical importance. As shown by
the present authors [5], the use of the MQM generally also results
in better estimations of the properties of ternary and higher-order
liquid alloys from binary model parameters. In the present work
the extension of the Mg–Mn binary liquid miscibility gap into
the ternary Mg–Mn–Si system is significantly smaller than would
be obtained by using a Bragg–Williams model. Also, the positive
deviations obtained in the present work along the Mg–MnSi join
would be over-estimated by a Bragg–Williams model. These better
predictions and estimations of phase equilibria will aid in the
design of novel magnesium alloys.
Acknowledgements
Financial support from General Motors of Canada Ltd. and
the Natural Sciences and Engineering Research Council of Canada
through the CRD grants program is gratefully acknowledged.
Fig. 17. Predicted liquidus projection of the Mg–Mn–Si system. Calculated
temperatures of invariants points are shown (◦ C).
the absence of any evidence for ternary compounds, none were
assumed.
The thermodynamic properties of the ternary liquid phase were
calculated by the MQM from the binary model parameters. No
additional ternary parameters were included. The ‘‘asymmetric
approximation’’ [14,59] with Si as ‘‘asymmetric component’’ was
used since the Mg–Mn liquid phase exhibit positive deviations
from ideal solution behavior whereas the Mg–Si and Mn–Si liquid
phases exhibit considerable negative deviations.
The Mg–Mn binary system exhibits a large liquid miscibility
gap (Fig. 16). With the addition of Si, this miscibility gap extends
into the ternary system. As shown by the present authors [5],
the use of the MQM generally results in better predictions of
ternary miscibility gaps than when a BW random-mixing model
is used. It should be noted that the consolute temperature of the
binary miscibility gap calculated in our previous optimization of
the Mg–Mn system [11], in which the liquid phase was modeled
by the MQM, is approximately 2000◦ lower than the calculated
consulate temperatures in earlier optimizations [61,62] in which
the BW random-mixing model was used. This results in a much
smaller extension of the calculated miscibility gap into the ternary
system in the present optimization.
In Fig. 17 the liquidus surface approximately along the
Mg–MnSi join is very flat. This is a result of the high degree of SRO
in the Mn–Si binary liquid phase which results in a tendency for Mn
and Si atoms to cluster and exclude Mg. As shown by the present
authors [5] such liquidus surfaces are generally predicted better by
the MQM than by BW or ‘‘associate’’ models.
5. Conclusions
Gibbs energy functions for all the phases in the binary Si–Zn and
Mn–Si systems have been obtained. All available thermodynamic
and phase equilibrium data have been critically evaluated in order
to obtain one set of optimized model parameters of the Gibbs
energies of all phases which can reproduce the experimental data
within experimental error limits. Evaluations and tentative phase
diagrams for the ternary systems Mg–Si–Zn and Mg–Mn–Si have
been given.
The use of the Modified Quasichemical Model results in a better
fitting of the partial enthalpy of mixing in the Mn–Si system than
is the case when a Bragg–Williams random-mixing model is used.
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