Crystallography of the Directionally Solidified NiO-Gd2O3 Eutectic

Crystallographic relations between the two phases of the eutectic in the system NiO-Gd₂O₃ were determined by X-ray diffraction and optical microscopy. A model of the atomic arrangement at the phase interface is proposed.     

Mechanical Properties of the Directionally Solidified MgO-MgAl2O4 Eutectic

Mechanical properties from room temperature to 1600°C are reported for the directionally solidified MgO-MgAl₂O₄ eutectic. Strength, although modest, showed little decrease to 1600°C, exhibited totally elastic behavior, and appeared to be related to colony size rather than fiber spacing. Work-of-fracture was low, increased only moderately at 1600°C, and failed to reveal any potential for the classical toughening of fiber composites. Microhardness, which exhibited classical Petch behavior related to the fiber spacing, exceeded the values for the constituent phases. It was concluded that these microstructures inhibit plastic flow and may prove most useful at high temperatures.     

Oriented Structure and Crystallography of Directionally Solidified LaB6—ZrB2 Eutectic

Directional solidification of LaB₆—ZrB₂, by use of an electron beam heating technique, yielded oriented ZrB₂ fibers in a LaB₆ matrix. The average diameter of the ZrB₂ fibers was ∼0.2–1.2 µm, with fiber lengths up to 100 µm. Primary platelike LaB₆ dendrites formed upon the solidification of an ingot with a composition of LaB₆—18 wt% ZrB₂. LaB₆ was the first phase to nucleate when eutectic growth occurred, and ZrB₂ showed nonfaceted growth. For the ingot solidified with planar growth the orientation relations of the phases were as follows: growth direction, [001]_LaB6∥[00.1]_ZrB2; interfacial plane, (11.0)_LaB6∥(11.0)_ZrB2.     

Fracture Behavior of Directionally Solidified Y3Al5O12/Al2O3 Eutectic Fiber

The strength and fracture of a directionally solidified Y₃Al₅O₁₂/Al₂O₃ eutectic fiber were investigated. The fiber was grown continuously by an edge-defined film-fed growth technique. The microstructure was characterized using X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The tensile strength and Weibull's modulus of the eutectic fibers were determined in the as-fabricated state and after extended thermal exposure at 1460°C in air. Fractographic analysis was used to identify and classify the strength-limiting mechanisms. The fracture toughness and crack growth behavior were characterized by an indentation technique. A fracture mechanics analysis was also used to establish the relationships between surface flaw size, tensile strength, and fracture toughness of the fiber.     

Crystallography of the Directionally Solidified NiO-CaO Eutectic

Crystallographic relations between the two solid solutions forming the eutectic of the system NiO-CaO were studied by X-ray diffraction and scanning electron microscopy. The ionic arrangement at the phase interface can be described by alternate (111) planes of metal and close-packed oxygen ions .     

Interface Crystallography and Structure in LaB6–ZrB2 Directionally Solidified Eutectics

Interfaces in LaB₆–ZrB₂ composites directionally solidified by a zone melting process were characterized by transmission electron microscopy (TEM). The nominal crystallographic orientation relationship between the two phases corresponded to a high-symmetry near-coincidence site lattice (NCSL). The small mistilt (2°–5°) from the high-symmetry orientation relationship was shown to result in an increased volume density of coincident sites. Furthermore, the dominant interface facet planes were predicted by the NCSL model. The configurations of interfacial misfit dislocations were analyzed by high-resolution TEM and showed a good agreement with predictions based on the displacement shift complete lattice and secondary original lattice (O2-lattice) models. These analyses suggested that interfaces were relaxed to relatively low-energy configurations.     

Effect of Moisture on the High-Temperature Stability of Unidirectionally Solidified Al2O3/YAG Eutectic Composites

The corrosion resistance of a unidirectionally solidified alumina/yttrium aluminum garnet (Al₂O₃/YAG) eutectic mixture was investigated at high temperature. Samples were exposed to high temperature (1200°–1800°C) in different atmospheres, which included argon, argon/water vapor, air, and air/water vapor. The most important microstructural changes occurred at the interface between the YAG and the Al₂O₃. Those changes consisted of localized thermal grooving, especially when the corrosive atmosphere contained water vapor. The samples exhibited significant weight loss at high temperature (1800°C) after 20 h of exposure. The calculated volume gain that was induced by the increased surface relief was low and limited, except when the corrosive atmosphere contained air, which indicated that the presence of air (particularly oxygen) induced a more-active corrosion process. On the other hand, no change in the flexural strength was observed, even after 100 h at 1800°C in a humid atmosphere, because of the cross-linked structure of the composite, which limited propagation of the groove.     

Eutectic Solidification of MgO-MgAl2O4

Directional solidification of the MgO-MgAl₂O₄ eutectic yields MgO whiskers in a spinel matrix. Microstructures produced at solidification rates of 0.4 to 30.0 cm/h were studied. The interlamellar spacing agrees with the inverse-square-root dependence on solidification rate. Colony-free microstructures formed in all ingots solidified at rates of <2.0 cm/h. For ingots solidified with planar growth, the orientation relation of the phases is:      

Eutectic Composition in the Pseudobinary of Y4Al2O9 and Y2O3

The eutectic composition between Y₄Al₂O₉ and Y₂O₃ was determined using electron probe microanalysis (EPMA) on directionally solidified specimens with hypo- and hypereutectic compositions. The microstructures of the specimens as a function of composition differ considerably with small deviation from the eutectic composition (70.5 mol% Y₂O₃ and 29.5 mol% Al₂O₃). Based on the current results and other published data, the pseudobinary system between Al₂O₃ and Y₂O₃ is revised.     

Consolidation of Eutectic Powder of Al2O3–GdAlO3

A eutectic solid of Al₂O₃–GdAlO₃ was prepared by arc discharge and crushed to a eutectic powder of 3–125 μm. The powder was consolidated by the spark plasma system (SPS). The consolidated powder duplicated the eutectic structure: Al₂O₃ and GdAlO₃ were joined to each crystal. There were no flaws such as cracks or pores in the eutectic composite. The bending strength was half that of the eutectic composite prepared by unidirectional solidification.     

Effects of Annealing on the Microstructure and Phase Chemistry of Directionally Solidified Bi2Sr2CaCu2O8

The effects of annealing on the microstructure and phase chemistry of directionally solidified Bi₂Sr₂CaCu₂O₈ (2212) were studied. Boules grown at fast growth rates are multiphase, contain no 2212, and exhibit some preferred orientation; Sr-deficient 2201 exhibits a (220) texture. These samples, however, do show a significant amount of 2212 after annealing, with (220) texture similar to the texture of the preexisting 2201 phase in the as-grown boules. In contrast, 2212 grown directly from the melt at slow growth rates has (200) texture. Boules grown at moderate growth rates are multiphase and contain 2212 as one of the phases; these boules show an increase in 2212 volume fraction after annealing. Microstructural observations in both cases indicate solidstate formation of 2212 occurs topotactically on or within the preexisting 2201 phase. Boules grown at fast growth rates are oxygen-deficient, since they show an increase in mass with annealing, whereas boules grown at moderate growth rates show no appreciable mass increase with annealing. An estimate of the Cu(I) concentration in the liquid zone during crystal growth at fast growth rates was obtained from gravimetric analysis.     

Effects of Sm2O3 and Gd2O3+ Nd2O3 on Electromechanical Properties of PbTiO3 Ceramics

The electromechanical properties of PbTiO₃ ceramics, modified by substitution of Sm or Gd + Nd (same average atomic radius as Sm) for Pb, were studied in the range of 6% to 14% substitution. The modified PbTiO₃ ceramics were stable, and the Curie temperature decreased linearly over this composition range. The 10% Sm composition had a large anisotropy in the coupling factor ratio, k _t / k _p , and a similar, but weaker, effect developed for 12% (1/2 Gd + 1/2 Nd). This indicates that other than average ion size may influence the electromechanical coupling factor ratio.     

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.     

Stable and Metastable Equilibria in the Systems Y2O2-Al2O3, and Gd2O3-Fe2O3

The phase equilibrium relations in the systems Y₂O₃-Al₂O₃ and Gd₂O₃-Fe₂O₃ were examined. Each system has two stable binary compounds. A 3:s molar ratio garnet-type compound exists in both systems. The 1:1 distorted perovskite structure is stable in the system Gd₂O₃-Fe₂O₃ but only metastable in the system Y₂O₃-AI₂O₃. This interesting example of metastable formation and persistence of a compound with ions of high Z/r values explains the discrepancies in the literature on the structure of the composition YA1O₃. A new 2:1 molar ratio cubic phase has been found in the system Y₂O₃-A1₂O₃. Since silicon can be completely substituted for aluminum in this compound, the aluminum ions are presumably in fourfold coordination.