High-Temperature Deformation of Stoichiometric 239PuO2

The deformation behavior of Stoichiometric ²³⁹PuO₂ was examined at 800° to 1500°C, using direct and diametral compression. Maximum ductility was observed at 1000°C, but above this temperature both strength and ductility decreased and the fracture mode changed from transgranular to inter-granular. The deformation activation energy measured at 1000°C was 598 kJ/mol. Comparison to the deformation behavior of hypostoichiometric ²³⁹PuO_2-x suggests that high-temperature dislocation motion becomes more difficult with increasing O/Pu ratio due to effects of stoichiometry on diffusion rates. Deformation mechanisms in ²³⁹PuO₂ appear to be a combination of dislocation motion and grain-boundary sliding.     

High-Temperature Tensile Strength of Er2O3-Doped ZrO2 Single Crystals

The deformation and fracture mechanisms in tension were studied in single-crystal Er₂O₃-doped ZrO₂ monofilaments processed by the laser-heated floating zone method. Tensile tests were carried out between 25° and 1400°C at different loading rates and the dominant deformation and fracture mechanisms were determined from the shape of the stress–strain curves, the morphology of the fracture surfaces, and the evidence provided by monofilaments deformed at high temperature and broken at ambient temperature. The tensile strength presented a minimum at 600°–800°C and it was controlled by the slow growth of a crack from the surface. This mechanism was also dominant in some monofilaments tested at 1000°C and above, while others showed extensive plastic deformation before fracture at these temperatures. The strength of plastically deformed monofilaments was significantly higher than those which failed by slow crack growth due to the marked strain hardening capacity of this material.     

Hot Deformation of YBa2Cu3O7-δ and Composite YBa2Cu3O7-δ/Ag

The response of ceramic superconductors and ceramic composites to compressive stresses at high temperatures has been examined. Monolithic YBa₂Cu₃O_7-δ and composite YBa₂Cu₃O₇-δ₆/Ag were tested at constant true strain rates from 10^-6 to 10^-3 s^-1 at temperatures from 800° to 950°C. Fine-grained monolithic YBa₂Cu₃O_7-δ appears to have a regime of superplastic deformation between temperatures of 850° and 950°C at strain rates from 10^-6 to 10^-4 S^-1. The addition of 20 vol% Ag to a coarser-grained material enhances the ductility of the ceramic and lowers the flow stress by a factor of 3 to 10. However, there is no evidence of superplasticity in the composite material in the range of temperature and strain rate where it was tested.     

Deformation Behavior of an Al2O3─Y3Al5O12 Eutectic Composite in Comparison with Sapphire and YAG

The relationship between a composite and its constituents with respect to high-temperature deformation behavior was investigated using the Al₂O₃-Y₃Al₅O₁₂ (YAG) eutectic system. The eutectic is essentially a composite with sapphire and YAG as the two phases with a volume fraction of YAG of 0.45. The deformation behavior of the eutectic and those of single-crystal sapphire and YAG were studied under identical conditions, using constant-strain-rate compression tests in air at 1530°C. The composite was also studied at 1650°C and compared with existing data on sapphire and YAG. The stronger YAG phase was found to reinforce the sapphire matrix at higher strain rates, but the composite crept faster than sapphire at lower strain rates. It is suggested using a simple semiempirical analysis that diffusional relaxation at the YAG-sapphire interface boundaries may cause the inferior creep behavior at lower strain rates.     

Thermal Expansion and Athermal Phase Transition of Y4AI2O9 Ceramics

Thermal expansion of Y₄A1₄O₉ ceramics prepared at 1600° and 1800°C was measured from room temperature to 1500°C in air. Volume changes at the phase transition of Y₄A1₂O₉, along with thermal hysteresis, were observed around 1400°C. The volume of the high-temperature phase was about 0.5% lower than that of the low-temperature phase. The hysteresis width for the sample prepared at 1600°C was 56°C, wider than that (14°C) for the sample prepared at 1800°C. The averages of phase transition start temperatures on heating and on cooling for these samples were, however, almost the same at 1377°C. The phase transitions did not occur at fixed temperatures, and the proportions of the high-temperature phase and the low-temperature phase did not change with time as long as the temperature remained constant ( athermal character). The sample prepared at 1800°C also showed another thermal hysteresis behavior from room temperature to about 1000°C.     

Mechanical Properties and Fracture Toughness of α-Al2O3-Platelet-Reinforced Y-PSZ Composites at Room and High Temperatures

YPSZ/Al₂O₃-platelet composites were fabricated by conventional and tape-casting techniques followed by sintering and HIPing. The room-temperature fracture toughness increased, from 4.9 MPa·m^1/2 for YPSZ, to 7.9 MPa·m^1/2 (by the ISB method) for 25 mol% Al₂O₃ platelets with aspect ratio = 12. The room-temperature fiexural strength decreased 21% and 30% (from 935 MPa for YPSZ) for platelet contents of 25 vol% and 40 vol%, respectively. Al₂O₃ platelets improved the high-temperature strength (by 110% over YPSZ with 25 vol% platelets at 800°C and by 40% with 40 vol% platelets at 1300°C) and fracture toughness (by 90% at 800°C and 61% at 1300°C with 40 vol% platelets). An amorphous phase at the Al₂O₃-platelet/YPSZ interface limited mechanical property improvement at 1300°C. The influence of platelet alignment was examined by tape casting and laminating the composites. Platelet alignment improved the sintered density by >1% d _th , high-temperature strength by 11% at 800°C and 16% at 1300°C, and fracture toughness by 33% at 1300°C, over random platelet orientation.     

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.     

Oxidation Behavior of Boron-Modified Mo5Si3 at 800°–1300°C

Mo₅Si₃ shows promise as a high-temperature creep-resistant material. The high-temperature oxidation resistance of Mo₅Si₃ has been found to be poor, however, limiting its use in oxidizing atmospheres. Undoped Mo₅Si₃ exhibits pest oxidation at 800°C. Mass loss occurs in the temperature range 900°–1200°C due to volatilization of molybdenum oxide, indicating that the silica scale that forms does not provide a passivating layer. The addition of boron results in protective scale formation and parabolic oxidation kinetics in the temperature range of 1050°–1300°C. The oxidation rate of Mo₅Si₃ was decreased by 5 orders of magnitude at 1200°C by doping with less than 2 wt% boron. Boron doping eliminates catastrophic pest oxidation at 800°C. The mechanism for improved oxidation resistance of borondoped Mo₅Si₃ is viscous sintering of the scale to close pores that form during the initial transient oxidation period, due to volatilization of molybdenum oxide.     

High-Temperature Phase Relations in the System Gd2O3-Ta2O5

The Phase relations of the system Gd₂O₃-Ta₂O₅ in the composition range 50 to100 mol% Gd2O3 was studied by solidstate reactions at 1350°, 1500°, or 1700°C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phase (W2 phase, space group C2221) with the composition of Gd3 TaO7 seems to melt incongruently; at about 2040°C, although this Gd₃TaO₇ Phase was previously reported to melt congruently. A new fluorite-type cubic phase (F phase, space group Fm3m ) was found for the first time above 1500°C in the system. It melts congruently with the composition of about 80mol% Gd2O3at 2318° 3°C. A phase diagram was proposed for the system Gd₂O3–Ta2O₅ in the Gd₂O₃–rich portion     

Kinetic Studies of the Thermal Synthesis of Calcium Silicates Above 1400°C: I, Dynamic Thermal Synthesis of Ca2SiO4

The dynamic reaction of SiO² with CaO (1:2 mol ratio) was studied at temperatures up to 1500°C, especially those > 1400°C. Simultaneous TG and DTA, high-temperature X-ray diffraction (horizontal sample), and high-temperature microscopy and dilatometry were used. The reactants (CaCO₃ and quartz) were finely ground and mixed in stoichiometric amounts. Thermogravimetry was used to ascertain that the samples were representative and homogeneous. The reaction mechanism in the dynamic thermal synthesis of Ca₂SiO₄ was elucidated and confirmed by high-temperature X-ray diffraction.     

Orientation Relationships of Reactively Grown Ba6Ti17O40 and Ba2TiSi2O8 on BaTiO3 (001) Determined by X-ray Diffractometry

Fresnoite grows at 700° and 800°C, and Ba₆Ti₇O₄₀ grows at 1200°C with definite orientations, which are determined by X-ray diffraction pole figure analysis. Partially textured fresnoite is formed at higher temperatures. The SiO₂ films react with the BaTiO₃ crystals, forming the phases Ba₂TiSi₂O₈ (fresnoite) and Ba₆Ti₁₇O₄₀. At 700° and 800°C, both phases grow with definite orientations, which are determined by X-ray diffraction pole figure analysis. Partially textured polycrystalline phases are formed at higher temperatures.     

High-Temperature Transmission Electron Microscopy and X-Ray Powder Diffraction Studies of Polymorphic Phase Transitions in Ba4Nb2O9

Polymorphic phase transitions in Ba₄Nb₂O₉ were studied by thermal analyses, high-temperature transmission electron microscopy and X-ray powder diffractometry. Two stable polymorphs were isolated, low-temperature α-modification and high-temperature γ-modification, with the endothermic phase transition at 1176°C. The α→γ transformation is accompanied by the formation of a 120° domain structure, which is a consequence of hexagonal→orthorhombic unit cell reconstruction. Reheating the presintered γ-Ba₄Nb₂O₉ results in the formation of a metastable γ'-modification (formerly known as β-polymorph) in the temperature range between 360° and 585°C, before the γ→α transformation at 800°C. Above ∼490°C Ba₄Nb₂O₉ becomes moderately sensitive to a loss of BaO. In air the surface of Ba₄Nb₂O₉ grains decomposes to nanocrystalline Ba₅Nb₄O₁₅ and BaO, which instantly reacts with atmospheric CO₂ to form BaCO₃. Surface reaction delays γ→α transformation up to 866°C in air. In vacuum the loss of BaO is even more enhanced and consequently the formation of minor Ba₃Nb₂O₈ phase is observed above 1150°C.     

Pressure-Assisted Combustion Synthesis of Dense Layered Ti3AlC2 and its Mechanical Properties

A near-single-phase Ti₃AlC₂ ternary carbide was synthesized from 3Ti–1.1Al–1.8C powder blend, both by the wave propagation and thermal explosion (TE) modes of self-propagating high-temperature synthesis. The application of a moderate (28 MPa) pressure immediately after TE at 800°C (reactive forging) yielded a 95% dense material containing, in addition to Ti₃AlC₂, an appreciable amount of TiC_{1− x} . By adjusting the starting composition, a 99% dense material containing up to 90 vol.% Ti₃AlC₂ was obtained. The material had a fine-layered microstructure with Ti₃AlC₂ grain size not exceeding 10 μm. The samples were readily machinable and had a high compressive strength of ∼800 MPa up to 700°C.     

The System TiO2-Bi2Ti4O11

The system TiO₂-Bi₂Ti₄O₁₁ was examined by Raman spectroscopy and X-ray diffraction to determine whether TiO₂ is soluble in Bi₂Ti₄O₁₁. The Raman spectral data obtained from preparations made at ∼ 1050°C and cooled to room temperature led us to conclude that TiO₂ is not soluble in the "high-temperature" form of Bi₂Ti₄O₁₁. It was also found that extensive grinding of the phase identified as the "high-temperature" form converts it to the "low-temperature" form, stable below 250°C.     

Lower-Temperature Modifications of Nb2C and V2C

The orthorhombic, low-temperature α-modification of Nb₂C has a structure similar to that of ζ-Fe₂N (a = 12.36 A, b = 10.89₅ A, c = 4.96₈ A) and transforms at ∼1200°C into the hexagonal ε-Fe₂N type. A second transition at approximately 2500°C is associated with the destruction of long range order in the carbon sublattice. Alpha-divanadium carbide (orthorhombic, a = 11.49 A, b = 10.06 A, c = 4.55 A) is isostructural with α-Nb₂C and transforms at ∼800°C into the hexagonal high-temperature modification. The structures of α-V₂C and α-Nb₂C are distorted modifications related to the ε-Fe₂N type.     

Densification of Cr2O3-ZrO2 Ceramics by Sintering

Cr₂O₃ and ZrO₂ were mixed in various ratios and pressed to form compacts, which were then sintered in carbon powder. Compacts with >30 wt% Cr₂O₃ were sintered to densities >98% of true density at 1500°C. This method of sintering in carbon powder can be used to prepare very dense Cr₂O₃-ZrO₂ ceramics at a relatively low temperature, (∼1500°C) without additives.     

Cyclic Fatigue-Crack Growth and Fracture Properties in Ti3SiC2 Ceramics at Elevated Temperatures

The cyclic fatigue and fracture toughness behavior of reactive hot-pressed Ti₃SiC₂ ceramics was examined at temperatures from ambient to 1200°C with the objective of characterizing the high-temperature mechanisms controlling crack growth. Comparisons were made of two monolithic Ti₃SiC₂ materials with fine- (3–10 μm) and coarse-grained (70–300 μm) microstructures. Results indicate that fracture toughness values, derived from rising resistance-curve behavior, were significantly higher in the coarser-grained microstructure at both low and high temperatures; comparative behavior was seen under cyclic fatigue loading. In each microstructure, Δ K _th fatigue thresholds were found to be essentially unchanged between 25° and 1100°C; however, there was a sharp decrease in Δ K _th at 1200°C (above the plastic-to-brittle transition temperature), where significant high-temperature deformation and damage are first apparent. The substantially higher cyclic-crack growth resistance of the coarse-grained Ti₃SiC₂ microstructure was associated with extensive crack bridging behind the crack tip and a consequent tortuous crack path. The crack-tip shielding was found to result from both the bridging of entire grains and from deformation kinking and bridging of microlamellae within grains, the latter forming by delamination along the basal planes.     

Phase Equilibrium Studies in the System Iron Oxide-Al2O3-Cr2O3

Subsolidus phase relations in the system iron oride-Al₂O₂-Cr₂O₃ in air and at 1 atm. O₂ pressure have been studied in the. temperature interval 1250° to 1500°C. At temperatures below 1318° C. only sesquioxides with hexagonal corundum structure are present as equilibrium phases. In the temperature interval 1318° to 1410°C. in air and 1318° to 1495° C. at 1 atm. O₂, pressure the monoclinic phase Fe₂O₃. Al₂O₃ with some Cr₂O₃ in solid solution is present in the phase assemblage of certain mixtures. At temperatures above 1380°C. in air and above 1445°C. at 1 atm. O₂ pressure a complex spinel solid solution is one of the phases present in appropriate composition areas of the system. X-ray data relating d- spacing to composition of solid solution phases are given.     

Effect of Li2CO3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Mg2V2O7 Ceramics

Li₂CO₃ was added to Mg₂V₂O₇ ceramics in order to reduce the sintering temperature to below 900°C. At temperatures below 900°C, a liquid phase was formed during sintering, which assisted the densification of the specimens. The addition of Li₂CO₃ changed the crystal structure of Mg₂V₂O₇ ceramics from triclinic to monoclinic. The 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic was well sintered at 800°C with a high density and good microwave dielectric properties of ɛ _r=8.2, Q × f =70 621 GHz, and τ_f=−35.2 ppm/°C. Silver did not react with the 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic at 800°C. Therefore, this ceramic is a good candidate material in low-temperature co-fired ceramic multilayer devices.     

High-Temperature Phase Transition of Y4Al2O9

Reversible thermal phase transition was observed for Y₄Al₂O₉ at 1377°C with a hysteresis width of 78°C using differential scanning calorimetry. The enthalpy of the transition (Δ H ) was about 300 cal/mol. Powder X-ray diffraction peaks of the high-temperature phase, as well as the peaks of the low-temperature phase, were characterized with a monoclinic unit cell. Thermal hysteresis was also confirmed by the temperature dependence of the lattice parameter. The unit cell volume of the high-temperature phase was 0.5% smaller than that of the low-temperature phase at 1400°C.