Deformation by the Stress-Induced Transport of Helium Bubbles in Silicon Carbide

Silicon carbide (SiC) containing He atoms expands by annealing above ∼ 1300°C, owing to the formation and growth of He bubbles. An external compressive stress applied during annealing retards expansion in the stress direction, while promoting it in the lateral directions. It is proposed that He atoms in the bubbles on grain boundaries perpendicular to the compression axis are transported to the bubbles on boundaries parallel to the stress axis. The transport of He accompanies changes in the volumes of the bubbles to maintain equal pressure in all the bubbles. The volume changes of the bubbles are caused by the flow of SiC-constituting atoms from or to the boundaries where the volume-changing bubbles exist. This process results in a jacking action which alters the thickness of boundary atom layers. It produces macroscopic strains. The plausibility of the proposed process is examined on the basis of energetics.     

Development of Surface Compressive Stresses in Zirconia–Alumina Composites by an Ion-Exchange Process

A two-step ion-exchange technique was developed for introducing compressive stresses on the surface of ZrO₂–Al₂O₃ composites. In the first step, a thin layer (∼250 μm) of Na-β"-Al₂O₃ was formed on the surface of the composite by a vapor-phase process at ∼1400°C. In the second step, Na⁺ ions were replaced by K⁺ ions by a heat treatment at ∼385°C for 2 h in a molten KNO₃ bath. Replacement of sodium by potassium led to the creation of surface compressive stresses. The flexural strength and Weibull modulus of ZrO₂–Al₂O₃ composite were ∼915 MPa and 10, respectively, for the as-sintered samples. By contrast, the flexural strength and Weibull modulus were ∼1140 MPa and 26, respectively, for the ion-exchanged samples. A residual surface compressive stress of ∼480 MPa was measured by a strain-gauge technique in K⁺-ion-exchanged samples. The presence of surface compressive stresses also was confirmed using an indentation technique. The technique developed here can be used to introduce compressive stresses on components of virtually any shape.     

Damage-Enhanced Creep in a Siliconized Silicon Carbide: Phenomenology

The creep behavior of a commercial grade of reaction-bonded silicon carbide was characterized at a temperature of 1300°C. Creep occurred more easily in tension than in compression. At a given applied stress, the steady-state creep rate in tension was found to be at least 20 times that obtained in compression. In both tension and compression, the stress exponent for steadystate creep was found to increase with increasing applied stresses. At low applied stresses, the stress exponent was ∼4, suggesting some kind of dislocation mechanism operating in the two-phase composite. At high stresses, the stress exponent was ∼11 in tension. The increase in the stress exponent was attributed to damage accumulation in the form of cavities. An effective threshold stress for cavitation of less than 100 MPa was suggested. In compression, the cause of the increase of stress exponent with stress cannot be attributed to cavitation.     

Lifetime-Applied Stress Response in Air of a SiC-Based Nicalon-Fiber-Reinforced Composite with a Carbon Interfacial Layer: Effects of Temperature (300° to 1150°C)

The lifetimes in air as a function of applied flexure stress and temperature (300–1150°C) are described for a Si–O–C based (Nicalon) fiber plain-weave cloth reinforced SiC-matrix composite (∼7% closed porosity) with an ∼0.3 µm thick carbon interfacial layer. The measured lifetimes of both samples with and without an external SiC seal coating were similar and decreased with applied flexural stress (for stresses greater than ∼90 MPa) and with temperature. At temperatures of ≥600°C, the external CVD SiC coating had negligible effect on the lifetimes; however, at 425°C, a detectable improvement in the lifetime was observed with an external SiC coating. When the applied stress was decreased below an apparent "threshold stress" (e.g., ∼90 MPa) for tests conducted at temperatures ≤950°C, no failures were observed for times of ≥1000 h. Electron microscopy observations show that the interfacial carbon layer is progressively removed during tests at 425° and 600°C. In these cases, failure is associated with fiber failure and pullout. At 950° and 1150°C, the carbon interface layer is eliminated and replaced by a thick silica layer due to the oxidation of the Nicalon fiber and the SiC matrix. This results in embrittling the composite.     

Flexure Strength of Melt-Infiltration-Processed Titanium Carbide/Nickel Aluminide Composites

TiC/Ni₃Al composites were prepared using a simple melt-infiltration process, performed at either 1300° or 1400°C, with the Ni₃Al content varied over the range of 8–25 vol%. Densities >96% of theoretical were obtained for all composites. Four-point flexure strengths at 22°C increased as the Ni₃Al content increased (i.e., ∼1100 MPa at 20 vol% Ni₃Al), with the highest strengths being observed for composites processed at 1300°C, because of reduced TiC grain size. Strengths at elevated temperatures increased with test temperature, up to ∼1000°C. As with the yielding behavior of the Ni₃Al alloy used, a maximum in composite strength (∼1350 MPa) versus temperature was observed; this occurred at 950°C, which is ∼300°C above the yield maximum for the alloy. Extensive plastic strain was achieved in the composites even at high loading rates at 1135°C, and the yield stress was dependent on the applied loading rate.     

Abrasive Wear Behavior of Heat-Treated ABC-Silicon Carbide

Hot-pressed silicon carbide, containing aluminum, boron, and carbon additives (ABC-SiC), was subjected to three-body and two-body wear testing using diamond abrasives over a range of sizes. In general, the wear resistance of ABC-SiC, with suitable heat treatment, was superior to that of commercial SiC. When the fine-scale (3 μm) diamond abrasives were used, it was found that thermal annealing at 1300°C increased the resistance to three-body wear by a factor of almost three, and two-body wear by a factor of almost two, compared with as-hot-pressed samples. Higher annealing temperatures, however, led to a decline in wear resistance from its highest value. Similar behavior was seen for 1300°C-annealed samples subjected to 15 μm diamond abrasive, although higher-temperature annealing at 1500°–1600°C enhanced the wear resistance again. When coarse abrasives (72 μm) were used, the wear resistance progressively increased with increased annealing temperature from ∼1000° to 1600°C. Corresponding transmission and scanning electron microscopy studies indicated that, whereas transgranular, conchoidal cracking was dominant in the mild abrasive wear with fine-scale (3 μm) abrasives, intergranular cracking and subsequent grain pullout was far more predominant in the more severe abrasive wear with coarse abrasives. Because the hardness and indentation toughness were barely altered during thermal treatment, the observed wear behavior was attributed mainly to the thermally induced microstructural changes, including the crystallization of glassy grain-boundary films, the possible strengthening of the boundaries due to the enhancement of the aluminum, and the formation of aluminum-rich, coherent nanoscale precipitates in the matrix grains above 1300°C.     

Mullite Development from Fibrous Kaolin Mineral

The development of mullite from kaolin minerals was studied using transmission electron microscopy and X-ray powder diffraction methods on the unusual long-fibered kaolin from Piedade, Sao Paulo, Brazil. The mullite crystals observed at ≥ 1200°C initially exhibited an elliptical shape and were randomly oriented inside the tubes; at 1250°C their shape changed to laths oriented either parallel to the tube length or at 120° to the adjacent lath. At 1300°C, the shape changed to a bipointed lath and the crystals developed a hexagonal configuration with preferred orientation at 120°, similar to mullite developed from kaolinite plates. Thus, the temperature at which mullite begins to develop from this tubular kaolin is ∼200°C above that normally observed in well-crystallized kaolinites and is 100°C above b axis disordered kaolinite. Cristobalite was not observed in the electron microscope in the range 1000° to 1300°C.     

Fabrication of Textured Alumina by High-Temperature Deformation

Texture formation via high-temperature deformation in sintered alumina was investigated. Fine-grained, normal-purity-alumina sintered bodies deformed under stresses up to 80 MPa in a temperature range of 1200°–1300°C. Fine, disklike grains formed in the equiaxial fine-grained matrix during high-temperature deformation and aligned unidirectionally via material flow during deformation. Highly textured sintered alumina bodies were obtained via high-temperature deformation and further annealing.     

Harper–Dorn and Power-Law Creep in Single-Crystalline Magnesium Oxide

The creep behavior of single-crystalline MgO tested in the 〈100〉 direction is reviewed in the temperature range 1300° to 1800°C. At low stresses, the stress exponent is equal to about unity, and the deformation process is attributed to Harper–Dorn creep. At high stresses, the stress exponent is equal to approximately 5 and the deformation process is attributed to dislocation glide controlled by climb. The creep behavior in both regions is successfully predicted by an internal stress model for Harper–Dorn creep.     

Strength of Silicon Nitride after Thermal Shock

A water-quenching technique was used to evaluate the thermal-shock strength behavior of silicon nitride (Si₃N₄) ceramics in an air atmosphere. When the tensile surface was shielded from air during the heating and soaking process, the quenched specimens showed a gradual decrease in strength at temperatures above 600°C. However, the specimens with the air-exposed surface exhibited a ∼16% and ∼29% increase in strength after quenching from 800° and 1000°C, respectively. This is because of the occurrence of surface oxidation, which may cause the healing of surface cracks and the generation of surface compressive stresses. As a result, some preoxidation of Si₃N₄ components before exposure to a thermal-shock environment is recommended in practical applications.     

Modifying Structures and Ferroelectric Properties of Pb(Zr0.6Ti0.4)O3 Thin Films by Constraint-Annealing of Precrystallized Film

A tensile or compressive mechanical constraint was applied, during annealing, on the Pb(Zr_0.6Ti_0.4)O₃ (PZT) ferroelectric films to investigate the effects of stress on its crystal structure and electric properties. The external stress was applied by bending the substrate into a circular section. By using both precrystallized film structure and high constraint strain (0.08%), the stress states of PZT during the crystallization process became controllable. Structural change of polycrystalline PZT was observed when crystallized under a compression constraint. Moreover, these films with compression constraint annealing exhibited enhanced remnant polarization by ∼70% and increased dielectric constant by ∼68%. The variations in ferroelectric behaviors were correlated to domain configuration, texture, amount of pyrochlore phase, grain size and residual stress, which are dependent on the stress state during annealing process.     

槽式太阳能集热管内相变微胶囊悬浮液的热力性能分析

针对传统使用水基和油基的太阳能集热器换热效果低和管壁热应力大的问题，以相变微胶囊悬浮液为工作流体，对抛物型槽式太阳能集热器进行了三维建模。采用蒙特卡罗射线追踪法结合有限容积法和有限元法的方法求解了太阳能集热管的光?热?力耦合问题，采用欧拉?欧拉多相流模型研究了相变微胶囊悬浮液在集热管内的流动换热特性。结果表明，相变微胶囊悬浮液强化了集热管内的对流换热，不仅降低了集热管的沿程壁温，且减少了集热管的周向温差，均化了集热管温度分布。集热管周向等效热应力呈花瓣型分布，对应的5个高温度梯度的位置附近(圆周角θ=5°, 90°, 175°, 225°和315°)出现等效应力局部峰值。吸热管内壁面θ=90°处轴向热应力为压应力，作用于整个管程，而径向热应力和切向热应力为拉应力，主要作用在进出口端。相变微胶囊悬浮液浓度越高，强化换热效果越好，集热管热应力越小，但产生的压降也随之增大。     

Effect of Thermal Exposures on the Strengths of Nextel™ 550 and 720 Filaments

The effects of thermal exposure on the strengths of Nextel™ 550 and 720 tows, bare and coated with carbon, were determined by room-temperature tensile testing of single filaments extracted from tows that had been exposed to different thermal environments (i.e., air or vacuum) at temperatures from 550° to 1400°C. The results help define the allowable composite processing conditions when using these tows. A 28% drop in the strength of Nextel 550 filaments occurred after a thermal exposure at 1100°C for 2 h in air. After an exposure of 1300°C/2 h/air, a strength degradation of ∼47% resulted. Filaments exposed above 1100°C under vacuum showed more severe strength degradation than filaments exposed in air. The observed strength degradation may stem from a combination of phase transformations of the alumina, the onset of mullite crystallization, and/or exaggerated mullite grain growth. Strength after heat treatment under vacuum at 1050° and 1150°C did not deteriorate as rapidly as after heat treatment under vacuum between 950° and 1050°C or between 1150° and 1250°C. This may be a result of the competition between healing of flaws by the amorphous silica and its evaporation (leading to an increase in its viscosity or loss) and/or densification of the filaments. Nextel 720 filaments exhibited about 9% strength loss after an exposure at 1100°C/2 h/air. The filaments maintained 75% of their strength after a 1300°C/2 h/air heat treatment. The observed strength degradation may stem from thermal grooving, grain growth, and/or annealing of the mullite subgrain boundaries. Thermal exposure of >10 h at 1300°C was required to produce measurable grain growth. Strength loss between 1200° and 1300°C (air heat treatment) was not as great as between 1100° and 1200°C or 1300° and 1400°C.     

High-Temperature Creep of Yttria-Stabilized Zirconia Single Crystals

Creep of 9.4-mol%-Y₂O₃-stabilized cubic ZrO₂ has been studied between 1300° and 1550°C. Conventional power-law creep (stress exponent n ∼ 4.5) is found at the higher temperatures, with an activation energy (∼6 eV) corresponding to cation diffusion. Transition to a different creep mechanism occurs at the lower temperatures, as indicated by higher values of the stress exponent ( n ∼ 7) and an activation energy (∼7.5 eV) higher than that for cation self-diffusion. The lower-temperature behavior is caused by a competition between cross-slip-controlled and recovery-controlled creep. Consideration of all the creep and diffusion data now available suggests that the rate-controlling high-temperature mass transport in Y₂O₃-stabilized ZrO₂ can be described by D = 10⁻³ exp(-5.0 eV/ kT ) m²·s⁻¹.     

Binary Titania-Silica Glasses Containing 10 to 20 Wt% TiO2

A series of glasses in the TiO₂-SiO₂ system was prepared by the flame hydrolysis boule process. Clear glasses containing as much as 16.5 wt% TiO₂ were obtained. More TiO₂ caused opacity due to phase separation and anatase/rutile crystallization during glass boule formation. Glasses in the 12 to ∼17 wt% TiO₂ range were metastable and showed structural rearrangements on heat treatment at temperatures as low as 750CEC (∼200° below the annealing point). These changes were accompanied by large changes in thermal expansion. Thermal treatment can be designed to produce almost any desired expansion between α_-200+700=−5 × 10^-7/°C and +10 × 10^-7/°C. Zero expansion between 0 and 550°C was obtained. Evidence that these changes are due to phase separation and anatase formation is presented. Viscosity data in the glass transition range, refractive index, and density are also presented.     

The measurement of thermal stress distributions along the flow path in injection-molded flat plates

Internal stresses in injection-molded parts are the result of thermal, flow, and pressure histories. Internal stresses can be roughly divided into thermal and flow-induced stresses. In this paper, a modified layer-removal method is presented to determine thermal stress distributions in injection-molded flat plates. With this method, the curvature of a rectangular specimen is determined after the removal of a layer from one surface. This curvature is converted into a stress via a mathematical relation, originally derived by Treuting and Read. By determining the local curvatures after successive layer removals, stress distributions along the flow path were obtained within a single specimen. Validation of this modified layer-removal method is described. A good reproductibility was obtained. The method can be regarded as semi-quantitative. Flat plates were injection-molded from three amorphous polymers: polystyrene, polycarbonate, and a polyphenylene ether/high-impact polystyrene blend. In general, the flat-plate cross-section shows a three-region stress distribution with a tensile stress region both at the surface and in the core of the flat plate and an intermediate region with compressive stresses. The modified layer-removal method was used to determine influences of mold temperature, annealing treatment, and pressure history on the thermal stress distributions. Increasing mold temperature results in a decreasing overall stress level, while the compressive stress region shifts to the surface. An annealing treatment significantly reduces the overall stress level, without affecting the stress pattern. Stress distributions along the flow path were influenced by the varying pressure histories from the entrance to the end of the mold cavity. The various features of the stress profiles are explained by the influence of the pressure decay rate in the injection-molding process.     

Expansion of Ag2O Lattice by Annealing

The lattice constants of Ag₂O are temperature-dependent, with an apparent maximum at ∼200°C; this effect is similar for annealing in vacuum and in O₂. The maximum appears to result from lack of thermal equilibrium in the oxide at temperatures <∼200°C. The lattice expansion was interpreted in terms of a change in the concentration of lattice vacancies in the Ag₂O structure.     

Effect of Internal Stress on the Strength of BaTiO3

The strength of BaTiO₃ which had been hot-pressed to ∼99% of theoretical density and tested above the Curie temperature was—9000 psi greater than at room temperature. This measured difference in strength is attributed to internal stress from the cubic→tetragonal phase transformation. Correction of measured strength to zero porosity predicts internal stresses of ∼11,000 psi, in good agreement with the dielectric theory for fine-grain BaTiO₃. The internal stress is nearly independent of grain size in the range ∼1.5 to 150 μm, showing that 90° domains do not reduce internal stresses causing failure. Lower internal stress in commercial BaTiO₃ is attributed to greater porosity, large flaw sizes, and possibly the effects of additives.     

Effect of Stress on Water Diffusion in Silica Glass

The diffusion of water in silica glass was measured as a function of applied uniaxial stress and hydrostatic pressure at selected temperatures. It was found that the diffusion coefficient of water increased exponentially with increasing tensile stress and decreased with increasing compressive stress and increasing hydrostatic pressure. The activation volume for water diffusion in silica glass was found to be ∼170 cm³/mol at 192°C and ∼72 cm³/mol at 350°C. The solubility of water in glass showed a trend opposite to the diffusion coefficient, namely, it decreased exponentially with increasing tensile stress and increased with increasing compressive stress and hydrostatic pressure. These stress (or pressure) dependences were attributed to the shift of the glass-water reaction equilibrium.     

Ferroelastic and Plastic Deformation of t'-Zirconia Single Crystals

Polydomain tetragonal ( t' ) zirconia was deformed in compression along a 〈100〉 orientation at various temperatures between 500° and 1400°C. The stress-strain curves showed a plateau corresponding to ferroelastic deformation, followed by plastic deformation at a higher yield stress level. In both ranges, the strain-rate sensitivity of the stress was measured by stress-relaxation tests. The microstructure of the tetragonal domains after ferroelastic deformation and the dislocation substructure were studied by transmission electron microscopy in a high-voltage electron microscope. As expected, ferroelastic deformation suppressed the tetragonal variant with its c -axis parallel to the loading direction. The dislocation structure consisted of intersecting dislocations on different slip systems with strongly bowed-out segments. The microprocesses of deformation are discussed here by comparing the deformation data with those of cubic zirconia deformed in the same orientation and based on the observed microstructure. The particular microstructure of t' zirconia seems to prevent recovery, so that the high flow stress of ∼700 MPa is preserved up to 1400°C.