Compressive Creep of SiC-Whisker-Reinforced Al2O3

Compressive creep of SiC-whisker-reinforced Al₂O₃ composites (0, 5, 15, and 25 wt% SiC) was measured in the temperature range of 1300° to 1500°C in air and argon. The creep resistance increased with increasing whisker concentration. The results indicated that the whiskers degraded in air, increasing strain rates compared to those in argon. Stress exponents between 1.0 and 2.0 and an activation energy of 620 ± 100 kJ/mol were measured. Transmission electron microscopy observations indicated that cavitation was minimal and that the deformed composites had the same dislocation structure as did the as-received samples.     

Compressive Creep and Creep Failure of 8Y2O3/3Al2O3-Doped Hot-Pressed Silicon Nitride

The compressive creep properties of hot-pressed Si₃N₄8Y₂O₃—3Al₂O₃ (wt%) have been investigated in the temperature range of 1543–1603 K in air. The stress exponent, n , of the power creep law was determined to be 1.5, and the activation energy was determined to be 650 kJ/mol. Transmission electron microscopy observations showed that grain-boundary sliding occurred with cavitation formation in the grain-boundary glassy phase. The quasi-steady-state creep results were consistent with that of the diffusion-controlled solution—diffusion—precipitation creep mechanism, and the distinguished failure mechanism was cavitation creep damage controlled by the viscosity of the boundary glassy phase. The compressive creep failure time, obtained at 1573 K, in the stress range of 175–300 MPa, followed the MonkmanGrant relation, indicating that cavity growth was mainly controlled by the creep response of the material.     

Creep of Doped Polycrystalline Al2O3

Creep behavior of polycrystalline A1₂O₃ doped with MgO + NiO, both as-hot-pressed in graphite dies and additionally annealed, was determined for 1300° to 1470°C and for 1000 to 15,000 psi in compression. The deformed specimens contained intergranular separations. Creep rates were proportional to stress to the 1.1 and 1.3 powers and were independent of grain size changes occurring during creep. The suggested creep mechanism is localized plastic deformation at stress concentration points accommodated by grain-boundary separations initiated by grain-boundary sliding.     

Dislocation-Induced Softening of MgO·2.9Al2O3 Single Crystals

Magnesium aluminate spinel (MgO·2.9Al₂O₃) single crystals were deformed in compression along the [001] direction at temperatures between 1100° and 1500°C. Dislocations introduced by plastic deformation were observed by transmission electron microscopy. These dislocations lead to plasticity at a temperature where undeformed spinel has no ductility. Further, room-temperature Vickers hardness decreases after plastic deformation at 1500°C.     

Creep Deformation in MgO-Saturated Large-Grain-Size Al2O3

Samples of 65 μm-grain-size AL₂O₃ containing 0.05 wt% Mg and 40 μm-grain-size AL₂O₃ containing 0.13 wt% Mg, both MgO-saturated, undergo compressive deformation in the range 1580° to 1800°C with results interpreted as diffusional creep rate-limited by grain-boundary diffusion with the Coble boundary-diffusion model giving
A lower deformation rate for an unsaturated composition indicates that MgO additions enhance grain-boundary diffusion.     

Bimodal Sintering of Al2O3/Al2O3 Platelet Ceramic Composites

The sintering behavior of an Al₂O₃ compact containing uniformly dispersed Al₂O₃ platelets has been investigated. The results reveal a significant decrease in the sintering rate as well as the formation of voids and cracklike defects in the presence of nonsinterable platelets. The addition of a small amount (2 vol%) of tetragonal-ZrO₂ particles enhances the sintering rate, increases end-point density (∼99.5% of theoretical density) and prevents formation of sintering defects.     

Dynamic Fracture Characterization of Al2O3 and SiCw/Al2O3

The dynamic stress intensity factors, which were determined with newly developed bar impact facilities and a new data reduction procedure, for an Al₂O₃ ceramic and 29 vol% SiC_w/Al₂O₃ composite were virtually identical, thus indicating that the short SiC whiskers were ineffective under dynamic fracture. SEM studies revealed five distinct fracture morphologies with increased percentage area of transgranular fracture in both materials with rapid crack propagation. Also, the high dynamic stress intensity factor caused multiple microscopic crack planes to form and then join as the crack advanced.     

Microstructure Development in Al2O3-Platelet-Reinforced Ce-ZrO2/Al2O3 Composites

Composites containing Ce-ZrO₂, Al₂O₃, and aligned Al₂O₃ platelets were produced by centrifugal consolidation and pressureless sintering, followed by heat treatments at 1600°C for varied duration. Constituents in the consolidated microstructures were either uniformly distributed throughout or segregated into gradient layers, depending critically on platelet content. Quantitative image analysis was used to examine microstructure development with heat treatment. Changes in the volume fraction, dimensional anisotropy, and gradient of pores and platelets, as well as changes in the phase gradient, were quantified. Microstructure development was strongly dependent on the initial microstructure design attained from suspension processing.     

Characterization of Freeze-Dried Al2O3 and Fe2O3

Oxide crystallite formation and growth from freeze-dried sulfates were studied for the representative materials Al₂O₃ and Fe₂O₃. Transmission and scanning electron micrographs showed the formation and growth of chainlike aggregates of crystallites. Aggregation occurred as part of the nucleation and growth of the oxide, and discrete oxide particles were never present. Orientation of the chain aggregates was related to the ice structure formed during freezing. X-ray line broadening data showed that crystallite size is a function of the 1/5 to 1/7 power of time for isothermal treatments. A qualitative analysis of material transport favored the surface diffusion mechanism.     

Effect of Neutron Irradiation on Knoop Microhardness Anisotropy in MgO·3Al2O3 Single Crystals

MgO·3Al₂O₃ single crystals were irradiated with neutron fluences of 8.3 × 10²² n/m² at 100°C and 2.4 × 10²⁴ n/m² at 470°C ( E > 1.0 MeV) in the Japan Materials Testing Reactor. The Knoop microhardness of several orientations on the (100) plane of both the irradiated and unirradiated crystals were measured with different indentation loads. The change in hardness profile of the crystals was almost the same after the two irradiation conditions. The hardness increased by 4–15% because of the irradiations depending on the crystallographic orientation, the larger change being observed at orientations between the (001) and (011) directions. While both the {111}     and {110}     slip systems are simultaneously active in the unirradiated MgO·3Al₂O₃, the {111}     system may be the dominant slip system in the neutron-irradiated crystals. It is concluded that the restriction of the {110}     slip system is caused by irradiation-induced interstitial ions.     

Measurement of Nonsymmetric Creep in Al2O3 with a Glass Phase

To check the development of nonsymmetry in bending creep tests an Al₂O₃ ceramic containing a glass phase is investigated in interrupted creep tests. The symmetric part of the strain is measured by the deflection and the nonsymmetric part is obtained by length measurements between two Knoop indentations situated at the centerline of bending bars. It is observed that nonsymmetry in creep occurs not only in the secondary creep range but also in the primary range.     

Wet Milling of Fe/Al/Al2O3 and Fe2O3/Al/Al2O3 Powder Mixtures

Wet milling of Al₂O₃-aluminide alloy (3A) precursor powders in acetone has been investigated by milling Fe/Al/Al₂O₃ and Fe₂O₃/Al/Al₂O₃ powder mixtures. The influence of the milling process on the physical and chemical properties of the milled powders has been studied. Particle refinement and homogenization were found not to play a dominant role, whereas plastic deformation of the metal particles leads to the formation of dislocations and a highly disarranged polycrystalline structure. Although no chemical reactions among the powder components in Fe₂O₃/Al/Al₂O₃ powder mixtures were observed, the formation of a nanocrystalline, ordered intermetallic FeAl phase in Fe/Al/Al₂O₃ powder mixtures caused by mechanical alloying was detected. Chemical reactions of Fe and Al particle surfaces with the atmosphere and the milling media lead to the formation of highly porous hydroxides on the particle surfaces. Hence the specific surface area of the powders increases, while the powder density decreases during milling. The fraction of Fe oxidized during milling was determined to be 0.13. The fraction of Al oxidized during milling strongly depends on the metal content of the powder mixture. It ranges between 0.4 and 0.8.     

Hydration of 3CaO·Al2O3 and 3CaO·Al2O3+] Gypsum With and Without CaCl2

Paste samples of tricalcium aluminate alone, with CaCl₂, with gypsum, and with gypsum and CaCl₂ were hydrated for up to 6 months and the hydration products characterized by SEM, XRD, and DTA. Tricalcium aluminate hydrated initially to a hexagonal hydroaluminate phase which then changed to the cubic form; the transformation rate depended on the size and shape of the sample and on temperature. The addition of CaCl₂ to tricalcium aluminate resulted in the formation of 3CaO · Al₂O₃· CaCl₂·10H₂O and 4CaO · Al₂O₃· 13H₂O, or a solid solution of the two. The chloride retarded the formation of the cubic phase 3CaO · Al₂O₃· 6H₂O; the addition of gypsum resulted in the formation of monosulfoaluminate with a minor amount of ettringite. When chloride was added to tricalcium aluminate and gypsum, more ettringite was formed, although 3CaO · Al₂O₃· CaSO₄· 12H₂O and 3CaO · Al₂O₃· CaCl₂· 10H₂O were the main hydration products.     

Interreaction Between Al2O3 and a CaO-Al2O3 Melt

Poly crystalline and single-crystalline α-alumina were reacted with a eutectic CaO-Al₂O₃ melt at 1530°C. A reaction zone develops in which a strongly textured CA₆ layer, as well as a CA₂ layer, forms, with a remaining layer of unreacted CaO-AI₂O₃ melt. Silica, an impurity in the α-alumina, is rejected by the advancing CA₆ phase and accumulates as calcium alumino-silicates in channels that assist in the reaction as fast transport paths. Reaction mechanisms and welding are briefly discussed.     

Slip Systems in Al2O3

Crystallographic notation for Al₂O₃ is reviewed, with particular reference to the correct basis to be used in describing slip systems. A Groves-and-Kelly calculation showed that the combination of pyramidal slip on {11¯02}<11¯01> and basal slip on (0001){112¯0} will allow homogeneous deformation of Al₂O₃ polycrystals. Furthermore, operation of either the {101¯1}<1¯011> or the {011¯2}<2¯021> slip system will also satisfy the Von Mises criterion, since each system is capable of 5 independent deformation modes. Electron microscopy of an Al₂O₃ polycrystal deformed ≅5% at 1150°C under a hydrostatic confining pressure confirmed that pyramidal slip had occurred.     

Homogeneous ZrO2–Al2O3 Composite Prepared by Nano-ZrO2 Particle Multilayer-Coated Al2O3 Particles

ZrO₂–Al₂O₃ nanocomposite particles were synthesized by coating nano-ZrO₂ particles on the surface of Al₂O₃ particles via the layer-by-layer (LBL) method. Polyacrylic acid (PAA) adsorption successfully modified the Al₂O₃ surface charge. Multilayer coating was successfully implemented, which was characterized by ξ potential, particle size. X-ray diffraction patterns showed that the content of ZrO₂ in the final powders could be well controlled by the LBL method. The powders coated with three layers of nano-ZrO₂ particles, which contained about 12 wt% ZrO₂, were compacted by dry press and cold isostatically pressed methods. After sintering the compact at 1450°C for 2 h under atmosphere, a sintered body with a low pore microstructure was obtained. Scanning electron microscopy micrographs of the sintered body indicated that ZrO₂ was well dispersed in the Al₂O₃ matrix.