Mechanical Properties of Chemically Strengthened Glasses at High Temperatures

Young's modulus, shear modulus, and modulus of rapture for two chemically strengthened glasses were determined at high temperatures. The Young's modulus and shear modulus decreased with increasing temperature, with a sharp inflection slightly above room temperature. The region of inflection indicated an internal friction peak. For comparison Young's modulus and shear modulus were determined as a function of temperature on a thermally tempered soda-lime-silica glass and on a semitempered borosilicate glass. Curves of these moduli, in contrast to those for the chemically strengthened glasses, did not reveal regions of inflection. The modulus of rupture is not affected by short exposure to heat up to 260°C., but decreases appreciably when exposed to temperatures above 204°C for 200 hr or more. Deflection measurements at room temperature showed that the two chemically strengthened glasses had about five times as much delayed elasticity as did thermally tempered soda-lime-silica glass.     

Elasticity and Anelasticity of Uranium Oxides at Room Temperature: I, Stoichiometric Oxide

The elastic modulus and internal friction of stoichiometric uranium oxide at room temperature were studied using a dynamic method. The elastic modulus of stoichiometric urania at room temperature increases with increasing density. When the volume fraction porosity is less than 0.1, either linear or exponential equations can he used to calculate the elastic modulus as a function of density. When the volume fraction porosity is more than 0.1, a linear equation seems to be more suitable. The elastic modulus of stoichiometric nonporous uranium oxide at room temperature was found, by extrapolation, to be 2243.56 ± 22.1 kbars when the exponential equation was used, and 2233.85 ± 22.05 kbars when the linear expression was used. The internal friction of stoichiometric urania decreases sharply as the grains become larger. The number, size, and position of pores may also affect the internal friction values.     

Elastic and Anelastic Response of Polycrystalline UO2-PuO2

A sonic resonance technique was used to investigate the room-temperature elastic and anelastic properties of physically mixed U_0.8PU_0.2O₂ as a function of density, stoichiometry, and cation homogeneity. The effect of porosity on the elastic moduli was linear and is described by E =2102.7 (1–2.03 P )± 13.5 Kbars for the Young's modulus, G =823.5(1–2.05 P )± 9.1 kbars for the shear modulus, and B = 1584.8(1–1.89 P )± 59.1 kbars for the bulk modulus, where P is the volume fraction porosity. Poisson's ratio was 0.28 and was not a function of porosity. The Debye temperature of U_0.8Pu_0.2O₂ computed from the Young's and shear moduli for theoretically dense specimens was 379°K. Variation of the O/M ratio from 1.968 to 2.006 produced no significant change in either the damping capacity or the elastic moduli of single-phase 80%UO₂-20% PuO₂ solid solutions. An approximate 24% decrease of the room-temperature Young's and shear moduli and an approximate increase by a factor of 14 in the internal friction were observed with gross modifications of plutonium cation homogeneity. Preliminary results suggest that internal friction measurements might be used to assay the homogeneity of UO₂-PuO₂ solid solutions.     

Crack Healing and Bending Strength of Aluminum Titanate Ceramics at High Temperature

Bending strength and Young's modulus of aluminum titanate ceramics at room temperature to 1300°C were examined. Bending strength increased from 62 MPa at room temperature to 280 MPa at 1100°C. Young's modulus also increased, to 99 GPa at 1100°C. These increments were caused by crack healing. In particular, crack cylinderization occurring at 1000° to 1100°C markedly increased the mechanical strength. The thermal-hysteresis curves also showed healing of grain-boundary cracks.     

High-Temperature Young's Modulus of Alumina During Sintering

High-temperature Young's modulus of a partially sintered alumina ceramic has been studied dynamically during the sintering process. Comparative, room-temperature Young's modulus data were obtained for a suite of partially sintered alumina compacts with different porosities. The dynamic Young's modulus of a 1200°C partially sintered material was observed to decrease linearly with temperature, but then above 1200°C it increased sharply as sintering and densification of the alumina became dominant. The evolution of the Young's modulus due purely to sintering exhibited an exponential relationship with porosity in excellent agreement with room-temperature measurements of equivalent porous alumina ceramics.     

Elastic Properties of Polycrystalline Monoclinic Gd2O3

The elastic properties of polycrystalline monoclinic Gd₂O₃ were determined by the sonic-resonance method. Volume-fraction porosity varied from 0.025 to 0.367 and temperature from room temperature to 1400°C. The Young's and shear moduli are linear functions of volume-fraction porosity, but the rate of their decrease with increasing porosity is less than that expected. The moduli decreased more rapidly than expected with increasing temperature. The Debye temperature is 362°K. With increasing temperature, the first Grueneisen constant, γ, decreases, whereas the second Grueneisen constant, δ, increases.     

Temperature Dependence of Young's Modulus and Internal Friction in Alumina, Silicon Nitride, and Partially Stabilized Zirconia Ceramics

The temperature dependence of Young's modulus and internal friction (Q⁻¹)in alumina, silicon nitride, and partially stabilized zirconia (Y-PSZ) ceramics was studied. Little change in Q⁻¹ was found for alumina, whereas Q⁻¹ for silicon nitride ceramics increased above 700°C. The Q⁻¹ of Y-PSZ increased markedly with increasing temperature up to a peak at ∼200°C.     

Temperature Dependence of Elastic Constants of Vitreous Silica

The relation between the elastic moduli and temperature from room temperature to about 1300°C was determined for a group of vitreous silica specimens by a dynamic resonance method. All the curves were approximately parabolic in shape, reaching a maximum near 1050° to 1200°C. At the maximum value, Young's modulus was more than 11% higher and the shear modulus was about 9% higher than their room-temperature values. Poisson's ratio was then computed to rise from about 1/6 at room temperature to about 1/5 at the temperature of maximum elastic modulus. Small but significant differences were observed in the temperature-modulus curves for specimens from different sources. These differences were found to be related to differences in the infrared transmission curves.     

Elasticity and Anelasticity of Uranium Oxides at Room Temperature: II, Hyperstoichiometric Oxides

The Young's modulus and internal friction of hyperstoichiometric uranium oxide at room temperature were determined by a sonic technique. Young's modulus decreased, whereas internal friction increased, with increasing O/U ratios. These effects are attributed to the precipitation of U₄O₉. A complete microstructural characterization is needed.     

Young's Modulus of Various Refractory Materials as a Function of Temperature

Young's modulus as a function of temperature was determined by a dynamic method for single-crystal sapphire and ruby and for polycrystalline aluminum oxide, magnesium oxide, thorium oxide, mullite, spinel, stabilized zirconium oxide, silicon carbide, and nickel-bonded titanium carbide. For the single crystals, Young's modulus was found to decrease linearly with increasing temperature from 100°C. to the highest temperature of measurement. For all the polycrystalline materials, except silicon carbide, stabilized zirconium oxide, and spinel, Young's modulus was found to decrease approximately linearly with increasing temperature until some temperature range characteristic of the material was reached in which Young's modulus decreased very rapidly and in a nonlinear manner with increasing temperature. This rapid decrease at high temperature is attributed to grain-boundary slip. Stabilized zirconium oxide and spinel were found to have the same rapid decrease in Young's modulus at high temperature, but they also had a decidedly nonlinear temperature dependence at low temperature.     

Influence of Chemical Reactions in Magnesia-Graphite Refractories: I, Effects on Texture and High-Temperature Mechanical Properties

Four formulations of magnesia-graphite-aluminum metal (antioxidant) bricks were prepared from the same raw materials, using the standard commercial practices. Chemical analysis and determination of room-temperature modulus of rupture and Young's modulus, as well as a complete microstructural characterization of the as-received materials, were performed. For high-temperature modulus-of-rupture and Young's modulus data, test samples of the four brick compositions were heated to 1000°, 1200°, and 1450°C in flowing argon (<1000 ppm oxygen at 1000°C) and then loaded mechanically in flexure. Modulus-of-elasticity values ranged from 3.7 to 16.2 GPa and reflected strong effects of aluminum-metal concentration and treatment temperature. Young's modulus evolution with temperature was determined by the evolution of the microstructure in the bulk of the specimens. Modulus-of-rupture values ranged from 6 to 21 MPa, and their evolution with temperature was determined by the evolution of the microstructure in the bulk of the specimens at the lower testing temperatures ( T lessthan equal to 1200°C) and by phase assemblages in the surface regions of the specimens-essentially by the presence of the dense MgO zone-at 1450°C.     

In Situ X-ray Diffraction and Young's Modulus Measurement during Heat Treatment of High-Alumina Cement Castables

In situ Young's modulus measurements and synchrotron radiation-energy dispersive diffraction have been used to study changes in high-alumina castables subjected to heat treatment from room temperature to 1600°C. Particular attention was paid to the hydrate "conversion" process and the effects of high temperature.     

Mechanical Properties of Magnesia–Spinel Composites

The mechanical properties of magnesia–spinel composite ceramics, which are candidate materials for supporting solid oxide fuel cells, have been measured as a function of porosity (up to 30%) and temperature (up to 900°C). The theory for the ring-on-ring test has been re-examined to resolve an inconsistency in the literature.
The Young's modulus shows an exponential dependence on porosity that is in agreement with the expectation of minimum solid area models. Fracture toughness, fracture energy, and flexural strength are all approximately proportional to Young's modulus.
The mechanical properties are not greatly dependent on temperature, but there is a detectable increase in fracture toughness with temperature, which could be due to some limited plasticity.     

Elastic Properties of Polycrystalline Yttrium Oxide, Holmium Oxide, and Erbium Oxide: High-Temperature Measurements

The Young's and shear moduli of polycrystalline yttrium oxide, holmium oxide, and erbium oxide were determined from room temperature to 1000°C using the sonic resonance technique. The bulk modulus and Poisson's ratio were computed as functions of temperature for each oxide. The Young's, shear, and bulk moduli decreased linearly with increasing temperature, whereas Poisson's ratio remained constant. The first and second Grüneisen constants, γ and δ, were calculated from the bulk modulus data and shown to be virtually independent of temperature. The Soga-Anderson equation adequately described the bulk modulus data for each oxide.     

Influence of Chemical Reactions in Magnesia-Graphite Refractories: II, Effects of Aluminum and Graphite Contents in Generic Products

A group of magnesia-graphite and magnesia-graphite-aluminum materials, the compositions of which represent a wide range of graphite contents (~10-16.4 wt%), aluminum contents (0-5.2 wt%), and MgO and graphite qualities, were fabricated, using standard commercial practices. Chemical analysis and determination of room-temperature modulus of rupture (MOR) and Young's modulus, as well as a complete microstructural characterization of the as-received materials, were performed. Mechanical characterization at high temperature (1000°, 1200°, and 1450°C) was done in terms of Young's modulus and MOR in an argon atmosphere (<1000 ppm oxygen at 1000°C). Modulus-of-elasticity values ranged from 4 to 16 GPa, and their evolution with temperature was determined by the evolution of the microstructure in the bulk of the specimens. A strong effect of aluminum-metal concentration on Young's modulus overrode other microstructural differences among the materials. MOR values ranged from 6 to 20 MPa, and their evolution with temperature was determined by the evolution of the microstructure in the bulk of the specimens at the lower testing temperatures ( T lessthan equal to 1200°C) and by phase assemblages in the surface regions of the specimens-essentially by the presence of the dense MgO zone-at 1450°C. The thickness of the dense MgO zone in the aluminum-containing materials was determined by the amount of aluminum and the MgO aggregate size.     

Mechanical and Thermal Properties of Antiperovskite Ti3AlC Prepared by an In Situ Reaction/Hot-Pressing Route

Bulk Ti₃AlC ceramic containing 2.68 wt% TiC was prepared by an in situ reaction/hot-pressing route. The reaction path, microstructure, mechanical and thermal properties were systematically investigated. At room temperature Vickers hardness of Ti₃AlC ceramic is 7.8 GPa. The flexural strength, compressive strength, and fracture toughness are 182, 708 MPa, and 2.6 MPa·m^1/2, respectively. Its apparent Young's modulus, shear modulus, bulk modulus and Possion's ratio are 208.9, 83.4, 140.4 GPa, and 0.25 at room temperature. Apparent Young's modulus decreases slowly with the increasing temperature, and at 1210°C the modulus is 170 GPa. The average coefficient of thermal expansion of Ti₃AlC ceramic is about 10.1 × 10⁻⁶ K⁻¹ in the temperature range of 150°–1200°C. Both the molar heat capacity and thermal conductivity increase with an increase in the temperature. At 300 and 1373 K, the molar heat capacities are 87 and 143·J·(mol·K)⁻¹, while the thermal conductivities are 8.19 and 15.6 W·(m·K)⁻¹, respectively.     

High-Temperature Mechanical Properties of Ceramic Materials: V, (Zn0.65, Mg0.35)2SiO4

The composition (0.65Zn,0.35Mg)₂ SiO₁ was investigated. Its thermal expansion was 32 × 10^-7/°C from room temperature to 1000°C. Modulus of rupture was approximately 7000 psi between room temperature and 800°C, whereas Young's modulus held at approximately 11 × 10° psi over the same range. The substitution of 0.35 m oles Mg⁺⁺ for Zn⁺⁺ in Zn₂Si0₄ causes little change in many of the physical properties, but the solid solution sinters much more readily than pure Zn₂Sio₄. The willemite solid solution studied has very good thermal shock resistance between room temperature and 1000°C.     

High-Temperature Properties of Unburned MgO-C Bricks Containing Al and Si Powders

Changes in the mechanical and thermal properties, as well as in the microstructure, of unburned MgO-C bricks containing Al and Si powders were investigated at selected temperatures. Specimens with heat treatments at 500°C shrank and exhibited higher apparent porosity than untreated specimens. The bending strength and elastic modulus at 500°C were much lower than those of untreated specimens, and the apparent porosity increased and the mechanical properties at 500°C decreased with each repeating heat treatment. It was predicted that when the volatile matter was no longer generated, microstructure shrinkage would stop and the mechanical properties become constant. The bending strength and static elastic modulus at 800°, 1000°, and 1300°C were higher than those at 500°C because of the binding effect of the reaction products (i.e., Al₄C₃, SiC, and MgAl₂O₄), although the apparent porosity was higher than at 500°C. Repeated heat treatment from room temperature (RT) to the respective temperature, however, degraded the properties to nearly the same level as at 500°C because of the increased apparent porosity and the cracks generated in magnesia particles by the reaction products. Plastic deformation appeared to occur at 1300°C just before bricks were fractured. In addition, the thermal expansion ratio decreased through repeated heating and cooling from RT to 500°, 1000°, or 1300°C, and finally decreased to a constant value, as predicted.     

Processing and Mechanical Properties of Dense Si3N4-SiC-Whisker Composites without Sintering Aids

Si₃N₄ with 20 vol% SiC whisker was fabricated without sintering aids by hot isostatic pressing. Density higher than 99.5% was attained after sintering at 2000°C and 170 MPa for 1 h. Careful mixing procedures and the use of an appropriate amount of a dispersant was found to be effective in avoiding whisker segregation and inhomogeneity. Mechanical properties of the composite were investigated by measurements of flexural strength, microhardness, frature toughness, and Young's modulus as a function of temperature. At room temerature, Vickers microhardness and Young's modulus increased from the matrix value about 20% and 5%, respectively. Toughness was about 30% higher, without reduction in flexural strength, up to 1400Deg;C.     

Some Physical Properties of High-Density Thorium Dioxide

The values for a number of physical properties are reported for a very high density form of thorium dioxide. When specimens of a mixture of 99½% ThO² and ½% CaO, by weight, were hydrostatically pressed at 30,000 lb. per sq. in. and heat-treated for 1 hour at 1800°C., they attained 99.0% of theoretical density. All the test specimens were extremely brittle. Physical-property values determined at room tempera- ture were the following: lattice constant; bulk and theoretical (X-ray) densities; compressive and impact strengths; Knoop hardness; modulus of rupture and Young's modulus, determined by a static method; Young's modulus and the shear modulus, determined by a dynamic method; Poisson's ratio and the bulk modulus, calculated from the dynamic-test data; and the velocity of sound through the material. The properties determined at elevated temperatures were the following : linear thermal expansion modulus of rupture and Young's modulus, determined by a static method; Young's modulus and the shear modulus, determined by a dynamic method; and Poisson's ratio, calculated from the elevated-temperature dynamic-test data. "Martin's diameter" grain counts were taken for the material both before and after heat-treatment.