Ternary System Al2O3—MgO—CaO: Part II, Phase Relationships in the Subsystem Al2O3—MgAl2O4—CaAl4O7

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·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.     

Thermodynamic Modeling of the MgO–Al2O3–CrO–Cr2O3 System

A complete critical evaluation and thermodynamic modeling of the phase diagrams and thermodynamic properties of the MgO–Al₂O₃–CrO–Cr₂O₃ system at 1 bar total pressure are presented. Optimized equations for the thermodynamic properties of all phases are obtained which reproduce all available thermodynamic and phase equilibrium data within experimental error limits from 25°C to above the liquidus temperatures at all compositions and oxygen partial pressures. The optimized thermodynamic properties and phase diagrams are believed to be the best estimates presently available. The database of the model parameters can be used along with software for Gibbs energy minimization in order to calculate any type of phase diagram section.     

Influence of the Characteristics of Spinels on the Slag Resistance of Al2O3–MgO and Al2O3–Spinel Castables

The XRD patterns at ambient temperature and at 1500°C showed that the spinel in the Al₂O₃–MgO castables fired at 1500°C for 3 h has the higher peak intensity, compared to those in Al₂O₃–spinel castables; the interplanar distance in the set (311) is 2.43 Å for the spinel in Al₂O₃–MgO castables as well as the spinels in Al₂O₃–spinel castables using spinels containing 73, 90, and 94 wt% Al₂O₃, respectively. The corresponding alumina contents of the spinels in these castables were estimated to be around 75 wt%. The smaller grain size of the spinel in Al₂O₃–MgO castables compared to that in Al₂O₃–spinel castables is evidenced by the recrystallization of the in situ spinel only occurring in Al₂O₃–MgO castables as revealed by the XRD patterns at ambient temperature and at 1500°C. The larger amount and smaller grain size of the in situ spinel in the matrix mostly account for the better slag resistance of Al₂O₃–MgO castables, compared to Al₂O₃–spinel castables.     

Determination of CaO–SiO2–MgO–Al2O3–CrOx Liquidus

In this work, the liquidus of synthetic CaO–SiO₂–MgO–Al₂O₃–CrO _x slags is evaluated in the industrially relevant compositional domain. Equilibrium experiments are carried out at 1500°C and partial oxygen pressure ( p _O2) 10^−11.04 atm, and at 1600°C and p _O2=10^−10.16 and 10^−9.36 atm. The studied basicities (CaO/SiO₂) are 1.2 and 0.5. Al₂O₃ levels range from 0 to 30 wt%. Oversaturated liquid is sampled and phase relations are measured with quantitative electron probe microanalysis–wavelength dispersive spectroscopy (EPMA–WDS). The results are compared with the commercially available FactSage thermodynamic databases. Qualitative agreement is always obtained. Also a good quantitative agreement is found at the higher basicity, especially for the spinel liquidus. A minor but systematic deviation can be observed for the eskolaite liquidus. At the lower basicity, the calculated phase diagram deviates strongly from the experimental results, probably due to missing ternary interactions in the database.     

Quaternary System Al2O3–CaO–MgO–SiO2: I, Study of the Crystallization Volume of Al2O3

Compatibility relations of Al₂O₃ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching followed by microstructural and energy-dispersive X-ray examination. A projection of the liquidus surface of the primary phase volume of Al₂O₃ was constructed in terms of the CaO, SiO₂, and MgO contents of the mixtures recalculated to 100 wt%. Two invariant points, where four solids coexist with a liquid phase, were defined, and the positions of the isotherms were tentatively established. The effect of SiO₂, MgO, and CaO impurities on Al₂O₃ growth also was studied.     

Crystallization of 60SiO2–20MgO–10Al2O3–10BaO Glass Ceramics

Three distinctly different microstructures of silica (as quartz and crystobalite), alumina, enstatite, and celsian, were found to develop in a 60SiO₂–20MgO–10Al₂O₃–10BaO glass ceramic. At 1010°C, growth of wormy fibrillar crystals was observed, indicating that crystal growth was diffusion controlled. At the intermediate temperature of 1080°C, a coarse cellular microstructure developed with multiple spherical particles nucleated on their surfaces and in the surrounding glass. At 1200°C, the glass crystallizes in a denderitic morphology but the dendrites were actually fragmented into multiple cube-shaped enstatite crystals, indicating a transition to interface-controlled growth. The crystals coarsen with time but maintain their order along the dendrite skeletons.     

Phase Equilibria in High-Alumina Part of the System CaO–MgO–Al2O3–SiO2

Results are presented of a study of phase equilibria among crystalline and liquid phases in the quaternary system CaO–MgO-Al₂O₃–SiO₂ at Al₂O₃ contents greater than 35%. Equilibrium diagrams shown are for the five triangular joins CaAl₂Si₂O₃-Ca₂Al₂SiO₇-MgAl₂O₄, Ca₂Al₂SiO₇-MgAl₂O₄-Al₂O₃, CaAl₂Si₂O₈-MgO-Al₂O₃, CaAl₂Si₂O₈-Mg₂SiO₄-MgAl₂O₄, and CaAl₂Si₂O₈-MgO-Mg₂SiO₄. The composition and nature of the four quaternary peritectic points and the relationships of univariant lines and primary phase volumes are discussed.     

Thermodynamic Assessment of the CaO–MgO–Al2O3 System

The system CaO–MgO–Al₃O₃ has been assessed with the Calphad technique using a computerized optimization procedure called parrot . The rather meager experimental information, mainly on liquidus relations, is described reasonably well, but the lack of data, especially on solid-phase relations, implies that the present assessment should be regarded as provisional. The system contains one stable ternary phase with the stoichiometry 3CaO·2Al₂O₃·MgO.     

Phase Relations in the System Al2O3–B2O3–Nd2O3

The phase relations at a temperature below "subsolidus" in the system Al₂O₃–B₂O₃–Nd₂O₃ are reported. Specimens were prepared from various compositions of Al₂O₃, B₂O₃, and Nd₂O₃ of purity 99.5%, 99.99%, and 99.9%, respectively, and fired at 1100°C. There are six binary compounds and one ternary compound in this system. The ternary compound, NdAl₃(BO₃)₄ (NAB), has a phase transition at 950°C ± 15°C. The high-temperature form of NAB has a second harmonic generation (SHG) efficiency of KH₂PO₄ (KDP) of the order of magnitude of the form which has been used as a good self-activated laser material, and the low-temperature form of NAB has no SHG efficiency.     

Subsolidus Phase Relationships in the Ga2O3–Al2O3–TiO2 System

Subsolidus phase relationships in the Ga₂O₃–Al₂O₃–TiO₂ system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO₂ (rutile), Ga_{2−2 x} Al_{2 x} O₃ ( x ≤0.78 β-gallia structure), Al_{2−2 y} Ga_{2 y} O₃ ( y ≤0.12 corundum structure), Ga_{2−2 x} Al_{2 x} TiO₅ (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga_{4−4 x} Al_{4 x} Ti _{n −4}O_{2 n −2} ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.     

Phase Equilibrium Relationships in a Portion of the System MgO–Al2O3–2CaO·SiO2

The system MgO–Al₂O₃–2CaO·SiO₂ comprises a plane through the tetrahedron CaO–MgO–Al₂O₃–SiO₂. A total of 108 compositions were prepared having an alumina content below the line joining 2CaO·Al₂O₃SiO₂ (gehlenite) and MgO·Al₂O₃ (spinel). Quenching experiments were carried out on 96 of these compositions at temperatures up to 1590°C. Three binary eutectic systems and two ternary eutectic systems are described. Compositions on this plane are of significance in an investigation of the constitution of basic refractory clinkers made from Canadian dolomitic magnesites. They also concern the compositions of certain blast furnace slags.     

Effect of Alkali Metal Oxide Addition on the Sinterability of MgO–B2O3–Al2O3 Glass–Al2O3 Filler Composites

The sintering of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler is terminated due to the crystallization of Al₄B₂O₉ in the glass. The densification of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler using pressureless sintering was accomplished by lowering the sintering temperature of the composite. The sintering temperature was lowered by the addition of small amounts of alkali metal oxides to the MgO–B₂O₃–Al₂O₃ glass system. The resultant composite has a four-point bending strength of 280 MPa, a coefficient of thermal expansion (RT—200°C) of 4.4 × 10⁻⁶ K⁻¹, a dielectric constant of 6.0 at 1 MHz, porosity of approximately 1%, and moisture resistance.     

Electroconductive Al2O3–NbN Composites

Electroconductive Al₂O₃–NbN ceramic composites were prepared by hot pressing. Dense sintered bodies of ball-milled Al₂O₃–NbN composite powders were obtained at 1550°C and 30 MPa for 1 h under a nitrogen atmosphere. The bending strength and fracture toughness of the composites were enhanced by incorporating niobium nitride (NbN) particles into the Al₂O₃ matrix. The electrical resistivity of the composites decreased with increasing amount of NbN phase. For a 25 vol% NbN–Al₂O₃ composite, the values of bending strength, fracture toughness, Vickers hardness, and electrical resistivity were 444.2 MPa, 4.59 MPa·m^1/2, 16.62 GPa, and 1.72 × 10⁻²Ω·cm, respectively, making the composite suitable for electrical discharge machining.     

Al2O3–ZrO2–SiO2 Ternary Glasses for Molybdenum Oxidation Barriers

The wettability of binary and ternary glasses belonging to SiO₂–Al₂O₃–ZrO₂ diagram has been studied using the sessile drop technique at 1750° and 1800°C. The ternary SiO₂–Al₂O₃–ZrO₂ (90–5–5 wt%) glass has proved to be well appropriated as a molybdenum oxidation barrier coating. The addition of 5 wt% of MoO₂ slightly improves its wettablity at higher temperatures without affecting its oxidation barrier properties. The Mo comes into the glass network as a mixture of Mo⁵⁺, Mo⁴⁺, and Mo⁶⁺. After oxidation at 1000°C in oxygen atmosphere, the molybdenum remains in the glass network as Mo⁶⁺.     

Quaternary System Al2O3–CaO–MgO–SiO2: II Study of the Crystallization Volume of MgAl2O4

Solid-state compatibility and melting relations of MgAl₂O₄ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching selected samples located in the 65 wt% MgAl₂O₄, plane followed by microstructural and energy dispersive X-ray analysis. A projection of the liquidus surface of the primary crystallization volume of MgAl₂O₄ was constructed from CaO, SiO₂ and exceeding Al₂O₃, not involved in stoichiometric MgAl₂O₄ formation; those three amounts were recalculated to 100 wt%. The temperature and character of six invariant points, where four solids co-exist with a liquid phase, were defined. One maximum point was localized and the positions of the isotherms were tentatively established. The effect of CaO, SiO₂, and Al₂O₃ impurities on the high temperature behavior of spinel materials was also discussed.     

Crystal Phases in the System Al2O3–AlN

Fused aluminum oxide containing a small amount of nitrogen (as added aluminum nitride) showed X-ray powder diffraction patterns similar to the δ-alumina obtained from the calcination of alumina mouohydrate in dry air. About 1% nitrogen was adequate for complete conversion of the α phase to the δ phase. A fused mixture containing 3.9% nitrogen showed a powder pattern identical to lithium ζ-alumina.     

Phase Equilibria in the System HfO2–Y2O3–CaO

Phase equilibria in the system HfO₂–Y₂O₃–CaO were studied in the temperature range 1250° to 2850°C by both experimental methods (X-ray phase analysis at 20° to 2000°C, petrography, annealing and quenching, differential thermal analysis in He at temperatures to 2500°C, thermal analysis in air using a solar furnace at temperatures to 3000°C, and electron microprobe X-ray analysis) and theoretical means (development of a mathematical model for the liquidus surface by means of the reduced polynomial method). Phase equilibria were determined by the structure of the restricting binary systems. No ternary compounds were found. The liquidus was characterized by the presence of six four-phase, invariant equilibria. Solid solutions were based on monoclinic (M), tetragonal (T), and cubic (F) modifications of HfO₂; C and H forms of Y₂O₃; CaO; and CaHfO₃ that crystallized in two polymorphous modifications, namely, the cubic and rhombic perovskite-type structure.