Orientation and Grain Boundary Microstructure of Alumina in Al/Al2O3 Composites Produced by Reactive Metal Penetration

The orientation and grain boundary microstructure of alumina in reactive metal penetration Al/Al₂O₃ composites are studied using orientation imaging microscopy and the results are compared with those of sintered polycrystalline Al₂O₃. The interconnected Al₂O₃ in the composite material is separated by Σ3 boundaries (twins) with a 60° rotation around the [0001] direction. A high frequency (∼100%) of Σ3 coincidence boundaries in composite alumina is remarkable since only ∼12% of boundaries in a sintered polycrystalline Al₂O₃ are of special nature. The coincidence boundaries in the in situ alumina grow in a coherent and faceted manner.     

Technique for Measuring Grain-Boundary Mobility: Application to MgO-Doped Al2O3

An experimental technique employing localized laser heating has been developed to seed abnormal grain growth in Mg-doped Al₂O₃ permitting the migration behavior of individual grain boundaries to be investigated. Nonuniform growth of both naturally nucleated and laser-nucleated abnormal grains was observed. Calculated boundary mobilities exceeded those characterizing grain growth in both doped and undoped porous Al₂O₃).     

Conversion of Polycrystalline Al2O3 into Single-Crystal Sapphire by Abnormal Grain Growth

In a given batch more than 30%–40% of polycrystalline, MgO-doped Al₂O₃ tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880°C. Most crystals grew 4–10-cm in length in tubes with wall thicknesses of 1/2 and 3/4 mm and outer diameters of 5 and 7 mm, respectively, and had their c -axes oriented ∼ 90° and 45° to the tube axis. Initiation of AGG was associated with low values of bulk MgO concentration near 50 ppm. The unconverted tubes did not develop centimeter-size crystals but instead exhibited millimeter-size grains. The different grain structures in converted and unconverted tubes may be related to nonuniform concentration of MgO in the extruded tubes. The growth front of the migrating crystal boundary was typically nonuniformly shaped, and the interface between the single crystal and the polycrystalline matrix was composed of many "curved" boundary segments indicative of classical AGG in a single-phase material. The average velocities of many migrating crystal boundaries were quite high and reached ∼1.5 cm/h. The average grain boundary mobility at 1880°C was calculated as 2 × 10⁻¹⁰ m³/(N·s), representing the highest value reported so far in Al₂O₃ and within a factor of 2.5 of the calculated intrinsic mobility. Under similar experimental conditions sapphire crystals did not grow when a codopant of CaO, La₂O₃, or ZrO₂ was added in concentrations of several hundred ppm.     

Electrical Conductivity of Polycrystalline A12O3 Doped with Silicon

Direct current conductivity was measured for polycrystalline Alz03 doped with silicon, which is found to act as a single donor, the donor level lying ∼165 kJ/mol (∼1.7 eV) below the conduction band. Silicon in excess of the solubility limit (∼220 ppm at 1500°C, 300 ppm at 1600°C) is present as a glassy aluminosilicate second phase. Silicon dissolved in Al₂O₃ tends to segregate at grain boundaries.     

Densification and Grain Growth of Al2O3 Nanoceramics During Pressureless Sintering

α - Al₂O₃ nanopowders with mean particle sizes of 10, 15, 48, and 80 nm synthesized by the doped α-Al₂O₃ seed polyacrylamide gel method were used to sinter bulk Al₂O₃ nanoceramics. The relative density of the Al₂O₃ nanoceramics increases with increasing compaction pressure on the green compacts and decreasing mean particle size of the starting α-Al₂O₃ nanopowders. The densification and fast grain growth of the Al₂O₃ nanoceramics occur in different temperature ranges. The Al₂O₃ nanoceramics with an average grain size of 70 nm and a relative density of 95% were obtained by a two-step sintering method. The densification and the suppression of the grain growth are achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. The densification was realized by the slower grain-boundary diffusion without promoting grain growth in second-step sintering.     

Electrical Conduction in Single-Crystal and Polycrystalline Al2O3 at High Temperatures

The electrical conductivity and ion/electron transference numbers in Al₃O₃ were determined in a sample configuration designed to eliminate influences of surface and gas-phase conduction on the bulk behavior. With decreasing O₂ partial pressure over single-crystal Al₂O₃ at 1000° to 1650°C, the conductivity decreased, then remained constant, and finally increased when strongly reducing atmospheres were attained. The intermediate flat region became dominant at the lower temperatures. The emf measurements showed predominantly ionic conduction in the flat region; the electronic conduction state is exhibited in the branches of both ends. In pure O₂ (1 atm) the conductivity above 1400°C was σ≃3×10³ exp (–80 kcal/ RT ) Ω⁻¹ cm⁻¹, which corresponds to electronic conductivity. Below 1400°C, the activation energy was <57 kcal, corresponding to an extrinsic ionic condition. Polycrystalline samples of both undoped hot-pressed Al₂O₃ and MgO-doped Al₂O₃ showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas-phase conduction above 1200°C increased drastically with decreasing O₂ partial pressure (below 10⁻¹⁰ atm).     

Strength Degradation and Crack Propagation in Thermally Shocked Al2O3

Strength degradation and crack propagation in Al₂O₃ are shown to depend on the initial strength and grain size of the material. The strengths of single-crystal sapphire and polycrystalline Al₂O₃ specimens with grain sizes of 10, 34, and 40 μ m decreased discontinuously at the critical quenching temperature. In contrast, the strength of polycrystalline Al₂O₃ with a grain size of 85 μm decreased gradually as the quenching temperature increased. The strength retained after thermal shock and the extent of crack propagation decrease with increasing initial strength and grain size, respectively, in Al₂O₃.     

Electric-Field-Induced Crack Growth Behavior in PZT/Al2O3 Composites

Hard lead zirconate titanate (PZT) and PZT/Al₂O₃ composites were prepared and the alternating-electric-field-induced crack growth behavior of a precrack above the coercive field was evaluated via optical and scanning electron microscopy. The crack extension in the 1.0 vol% Al₂O₃ composite was significantly smaller than that in monolithic PZT and the 0.5 vol% Al₂O₃ composite. Secondary-phase Al₂O₃ dispersoids were found both at grain boundaries and within grains in the composites. A large number of dispersoids were observed at the grain boundaries in the 1.0 vol% Al₂O₃ composite. It appears that the Al₂O₃ dispersoids reinforce the grain boundaries of the PZT matrix as well as act as effective pins against microcrack propagation.     

Ionically Conducting Composite Membranes from the Li2O–Al2O3–TiO2–P2O5 Glass–Ceramic

This paper reports processing of lithium ion-conducting, composite membranes comprised of 14Li₂O·9Al₂O₃·38 TiO₂·39P₂O₅ glass–ceramic and polyethylene. The processing involved tape casting of 14Li₂O·9Al₂O₃·38TiO₂·39P₂O₅ glass powder with organic additives into tapes, subjecting the green tape to binder burnout and thermal soaking in the temperature range of 950°–1100°C, and finally infiltrating the porous tape with polyethylene solution. The ionic conductivity and microstructure of 150–350 μm thick membranes were characterized and are discussed in this paper. The crystallites of the glass–ceramic show liquid-like conductivity at ambient temperature, whereas the grain boundary conductivity is lower by a factor of five. The lower grain boundary conductivity is explained on the basis of crystallographic mismatch and the existence of AlPO₄ at the grain boundary. The polyethylene infiltration in the porous membrane improved mechanical resilience with a minor adverse effect on conductivity.     

Discontinuous Dissolution and Grain-Boundary Migration in Al2O3-Fe2O3 by Oxygen Partial Pressure Change

When sintered 95Al₂O₃-5Fe₂O₃ (wt%) specimens constituting corundum grains and iron aluminate spinel precipitates were annealed under high oxygen partial pressure (P_o2) where only a corundum phase is stable, fast dissolution of particulate spinel precipitates occurred, together with the migration of corundum grain boundaries. Behind the migrating boundaries, a corundum solid solution enriched with Fe₂O₃ formed. Discontinuous dissolution (DD) of particulate spinel precipitates thus occurred by P_o2 increase. In contrast, when 95Al₂O₃-5Fe₂O₃ specimens constituting only corundum grains were annealed under low P_o2 where both corundum and spinel phases are stable, grain boundaries migrated without spinel precipitation, leaving behind a corundum phase depleted of Fe₂O₃, similar to chemically induced grain-boundary migration (CIGM) observed during solute depletion. The volatilization of Fe₂O₃ appeared to cause the boundary migration without precipitation. The observed CIGM and DD would suggest various possibilities of microstructure control in other oxide systems through oxygen partial pressure change.     

Effects of Chemical Inhomogeneities on Grain Growth and Microstructure in Al2O3

Effects of chemical inhomogeneities and single-crystal seeds on normal and discontinuous grain growth were investigated in both undoped and MgO-doped Al₂O₃. The chemical impurities in the samples were exsolved at a lower temperature than the sintering temperature and measured by SEM/EDS to determine the correlation between the distribution of impurities and the microstructure in Al₂O₃. A feature of this study was the use of clean-room processing and firing procedures to maintain sample composition at the levels initially present in the starting powders. As the local concentrations of chemical impurities (i.e., Si, Ca) increased, the grain boundary–grain boundary dihedral angle distribution widened, with many angles at 180°, the grain-size distribution widened, and pore–boundary separation was enhanced. Discontinuous grain growth was observed in regions of undoped Al₂O₃ containing the largest Ca and Si concentrations. It is suggested that doping with MgO solute reduces the effects of impurities on grain growth by increasing the bulk solubility and decreasing interfacial segregation of impurities, especially Si, and by narrowing the distribution of grain boundary–grain boundary dihedral angles.     

Microstructural Analysis of Sintered High-Conductivity Zirconia with Al203 Additions

Transmission electron microscopy (at 100 and 1000 kV potential) and analytical scanning transmission electron microscopy were used to study α-Al₂0₃ second-phase particles and their interactions with grain boundaries in two high-conductivity Y₂0₃/Yb₂0₃ stabilized zirconia ceramics containing deliberate additions of the alumina as a sintering aid. Most of the Al₂0₃ particles were intragranular and microanalysis showed that they contained inclusions rich in Zr or Si plus Zr. Al₂O₃ particles at grain boundaries were frequently associated with amorphous cusp areas rich in Si and Al. The results suggest that the Al₂0₃ acts as a scavenger for SiO₂, removing it from grain-boundary localities. A model is proposed whereby this process occurs as the boundaries meet the second-phase particles, assisted by rapid grain-boundary diffusion. Such an ZrO₂-Al₂O₃-SiO₂ interaction and partitioning is predicted thermodynamically and offers a possible explanation for the improvements in ionic conductivity brought about by Al₂O₃ additions, as reported in the literature.     

Discontinuous Dissolution of Iron Aluminate Spinel in the Al2O3–Fe2O3 System

When sintered 85Al₂O₃–15Fe₂O₃ (in wt%) specimens consisting of corundum grains and spinel particles were annealed at temperature where only a corundum phase was stable, phase transformation of spinel into metastable FeAIO₃ and subsequently complete dissolution of the metastable phase occurred together with the migration of grain boundaries at the surface of the specimens. Since the grain boundary migration was induced by grain boundary diffusion of Fe₂O₃ from the transforming and dissolving particles, the boundary migration by temperature decrease corresponds to a discontinuous dissolution of the spinel particles and a chemically induced grain boundary migration by temperature change. Inside the specimens, however, the transformation—dissolution and the grain boundary migration were suppressed because of unavailable accommodation of the volume expansion due to the transformation.     

Effect Of Grain Boundaries on Diffusion-Controlled Processes in Aluminum Oxide

Oxygen ion diffusion coefficients in single-crystal Al₂O₃ are several orders of magnitude less than alminum ion diffusion coefficients in polycrystalline Al₂O₃. In polycrystalline Al₂O₃, oxygen ion diffusion is enhanced by the presence of grain boundaries as in the chloride ion diffusion in the alkali halides. Creep and sintering of polycrystalline Al₂O₃ occur at a faster rate than is possible through control by lattice diffusion of oxygen; the rates are in fair quantitative agreement with cation diffusion. It is tentatively concluded that enhanced oxygen diffusion in regions adjacent to boundaries allows aluminum ion bulk diffusion to be rate controlling for these processes. The electrical conductivity in Al₂O₃ is too high to be related to either anion or cation diffusion and is predominantly electronic.     

Fabrication and Evaluation of Al2O3–AgCl–Agl Composites by Use of Electrochemical Deposition

Al₂O₃–AgCl–AgI composites were fabricated by an electrochemical deposition technique utilizing porous α–Al₂O₃ ceramics as a matrix. The growth process was presumed to be dominated by the diffusion rate of the Ag⁺ ion. The phases identified in the composites were α-Al₂O₃, AgCl, and β-AgI. The deposited AgCl and AgI showed enhanced conductivity compared with the reported bulk values. The deposited mixture exhibited enhanced conductivity compared with pressed polycrystalline AgCl or AgI. The enhancement was explained using a model that a deposit with a high conductivity grows much faster than that with a low conductivity.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

Influence of Dopant Concentration on Creep Properties of Nd2O3-Doped Alumina

The microstructural features and tensile creep behavior of Al₂O₃ doped with Nd₂O₃ at levels ranging from 100 to 1000 ppm (Nd:Al atomic ratio) were systematically investigated. Compositional mapping, using both high-resolution scanning transmission electron microscopy and secondary ion mass spectroscopy revealed that, for all of the compositions studied, the Nd³⁺ ions were strongly segregated to the Al₂O₃ grain boundaries. Microstructural observations revealed that the solubility of Nd₂O₃ was between 100 and 350 ppm. Tensile creep tests were conducted over a range of temperatures (1200°–1350°C) and stresses (20–75 MPa). Both the stress and grain-size exponents were analyzed. In selected experiments, controlled grain-growth anneals were used to enable creep testing of samples of the same average grain size but different neodymium concentrations. Independent of dopant level, the neodymium additions decreased the creep rate by 2–3 orders of magnitude, compared with that of undoped Al₂O₃. The value of the apparent creep activation energy increased with increased dopant concentration and then saturated at dopant levels exceeding the solubility limit. Overall, the results of the present study were consistent with a creep-inhibition mechanism whereby oversized segregant ions reduce grain-boundary diffusivity by a site-blocking mechanism.     

Effect of Oxygen Partial Pressure on the Creep of Polycrystalline Al2O3 Doped with Cr, Fe, or Ti

Steady-state creep was studied in hot-forged polycrystalline Al₂O₃ (3 to 42 μm) of nearly theoretical density doped with≤1 cation % of Fe, Ti, or Cr. Tests were conducted at stresses between 10 and 550 kg/cm² at 1375° to 1525°C under O₂ partial pressures of 0.88 to 10⁻¹⁰ atm. Except in the 10-μm Fe-doped material tested at very small stresses, slightly nonviscous creep behavior was generally observed. The effects of P _o2 on the creep rate indicated that increased concentration of a divalent (Fe²⁺) or quadrivalent (Ti⁴⁺) impurity in solid solution enhances the creep rate of polycrystalline Al₂O₃. The activation energies for the creep of Fe- and Ti-doped Al₂O₃ samples (148 and 145 kcal/mol, respectively) were significantly higher than that for Cr-doped material (114 kcal/mol). Taking into account the effects of P_o2, temperature, and grain size, it was concluded that the steady-state creep of transition-metal-doped Al₂O₃ is controlled by cation lattice diffusion.     

Intragranular Particle Residual Stress Strengthening of Al2O3–SiC Nanocomposites

Al₂O₃/SiC ceramic nanocomposites were fabricated from nanocrystalline Al₂O₃ (10 nm in diameter) and SiC (15 nm in diameter) powders, and a theoretical model of intragranular particle residual stress strengthening was investigated. The SiC nanoparticles in the Al₂O₃ grains create a normal compressive stress at the grain boundaries and a tangential tensile stress in the Al₂O₃ grains, resulting in the "strengthening" of the grain boundaries and "weakening" of the grains. The model gives a good explanation of the experimental results of the authors and others which are difficult to be explained by the existing strengthening models, i.e. the maximum strength is normally achieved at about 5 vol% of SiC particles in the Al₂O₃–SiC ceramic nanocomposites. According to the model, there exists an optimum amount of SiC for strengthening, below which the grain boundaries are not fully "strengthened" and the fracture is mainly intergranular, above which the grains are "weakened" too much and the fracture is mainly transgranular, and at which the fracture is a mixture of intergranular and transgranular.     

Densification and Mechanical Properties of B4C with Al2O3 as a Sintering Aid

The densification behavior and mechanical properties of B₄C hot-pressed at 2000°C for 1 h with additions of Al₂O₃ up to 10 vol% were investigated. Sinterability was greatly improved by the addition of a small amount of Al₂O₃. The improvement was attributed to the enhanced mobility of elements through the Al₂O₃ near the melting temperature or a reaction product formed at the grain boundaries. As a result of this improvement in the density, mechanical properties, such as hardness, elastic modulus, strength, and fracture toughness, increased remarkably. However, when the amount of Al₂O₃ exceeded 5 vol%, the level of improvement in the mechanical properties, except for fracture toughness, was reduced presumably because of the high thermal mismatch between B₄C and Al₂O₃.