Elevated-Temperature Toughening Mechanisms in a SiCw/Al2O3 Composite

Crack growth resistance studies of a Sic-whisker-reinforced Al₂O₃-matrix composite have been correlated with the composite microstructure to determine the active fracture toughening mechanism, at each of three test temperatures through 1400°C. Evidence of cumulative toughening at all temperatures, as reported in the literature, was validated by R -curves; however, isolation of the following wake zone effects from that of the frontal process zone elicits a departure from published assumptions. A frontal zone mechanism, presumably microcrack toughening, dominates at room temperature, while a following wake zone mechanism of crack face whisker-bridging controls at temperatures near 1200°C.     

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.     

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.     

Evaluation of Whisker Alignment in Axisymmetric SiCw-Reinforced Al2O3 Composite Material

A quantitative method to evaluate the degree of whisker alignment in axisymmetric composite materials was developed. The angular distribution of the whiskers was analyzed by measuring the aspect ratios of the whiskers observed on a planar section. However, due to the large difference in the probability of whiskers being detected on the planar section (depending on whisker length and degree of alignment), the angular distribution of the whiskers observed on the planar section was significantly different from the actual distributions of the whiskers in three dimensions. Three-dimensional angular distributions were evaluated by comparing the aspect ratios observed on the planar section with those calculated under the assumption that the whisker angle fell in a Gaussian distribution with average angle of 90°. By this method, the degree of preferential alignment is expressed as the standard deviation of the Gaussian distribution. This quantification of whisker alignment is useful in analyzing the mechanical behaviors of composite materials reinforced with elongated particles.     

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.     

Dynamic Compaction of Al2O3-ZrO2 Compositions

Shock compaction of Al₂O₃-ZrO₃ binary and ternary powder compositions resulted in dense, one-piece samples without visible cracks for pressures ≤12.6 GPa. Dynamic pressures were achieved by using a 6.5-m-long two-stage gas gun. It is believed that plastic deformation by dislocation slip of α-Al₂O₃ partially accommodates the tensile stresses created during the release of shock pressures. A fine and narrow particle size distribution is necessary to achieve high bulk densities, but the bulk structural integrity was not strongly related to the distribution. A high-pressure phase of ZrO₂, which was formed from the monoclinic polymorph, was found at and above shock pressure of 6.3 GPa. No evidence of the orthorhombic cotunnite structure was observed. Compaction of glassy and submicrocrystalline rapidly solidified starting materials showed good structural integrity, although the bulk density was relatively low. It is not clear what the densificationhonding mechanism is in these materials, although it appears not to be plastic deformation. Microstructural analysis showed that fine and uniform microstructures are retained after compaction at appropriate dynamic pressures for all compositions, with some interparticle cohesion present.     

High-Temperature Strength and Fracture Toughness of Y2O3-Partially-Stabilized ZrO2/Al2O3 Composites

The temperature dependence of bending strength, fracture toughness, and Young's modulus of composite materials fabricated in the ZrO₂ (Y₂O₃)-Al₂O₃ system were examined. The addition of A1203 enhanced the high-temperature strength. Isostatically hot-pressed, 60 wt% ZrO₂ (2 mol% Y₂O₃)/40 wt% Al₂O₃ exhibited an extremely high strength, 1000 MPa, at 1000°C.     

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.     

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.     

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.     

Enhanced Fracture Toughness in Layered Microcomposites of Ce-ZrO2 and Al2O3

Laminar composites, containing layers of Ce-ZrO₂ and either Al₂O₃ or a mixture of Al₂O₃ and Ce-ZrO₂, have been fabricated using a colloidal method that allowed formation of layers with thicknesses as small as 10 μm. Strong interactions between these layers and the martensitic transformation zones surrounding cracks and indentations have been observed. In both cases, the transformation zones spread along the region adjacent to the layer, resulting in an increased fracture toughness. The enhanced fracture toughness was observed for cracks growing parallel to the layers as well as for those that were oriented normal to the layers.     

Strength and Fracture Toughness of Isostatically Hot-Pressed Composites of Al2O3 and Y2O3-Partially-Stabilized ZrO2

Composites of Al₂O₃ and Y₂O₃ partially-stabilized ZrO₂ were isostatically hot-pressed using submicrometer powders as the starting material. The addition of Al₂O₃ resulted in a large increase in bending strength. The average bending strength for a composite containing 20 wt% Al₂O₃ was 2400 MPa, and its fracture toughness was 17 MN·w^−3/2     

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.     

Defect Models for Sintering and Densification of Al2O3: Ti and Al2O3: Zr

A defect model proposed to explain the effect of titanium doping on the rate of sintering of Al₂O₃ is revised to fit the oxidizing conditions of the experiments. The model accounts for the observed change in sintering rate by a change from rate limitation by ions to rate limitation by electrons, but requires the presence of an unusually large concentration of acceptor impurities in the material. Models similar to the ones originally proposed account for the rate of densification of Al₂O₃:Zr by hot-pressing in vacuo, provided it is extended by including electronics defects.     

Fracture Induced in Al2O3 Bicrystals by Anisotropic Thermal Expansion

Bicrystals of Al₂O₃ were fabricated to study the effects of thermal expansion anisotropy on fracture in a model system containing one grain boundary. Fractures occurred perpendicular to the directions of maximum tensile stress in bicrystals with thermal expansion coefficient differences as low as 0.31×10⁻⁶⁰C⁻¹ and originated at the boundary, probably in areas of high residual stress associated with pores. Numerical stress analysis of two-dimensional model bicrystal configurations showed that stresses induced by thermal expansion anisotropy are maximal in a region localized along the bicrystal boundary.