J. Am. Ceram. Soc., 83 [4] 919 –27 (2000)
journal
Ternary System Al2O3–MgO–CaO: Part II, Phase Relationships in the
Subsystem Al2O3–MgAl2O4–CaAl4O7
Antonio H. De Aza,† Juan E. Iglesias,‡ Pilar Pena,† and Salvador De Aza*,†
Instituto de Cerámica y Vidrio, CSIC, Madrid, Spain; and Instituto de Ciencia de Materiales, CSIC, Madrid, Spain
(CaAl12O19 or CA6§) or through the existence of a new ternary
phase within the above-mentioned ternary system, whose stoichiometric composition is tentatively assigned as CaMg2Al16O27
(CM2A8).
Göbbels et al.2 and Iyi et al.3 have published articles on this
topic. In the first article, the authors studied the subsolidus phase
relations in the Al2O3-rich part of the system Al2O3–MgO–CaO
and found two new compounds. These compounds are located on
the joint connecting CA6 and spinel (MgAl2O4). Their stoichiometric compositions are given as Ca2Mg2Al28O46 (C2M2A14) and
CaMg2Al16O27 (CM2A8), denoted as CAM I and CAM II by
Göbbels et al. Both compositions show limited solid solubility
range. On the other hand, in the second article, the structure
models of both compounds were established by high-resolution
electron microscopy, and structure refinements were conducted
using single-crystal X-ray diffractometry (XRD) data.
However, these works provide no detailed information on the
melting relationships of the new crystalline phases. Furthermore, a
theoretical study of the solid-state compatibilities in the Al2O3-rich
part of the system Al2O3–MgO–CaO published by Göbbels et al.2
showed that, once the “Law of Adjoining Phase Regions” of
Palatnick and Landau4 was taken into account, several errors in the
isothermal sections were obvious. Despite these errors, these
works were of significant assistance on clarifying the results of the
above-mentioned preliminary experiments conducted in the
present investigation.
The goal of the present work is to determine solid-state
compatibilities and melting relationships in the subsystem Al2O3–
MgAl2O4–CaAl4O7. The results obtained in Part I1 and literature
data are used to establish a phase diagram of the entire system
Al2O3–MgO–CaO.
Solid-state compatibility and melting relationships in the subsystem Al2O3–MgAl2O4–CaAl4O7 were studied by firing and
quenching selected samples located in the isopletal section
(CaO䡠MgO)–Al2O3. The samples then were examined using
X-ray diffractomtery, optical microscopy, and scanning and
transmission electron microscopies with wavelength- and
energy-dispersive spectroscopies, respectively. The temperature, composition, and character of the ternary invariant
points of the subsystem were established. The existence of two
new ternary phases (Ca2Mg2Al28O46 and CaMg2Al16O27) was
confirmed, and the composition, temperature, and peritectic
character of their melting points were determined. The isothermal sections at 1650°, 1750°, and 1840°C of this subsystem
were plotted, and the solid-solution ranges of CaAl4O7,
CaAl12O19, MgAl2O4, Ca2Mg2Al28O46, and CaMg2Al16O27
were determined at various temperatures. The experimental
data obtained in this investigation, those reported in Part I of
this work, and those found in the literature were used to
establish the projection of the liquidus surface of the ternary
system Al2O3–MgO–CaO.
I.
Introduction
in the Part I of this work,1 the ternary system
Al2O3–MgO–CaO is extremely important in many technological applications, in particular, refractories.
In Part I, the spinel primary phase field of crystallization in the
subsystem MgAl2O4–CaAl4O7–CaO–MgO was established to understand why spinel addition to high-Al2O3 concretes significantly
improves the wear resistance of these materials in secondary steel
refining ladles. In the present research, the work is mainly focused
on determining the solid-state compatibility and melting relationships in the subsystem Al2O3–MgAl2O4–CaAl4O7 to understand
properly phase constitution of the matrices of these materials and,
consequently, their melting behavior.
Preliminary experiments, conducted in the subsolidus region of
the system, showed that the solid-state compatibilities and melting
relationships within this part of the system Al2O3–MgO–CaO are
complexed, mainly because of the existence of phases whose
microanalyses (scanning electron microscopy–wavelengthdispersive spectroscopy (SEM-WDS)) agreed with none of the
compositions of the expected phases. The data could be explained
either as a wide but limited solid solution of MgO in the
magnetoplumbite structure of the calcium hexa-aluminate
A
S DISCUSSED
II.
Experimental Procedure
With the purpose of establishing the Al2O3-rich part of the
system Al2O3–MgO–CaO, nine selected compositions (Table I)
located in the isopletal section doloma¶–alumina ((CaO䡠MgO)–
Al2O3) in the range 76.04 –100 wt% Al2O3 were prepared and
studied.
An additional composition to those studied in the isopletal
section (CaO䡠MgO)–Al2O3 was also prepared. This composition
corresponded to the compound CaMg2Al16O27 (CM2A8). Calculated batches were weighed, starting from the materials described
in Part I of this work,1 and were processed in the manner described
in the previous paper. The samples were thermally treated between
1625° and 1950°C. Treatments up to 1725°C were made under the
conditions described in Part I. Thermal treatments between 1725°
and 1950°C were conducted in an argon atmosphere in a Brew
furnace (200 mm diameter and 200 mm high; LBL, Berkeley, CA)
with tantalum heating elements. The samples (5 mm diameter and
R. S. Roth—contributing editor
Manuscript No. 189562. Received February 12, 1999; approved September 20,
1999.
Supported by Plan Nacional de Materiales CICYT and European Union under
Project Nos. MAT97–0728 and BRPR-CT97–0427, respectively.
*Member, American Ceramic Society.
†
Instituto de Cerámica y Vidrio.
‡
Instituto de Ciencia de Materiales.
§
Cement notation is used in the text and figure captions of this article; i.e., C is
CaO, M is MgO, and A is Al2O3 (e.g., CA6 is CaO䡠6Al2O3 or CaAl12O19).
¶
Product derived from the burning or calcining of dolomite or dolomite rock. It
contains a mixture of MgO and CaO relative to proportions of originally existing
carbonates.
919
920
Journal of the American Ceramic Society—De Aza et al.
Table I. Selected Compositions in the Isopletal Section
(CaO䡠MgO)–Al2O3† in the Range of 76.04 wt% up to
100 wt% Al2O3
Designation
CaO
Composition (wt%)
MgO
Al2O3
77
79
81
83
85
88 (Ca2Mg2Al28O46)
89
91
94
CaMg2Al16O27‡
13.3815
12.2179
11.0543
9.8907
8.7270
6.9225
6.3998
5.2362
3.4908
5.8884
9.6185
8.7821
7.9457
7.1093
6.2730
4.9759
4.6002
3.7638
2.5092
8.4651
77
79
81
83
85
88.1016
89
91
94
85.6465
†
In the range 76.04–100 wt% Al2O3. ‡Additional composition outside of the
isopletal section.
6 mm long) were placed into molybdenum crucibles (15 mm long
and 8 mm in diameter) that were hermetically sealed by welding.
Temperature was controlled by a W5Re–W26Re thermocouple
(maximum temperature of 2320°C) encapsulated in a beryllium
sheath and connected to an electronic digital controller (⫾1°C;
Vol. 83, No. 4
Model Eurotherm 900, EPC, Edinburgh, U.K.). The temperature of
the furnace was frequently checked against the melting points of
platinum and rhodium, using an optical pyrometer.
At temperatures ⬍1725°C, the samples, after heat treatment,
were air quenched.1 In thermal treatments conducted above
1725°C in the Brew furnace, the samples were quenched inside the
furnace by switching off the power. This provided a very fast
cooling of the furnace chamber and of the samples, because the
furnace walls were refrigerated by a continuous flux of cold water.
Occasionally, the samples were reground after quenching, then
pressed and fired again to ensure the attainment of equilibrium.
After the samples were quenched, they were removed from the
platinum or molybdenum crucibles and mounted in epoxy resin.
The samples then were polished to 1 ␮m.
The phases present in the equilibrated specimens were determined and analyzed using the techniques described in Part I.
However, the two new phases (Ca2Mg2Al28O46 and CaMg2Al16O27)
were prepared by ion-beam thinning and were studied by transmission electron microscopy (TEM; Model JEM-2010, JEOL,
Tokyo, Japan) at 200 kV using a high-resolution technique
(HR-TEM) involving lattice plane imaging. All TEM samples
were carbon coated to ensure specimen stability in the electron
beam. X-ray powder pattern lattice parameter refinements of the
two new compounds were conducted (see Appendix).
Fig. 1. Experimental isopletal section (CaO䡠MgO)–Al2O3 in the range 76.04 –100 wt% Al2O3. Symbols represent samples studied (see Table I).
April 2000
Ternary System Al2O3–MgO–CaO
921
Fig. 2. SEM micrographs of typical samples within various fields of crystallization: (a) sample 77 at 1712° ⫾ 1°C for 4 ⫹ 7 h with coexisting phases
CaAl4O7 (CA2) ⫹ MgAl2O4(ss) (MA); (b) sample 79 at 1712° ⫾ 1°C for 4 ⫹ 7 h with coexisting phases CaAl2O4 (CA2) ⫹ MgAl2O4(ss) (MA) ⫹
CaMg2Al16O27(ss) (CM2A8); (c) sample 88 at 1725° ⫾ 1°C for 4 ⫹ 4 h with coexisting phases Ca2Mg2Al28O46(ss) (C2M2A14) ⫹ CaMg2Al16O27(ss)
(CM2A8); and (d) sample 94 at 1750° ⫾ 1°C for 12 ⫹ 12 h with coexisting phases Al2O3 ⫹ MgAl2O4(ss) (MA) ⫹ Ca2Mg2Al28O46(ss) (C2M2A14).
Solid-state compatibilities and melting relationships in the
subsystem Al2O3–MgAl2O4–CaAl4O7 were established using the
experimental procedure described in Part I.1
III.
Results and Discussion
The experimental segment 76.04 –100 wt% Al2O3 of the isopletal section of(CaO䡠MgO)–Al2O3 that was plotted with the
results obtained, after heat treatment at various temperatures of the
selected compositions, is shown in Fig. 1.
The data obtained confirmed the existence of the two new
compounds Ca2Mg2Al28O46 and CaMg2Al16O27 proposed by
Göbbels et al.2 These compounds were clearly identified by XRD
(see Appendix), SEM-WDS, and HR-TEM. HR-TEM also allowed
us to confirm the laminar structure of both compounds, being
polytypoids with structures derived from that of CA6. In the case
of the CaMg2Al16O27, the stacking sequence (MS)n†† also has
††
M is CA6 blocks; S is spinel blocks.
been confirmed; however, this is not the case for Ca2Mg2Al28O46,
which requires further studies (see Appendix).
The obtained section provides new information that is in
disagreement with published literature data.
The isopletal section shows the existence of the solid-state
compatibility Ca2Mg2Al28O46(ss)–Al2O3 that was omitted by
Göbbels et al.2 These phases are compatible up to 1725° ⫾ 10°C.
Above this temperature, the solid-state compatibility changes; the
phases Al2O3, MgAl2O4(ss), and Ca2Mg2Al28O46(ss) coexist up to
1830°⫾10°C. At this temperature, the compatibility changes
again; Al2O3, MgAl2O4(ss), and CaAl12O19(ss) are in equilibrium
with each other. This compatibility change is not indicated by
Göbbels et al. Finally, the phases Al2O3, MgAl2O4(ss), and
CaAl12O19(ss) coexist up to 1850° ⫾ 10°C, which is the temperature of the peritectic melting point of this subsystem.
The solid-state compatibility changes can be explained by
changes in the free energy of formation of the phases
Ca2Mg2Al28O46, CaMg2Al16O27, and MgAl2O4 as a consequence
of the variation of their solid solutions with the temperature.
The remaining ternary subsystems, indicated in the isopletal
section (Fig. 1), are in agreement with those reported by Göbbels
922
Journal of the American Ceramic Society—De Aza et al.
Vol. 83, No. 4
Table II. Samples Analyzed by SEM–WDS
Temperature
(°C)
Time of
treatment (h)
1650
26 ⫹ 25
26 ⫹ 25
26 ⫹ 25
26 ⫹ 25
26 ⫹ 25
26 ⫹ 25
1750
10 ⫹ 14
10 ⫹ 14
10 ⫹ 14
10 ⫹ 14
4
10 ⫹ 14
10 ⫹ 14
10 ⫹ 14 ⫹ 20
1840
4
4
4
3⫹7
Composition
Phases in equilibrium
77
79
85
88
91
CaMg2Al16O27
MgAl2O4(ss) ⫹ CaAl4O7
MgAl2O4(ss) ⫹ CaAl4O7 ⫹ CaMg2Al16O27
CaAl4O7 ⫹ CaMg2Al16O27(ss) ⫹ Ca2Mg2Al28O46
CaMg2Al16O27(ss) ⫹ Ca2Mg2Al28O46(ss)
Ca2Mg2Al28O46(ss) ⫹ Al2O3
CaMg2Al16O27(ss) ⫹ MgAl2O4(ss)?
77
79
81
83
85
88
89
91
MgAl2O4(ss) ⫹ liquid
MgAl2O4(ss) ⫹ CaMg2Al16O27(ss) ⫹ liquid
MgAl2O4(ss) ⫹ CaMg2Al16O27(ss) ⫹ liquid
MgAl2O4(ss) ⫹ CaMg2Al16O27(ss) ⫹ liquid
CaMg2Al16O27(ss) ⫹ Ca2Mg2Al28O46(ss) ⫹ liquid
CaMg2Al16O27(ss) ⫹ Ca2Mg2Al28O46(ss)
CaMg2Al16O27(ss) ⫹ Ca2Mg2Al28O46(ss)
MgAl2O4(ss) ⫹ Ca2Mg2Al28O46(ss) ⫹ Al2O3
81
88
89
94
Liquid
MgAl2O4(ss) ⫹ CaAl12O19(ss) ⫹ liquid
MgAl2O4(ss) ⫹ CaAl12O19(ss) ⫹ liquid
MgAl2O4(ss) ⫹ CaAl12O19(ss) ⫹ Al2O3
et al.2 The temperature and character of the ternary subsystem
invariant points also are indicated in Fig. 1. (These data and their
compositions are presented later in Table V.)
Figures 2(a)–(d) show typical SEM microstructures, within
different fields of crystallization, of samples after quenching from
various annealing temperatures.
To determine the projection of the liquidus surface of the
subsystem Al2O3–MgAl2O4–CaAl4O7, the composition of the
various phases coexisting at equilibrium in selected samples,
within the isopletal section (Fig. 1) at different temperatures, were
quantitatively determined by SEM-WDS. Table II shows the
selected compositions, their temperatures, the times of treatment,
and the coexisting phases.
For the sake of simplicity, the analyzed samples and the results
obtained for a series of isothermal sections at 1650°, 1750°, and
1840°C are discussed below.
(1) Isothermal Section at 1650°C
The isothermal section at 1650°C was plotted by projecting the
data obtained from compositions treated at 1650°C (see Fig. 1)
together with data reported by Göbbels et al.2 This section, shown
in Fig. 3, is subsolidus throughout. The various solid-state compatibility relationships have been indicated and described in the
figure caption. Numbers in italic represent experimental samples
used to construct the figure (see Fig. 1 and Table II). The data of
the solid-solution ranges determined and used to plot the isothermal section are shown in Table III. This experimental section
obeys the “Law of the Adjoining Phase Regions.”4
(2) Isothermal Section at 1750°C
The isothermal section at 1750°C is shown in Fig. 4. It was
plotted using data obtained in the study of compositions treated at
Fig. 3. Isothermal section at 1650°C of the subsystem Al2O3–MgAl2O4–CaAl4O7. Coexisting phases are (a) CaAl4O7 ⫹ MgAl2O4(ss), (b) CaAl4O7 ⫹
MgAl2O4(ss) ⫹ CaMg2Al16O27(ss), (c) MgAl2O4(ss) ⫹ CaMg2Al16O27(ss), (d) MgAl2O4(ss) ⫹ CaMg2Al16O27(ss) ⫹ Al2O3, (e) CaMg2 Al16O27(ss) ⫹
Al2O3, (f) CaMg2Al16O27(ss) ⫹ Al2O3 ⫹ Ca2Mg2Al28O46(ss), (g) Al2O3 ⫹ Ca2Mg2Al28O46(ss), (h) Al2O3 ⫹ Ca2Mg2Al28O46(ss) ⫹ CaAl12O19 (ss), (i)
Al2O3 ⫹ CaAl12O19(ss), (j) CaAl12O19(ss), (k) CaAl12O19(ss) ⫹ CaAl4O7, (l) CaAl12O19(ss) ⫹ CaAl4O7 ⫹ Ca2Mg2Al28O46(ss), (m) CaAl4O7 ⫹
Ca2Mg2Al28O46(ss), (n) CaAl4O7 ⫹ Ca2Mg2Al28O46(ss) ⫹ CaMg2Al16O27(ss), (o) CaAl4O7 ⫹ CaMg2Al16O27(ss), (p) CaMg2Al16O27(ss), (q)
CaMg2Al16O27(ss) ⫹ Ca2Mg2Al28O46(ss), (r) Ca2Mg2Al28O46(ss), and (s) Ca2Mg2Al28O46(ss) ⫹ CaAl12O19(ss). Numbers in italics are experimental
compositions (see Fig. 1).
April 2000
Ternary System Al2O3–MgO–CaO
923
Table III. Solid-Solution Ranges at 1650°C
Solid-state compatibilities
†
Mg1–3zAl2⫹2zVazO4
(1) CaAl4O7–MgAl2O4(ss)
(2) CaMg2Al16O27(ss) ⫹ MgAl2O4(ss) ⫹ CaAl4O7
(3) CaMg2Al16O27(ss)–MgAl2O4(ss)
(16) CaMg2Al16O27(ss)
(18) Ca2Mg2Al28O46(ss)
Ca2Mg2–3xAl28⫹2xO46
0 ⱕ z ⱕ 0.07
z ⫽ 0.07
0.07 ⱕ z ⱕ 0.12
0 ⱕ x ⱕ 0.30
CaMg2–3yAl16⫹2yO27
y⫽0
0 ⱕ y ⱕ 0.15
0 ⱕ y ⱕ 0.20
CaAl4O7
CaAl4O7
CaAl4O7
†Numbers in parantheses correspond to those assigned in to the different solid-state compatibilities Fig. 3.
Fig. 4. Isothermal section at 1750°C of the subsystem Al2O3–MgAl2O4–CaAl4O7. Coexisting phases are (a) MgAl2O4(ss) ⫹ liquid, (b) MgAl2O4(ss) ⫹
CaMg2Al16O27(ss) ⫹ liquid, (c) MgAl2O4(ss) ⫹ CaMg2Al16O27(ss), (d) MgAl2O4(ss) ⫹ CaMg2Al16O27(ss) ⫹ Ca2Mg2Al28O46(ss), (e) MgAl2O4(ss) ⫹
Ca2Mg2Al28O46(ss), (f) MgAl2O4(ss) ⫹ Ca2Mg2Al28O46(ss) ⫹ Al2O3, (g) Ca2Mg2Al28O46(ss) ⫹ Al2O3, (h) Ca2Mg2Al28O46(ss) ⫹ Al2O3 ⫹ CaAl12O19(ss),
(i) Al2O3 ⫹ CaAl12O19(ss), (j) CaAl12O19(ss), (k) CaAl12O19(ss) ⫹ CaAl2O7, (l) CaAl12O19(ss) ⫹ CaAl2O7 ⫹ Ca2Mg2Al28O46(ss), (m) CaAl2O7 ⫹
Ca2Mg2Al28O46(ss), (n) CaAl2O7 ⫹ Ca2Mg2Al28O46(ss) ⫹ liquid, (o) Ca2Mg2Al28O46(ss) ⫹ liquid, (p) Ca2Mg2Al28O46(ss) ⫹ CaMg2Al16O27(ss) ⫹ liquid,
(q) CaMg2Al16O27(ss) ⫹liquid, (r) CaMg2Al16O27(ss), (s) CaMg2Al16O27(ss) ⫹ Ca2Mg2Al28O46(ss), (t) Ca2Mg2Al28O46(ss), and (u) Ca2Mg2Al28O46(ss) ⫹
CaAl12O19(ss). Numbers in italics are experimental compositions (see Fig. 1).
1750°C within the isopletal section (Fig. 1). The different phase
relationships are indicated in the figure and described in the figure
caption. The solid-solution ranges determined and used to plot the
isothermal section are shown in Table IV. In the isothermal section
at 1750°C, liquid phases already appear, which implies that the
isopletal section at 1800°C proposed by Göbbels et al.2 is not
accurate. In this section the “Law of the Adjoining Phase Regions”4 is also fulfilled.
The composition of the spinel and CA6 coexisting with liquid at
1840°C correspond to Mg1–3zAl2⫹2zVazO4 with z ⫽ 0.24 and
CaMgxAl12–2x/3O19 with x ⫽ 0.18. However, the solid solutions in
Al2O3 and CA6 coexisting with spinel at 1840°C are negligible and
within experimental error. For simplicity, as previously presented,
the different phase relations have been marked in the figure and
described in the figure caption This experimental section also
obeys the “Law of the Adjoining Phase Regions.”4
(3) Isothermal Section at 1840°C
The isothermal section at 1840°C was plotted with data obtained from experimental compositions treated at the mentioned
temperature (see Fig. 1). Figure 5 shows this isothermal section.
(4) Liquidus Surface of the Subsystem Al2O3–MgAl2O4–
CaAl4O7
With all the information obtained, the projection of the liquidus
surface of the subsystem Al2O3–MgAl2O4–CaAl4O7 was established. Figure 6 shows the outline of this liquidus surface. The
Table IV. Solid-Solution Ranges at 1750°C
Coexisting phases
†
(1) MgAl2O4(ss) ⫹ liquid
(2) MgAl2O4(ss) ⫹ CaMg2Al16O27(ss) ⫹ liquid
(3) MgAl2O4(ss) ⫹ CaMg2Al16O27(ss)
(6) MgAl2O4(ss) ⫹ Ca2Mg2Al28O46(ss) ⫹ Al2O3
(16) CaMg2Al16O27(ss) ⫹ Ca2Mg2Al28O46(ss) ⫹ liquid
†
Mg1–3zAl2⫹2zVazO4
0 ⱕ z ⱕ 0.09
z ⫽ 0.09
0.09 ⱕ z ⱕ 0.19
z ⫽ 0.21
Numbers in parentheses correspond to those assigned to the different phase compatibilities in Fig. 4.
Ca2Mg2–3xAl28⫹2xO46
x ⫽ 0.26
x⫽0
CaMg2–3yAl16⫹2yO27
y⫽0
0 ⱕ y ⱕ 0.20
y ⫽ 0.05
Al2O3
924
Journal of the American Ceramic Society—De Aza et al.
Vol. 83, No. 4
Fig. 5. Isothermal section at 1840°C of the subsystem Al2O3–MgAl2O4–CaAl4O7. Coexisting phases are (a) MgAl2O4(ss) ⫹ liquid, (b) MgAl2O4(ss) ⫹
CaAl12O19(ss) ⫹ liquid, (c) MgAl2O4(ss) ⫹ CaAl12O19(ss), (d) MgAl2O4(ss) ⫹ CaAl12O19(ss) ⫹ Al2O3, (e) CaAl12O19(ss) ⫹ Al2O3, (f) CaAl12O19(ss), and
(g) CaAl12O19(ss) ⫹ liquid. Numbers in italics are experimental compositions (see Fig. 1).
Fig. 6. Projection of the liquidus surface of the subsystem Al2O3–MgAl2O4–CaAl4O7. For composition, temperature, and character of the invariant points,
see Table V.
compositions, temperatures, and character of the various invariant
points within the subsystem are shown in Table V. For the sake of
simplicity, the solid-state compatibility triangles have been omitted at the temperature of the corresponding invariant points.
The new phases Ca2Mg2Al28O46 and CaMg2Al16O27 (see
Appendix) melt incongruently at 1830° ⫾ 10°C and 1820° ⫾
10°C, respectively, according to the reactions
Ca2Mg2Al28O46 3 MgAl2O4 ⫹ CaAl12O19 ⫹ liquid
CaMg2Al16O27 3 MgAl2O4 ⫹ Ca2Mg2Al28O46 ⫹ liquid
High temperatures are observed for the first liquid formation
(ⱖ1730°C) in this region of the ternary system Al2O3–MgO–CaO.
This justifies the high refractoriness of materials designed in this
area of the system, especially if they are formulated in the
subsystem Al2O3–MgAl2O4(ss)–CaAl12O19, where the temperature of first liquid formation is 1850°⫾10°C (Table V).
(5) Liquidus Surface of the System Al2O3–MgO–CaO
The experimental data obtained in Parts I and II of this
investigation and the data collected from the literature were used to
establish the entire projection of the liquidus surface of the ternary
system Al2O3–MgO–CaO (Fig. 7). The composition, temperatures, and character of all the invariant points within the system are
indicated in Table V.
The liquidus surface shows a large primary field of crystallization of the spinel within the ternary system Al2O3–MgO–CaO, as
compared with those of calcium aluminates, which means low
April 2000
Ternary System Al2O3–MgO–CaO
925
Table V. Composition, Temperature, and Character of Invariant Points within Ternary System Al2O3–MgO–CaO†
Points
p1
e1
e2
e3
e4
p2
e5
e6
e7
P1
E1
E2
E3
P2
P3
P4
E4
P5
P6
P7
P8
P9
Al2O3
CaO
MgO
†
Temperature
(°C)
1539
1400
1390
1590
1776
1883
1995
1975
2370
1450
1321
1346
1344
1350
1372
1567
1730
1760
1740
1820
1830
1850
2045
2625
2825
Al2O3
⫾5
42.8
48.31
53.03
65.75
79.74
87.37
55
96.01
⫾5
42.3
47.85
50
50.74
51.11
52.5
63.2
78.25
79.25
78.75
80.0
81.2
86.0
100
⫾3
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
2
2
10
10
10
10
10
10
5
Composition (wt%)
CaO
MgO
57.2
51.69
46.97
34.25
20.26
12.63
67
51.5
46.40
44.26
43.88
42.59
40.55
33.3
18.0
19.75
19.0
14.5
12.5
6.5
45
3.99
33
6.20
5.75
5.74
5.38
6.30
6.95
3.50
3.75
1.0
2.25
5.5
6.3
7.5
100
100
Comments and references
Peritectic (Nurse et al.6)
Eutectic (Chatterjee and Zhmoidin7)
Eutectic (Chatterjee and Zhmoidin7)
Eutectic (Rolin and Pham8,9)
Eutectic (Rolin and Pham8,9 and Hallstedt10)
Peritectic (Hallstedt10)
Eutectic (Alper et al.11)
Eutectic (Viechnicki et al.12)
Eutectic (Doman et al.13)
Peritectic (Ranking and Merwin14)
Eutectic (Majundar15)
Eutectic (Majundar15)
Eutectic (Majundar15)
Peritectic (Majundar15)
Peritectic
Peritectic
Eutectic
Peritectic
Peritectic
Peritectic
Peritectic
Peritectic
In Ar (Viechnicki et al.12)
(Doman et al.13)
In N2 (McNally et al.16)
Points whose text is italicized are those determined in the present investigation and in Part I of this work.1
solubility and adequate chemical resistance to slags containing
CaO–MgO.
IV.
Conclusions
(a) The projection of the liquidus surface of the subsystem
Al2O4–MgAl2O4–CaAl4O7 has been experimentally established.
The solid-state compatibility relations, compositions, temperatures, and character of the various invariant points and the
extension of the primary phase fields of crystallization of the
various phases also have been determined.
(b) The existence of two new ternary phases Ca2Mg2Al28O46
and CaMg2Al16O27 within the subsystem have been confirmed.
The temperature, composition, and peritectic character of their
melting points also have been established.
(c) The range of the solid solutions, at different temperatures,
of the phases CaAl4O7, CaAl12O19, MgAl2O4, Ca2Mg2Al28O46,
and CaMg2Al16O27 also have been determined within the various
compatibility triangles.
(d) The high temperatures of first liquid formation involved
(ⱖ1730° ⫾ 10°C) in the Al2O3-rich part of the ternary system
Al2O3–MgO–CaO justifies the high refractory characteristics of
materials designed in this area of the system, particularly if they
are formulated within the subsystem Al2O3–MgAl2O4–
CaAl12O19, where the temperature of first liquid formation is
1850°⫾10°C.
(e) The results obtained confirm the greater refractoriness of
the high-Al2O3 refractory concretes bonded with spinel and
calcium aluminate cements, as compared with that of the traditional high-Al2O3 concretes bonded only with calcium aluminate
cements, in which the highest temperature of initial liquid-phase
formation is ⬃1547° ⫾ 5°C.
(f) The phase diagram of the entire ternary system Al2O3–
MgO–CaO has been established using all the experimental and
literature information obtained.
Appendix
(1) Phase Characterization of Ca2Mg2Al28O46 and
CaMg2Al16O27
Ca2Mg2Al28O46 and CaMg2Al16O27 were obtained from stoichiometric amounts of 99.99-wt%-pure Al2O3 (Fluka AG, Buchs,
Switzerland), 99.5-wt%-pure CaCO3, and 99.9-wt%-pure MgO (E.
Merk, Darmstadt, Germany). The samples were homogenized,
isostatically pressed at 200 MPa, and solid-state reacted at 1700°C
for various lengths of time with repeated millings and reheatings.
The powder patterns of these materials were obtained on a
diffractometer (R ⫽ 230 mm, Model X⬘pert, Philips, Eindhoven,
The Netherlands) fitted with an incident-beam Ge (111) monochromator of the symmetric Johansson type using CuK␣1 (␭ ⫽
1.5405981 Å) radiation. Soller slits of 1.1° axial divergence were
placed in the path of the diffraction beam; the equatorial divergence was 0.5°. The ␪/2␪ scan mode was used, and intensities were
measured for 10 s at 0.03° intervals while the sample was rotated
around an axis normal to its plane at ⬃2 Hz. The patterns were
obtained using the silicon powder 2␪/d spacing standard for XRD
(Reference Material 640b, a ⫽ 5.430940 Å, National Institute for
Standards and Technology, Gaithersburg, MD).17
(2) CaMg2Al16O27
The powder pattern in Göbbels et al.2 was used to assign Miller
indexes to the strongest reflections at low Bragg angles. With our
2␪ values for 16 reflections thus indexed, lattice parameters were
determined by a least-squares procedure. With these parameters
and the atom coordinates found in space group P⫺6m2 by Iyi et
al.,3 the powder pattern was computed. Subsequently weaker
reflections were unambiguously assigned. The procedure was
repeated several times, finally obtaining 80 indexed reflections,
11° ⱕ 2␪ ⱕ 90°, 39 of which are not listed in the pattern of
Göbbels et al. The reflections 2 0 15 and 1 2 10 are calculated as
moderately strong, and their Bragg angles differ by ⬍0.03°;
consequently, the uniquely observed 2␪ value was assigned to both
reflections, and they were each given half-weight in the leastsquares lattice parameters refinement. Other incompletely resolved
reflections are duly marked in Table AI,‡‡ where the indexed
pattern is presented.
The quality of the indexing can be judged from the estimator
R ⫽ 兺兩2␪obs – 2␪calc兩/兺2␪obs, R ⫽ 0.00017, and from the usual5
figure of merit M20 ⫽ 50 (具ε典 ⫽ 0.000034(sin2 ␪), N ⫽ 30). The
‡‡
For Table AI, order ACSD–333 from Data Depository Service, The American
Ceramic Society, 735 Ceramic Place, Westerville, OH 43081.
926
Journal of the American Ceramic Society—De Aza et al.
Vol. 83, No. 4
Fig. 7. Projection of the liquidus surface of the system Al2O3–MgO–CaO. For composition, temperature, and character of the invariant points, see Table V.
maximum deviation between calculated and observed values is
兩⌬2␪兩max ⫽ 0.04°, which occurs four times in 80 reflections; the
average value is 兩⌬2␪兩 ⫽ 0.01°. From this indexing, we obtained
the lattice parameters a ⫽ 5.6002(2) Å and c ⫽ 31.350(1) Å.
Although the sample used was quite pure, seven weak reflections were impossible to index, and they were later recognized as
the seven strongest reflections of spinel, MgAl2O4; two more
reflections at 2␪ ⫽ 23.53° (I ⫽ 2) and 63.22 (I ⫽ 9) were left
unindexed.
(3) Ca2Mg2Al82O46
The indexing of the powder pattern was more difficult in this
case because of the presence of a significant quantity of
CaMg2Al16O27 in our sample. In fact, of 68 peaks measured to
2␪ ⫽ 60°, at least 26 belong to CaMg2Al16O27. Nevertheless, the
successive-approach method outlined above for CaMg2Al16O27
also converged adequately, producing an indexing with many
differences from that of Göbbels et al.,2 which appeared to be
indexed on cell geometric information only, before structure
determination. Calculated intensities in space group R3៮ m, using
the model of Iyi et al.,3 lead, for many observed peaks, to Miller
indexes different from those assigned by Göbbels et al. Our lattice
parameters, obtained from least-squares refinement of 39 reflections unambiguously assigned to Ca2Mg2Al82O46 are a ⫽
5.5812(3) Å and c ⫽ 79.921(5) Å.
(4) Discussion
We obtained lattice parameters for CaMg2Al16O27 greater
than those given by Göbbels et al.2 (a ⫽ 5.5926(2) Å and c ⫽
31.297(14) Å). The difference in a is 0.007 Å, which represents
⬃35␴, independent of standard deviation used; the difference in c
is 0.05 Å, which represents 5␴ if the ␴ value of Göbbels et al. is
used, or 50␴ with our value of the standard deviation. In this
connection, our ␴ values correspond to relative errors of the same
order of magnitude in both parameters, i.e., ␴c/␴a ⫽ 5, because
c/a ⫽ 5.6, which is considered normal. On the contrary, ␴c/␴a ⫽
70 in Göbbels et al. indicates some pathologic condition in their
data; moreover, Göbbels et al. offer no explanation for the
differences between their powder lattice parameters and those
obtained (by Iyi et al.3) from single-crystal data. We conclude that
our indexed pattern for CaMg2Al16O27 (see Table AI) represents
an improvement over that previously published.
The same conclusion cannot drawn for Ca2Mg2Al82O46, because we could not obtain a sample of sufficient purity. Our larger
values for a and c (Göbbels et al.2 found a ⫽ 5.5710(1) Å and c ⫽
April 2000
Ternary System Al2O3–MgO–CaO
79.770(12) Å) represent a difference of ⬃30␴ if our values of the
standard deviation are used, which can be attributed to the fact that
we were measuring, in all probability, a solid solution. Again, our
␴ ratio, ␴c/␴a ⫽ 17, is more reasonable (c/a ⫽ 14) than that of
Göbbels et al., ␴c/␴a ⫽ 120. The Göbbels et al. powder pattern can
be improved upon, because many reflections are incorrectly
indexed.
Acknowledgments
The authors wish to thank Dr. A. P. Tomsia and Dr. Eduardo Saiz of Lawrence
Berkeley Laboratory for the use of the Materials Science Division facilities and their
help and assistance during the SEM-WDS analyses. Thanks are also expressed to Dr.
Z. B. Luklinska of the Materials Science Department, Queen Mary and Westfield
College, University of London, for providing help with HR-TEM experiments.
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