Flexural Creep of an in Situ-Toughened Silicon Carbide

Flexural creep behavior is reported for an in situ -toughened SiC between 1100° and 1500°C in four-point bending. The flexural creep rate of this SiC, sintered with aluminum, boron, and carbon (ABC-SiC), exhibits linear stress dependence, low apparent activation energy, and low incidence of cavitation and dislocation production. Most grain boundaries in this ceramic contain 1–5 nm intergranular films. The creep rate is consistent with a grain-boundary transport mechanism involving diffusion along the grain-boundary film-SiC interfaces. The microstructure and grain boundaries have been examined using transmission electron microscopy to assess possible changes during creep, particularly in relation to the applied stress direction.     

The Influence of Substructures on Tensile Creep of MgO Single Crystals

Tensile creep tests were performed at 1400°C and a stress of 44.1 MN/m² on <110> oriented MgO single crystals which had been prestrained at 1800°C in tension to produce various dislocation substructures. Several substructures, ranging from blocky (somewhat diffuse) subgrains to random dislocation arrays, were developed by varying the tensile strain rate at 1800°C. Introducing substructure had no significant effect on the early stages of primary creep (to ∼1% strain), but increased the minimum creep rate compared to that in pristine (substructure-free) material. A precreep substructure of blocky subgrains was more creep resistant than one consisting of random dislocation arrays.     

Creep of Dense, Polycrystalline Magnesium Oxide

Creep of polycrystalline MgO was studied using four-point transverse bending at 1380° to 1800°K and stresses from 1000 to 5000 psi. The effects of temperature, stress, and grain size on the creep rate were determined for grain sizes from 2 to 20μ. Activation energies for creep decreased sharply with increasing grain size from 96,000 cal/mole at 2μ to 54,100 cal/mole at 5.5μ and then remained constant over the grain-size range 5.5 to 20μ. Creep was attributed in part to a stress-directed diffusional mechanism controlled by extrinsic oxygen ion diffusion in the 5.5 to 20μ grain sizes, although the calculated ionic self-diffusion rates were higher than those predicted by the Nabarro-Herring theory. It is suggested that the discrepancy may be due to a vacancy formation mechanism, which is consistent with the observed formation of dislocation substructure and preferentially distributed porosity during creep, as well as with the observed decrease in creep rate with increasing creep strain.     

Deformation Behavior of Polycrystalline Aluminum Oxide

The steady-state creep behavior of polycrystalline aluminum oxide from 97 to 100% dense has been examined at temperatures from 1600° to 1800°C over a range of stresses from 100 to 2000 psi under four-point transverse bending. Grain size is shown to have an important effect on the deformation behavior. Stress-strain rate dependence for the fine-grained aluminum oxide (3 to 13μ) is of viscous behavior (°_ε∼σ). The strain rate is also inversely proportional to the square of the grain size (°_ε∼ 1/(GS)²). The creep behavior can be described by the Nabarro-Herring diffusional flow model although the diffusion coefficient does not agree with that for oxygen diffusion in sapphire. When the grain size becomes coarse, the Nabarro-Herring model no longer applies. Coarse-grained aluminum oxide (50 to 100μl) behaves plastically when deformed (°_ε∼σ). The strain rates observed on coarsegrained aluminum oxide are higher than one would predict from the deformation behavior of fine-grained aluminum oxide. This plastic flow probably occurs through some dislocation movement or glide mechanism.     

Recovery of High-Temperature Creep-Resistant Substructure in Rutile

Recovery of creep-resistant substructure in rutile was studied at 1000°, 1020°, and 1040°C. Specimens were crept under a stress of 10,000 psi to a strain early in the secondary stage of creep and then allowed to recover for varying periods under a residual stress of 400 psi. Recovery was detected by the increased creep strain which occurred when the 10,000 psi stress was reapplied. An apparent activation energy of 135,000 cal/mol was obtained for the recovery process. Experimental evidence suggests that the primary recovery mechanism involves the sweeping out of dislocation barriers within the material by the migration of dislocation walls or subgrain boundaries.     

Compressive Creep of CoO Single Crystals

Uniaxial compressive creep of single-crystal CoO was investigated at 1000° to 1290°C and stresses of 1200 to 2100 psi. The activation energy for creep at inflection and steady state was ≅50 kcal/mol. The stress exponent was ≅5 at both inflection and steady state. The results are explained in terms of diffusion-controlled dislocation motion.     

High Temperature Tensile Creep of Magnesium Oxide Single Crystals

The creep behavior and the dislocation substructure developed during creep were investigated for 〈011〉 oriented MgO single crystals creep tested in tension. Creep deformation was studied over stress and temperature ranges of 29.0 to 86.2 MN/m² and 1200 to 1500°C, and the minimum creep rate, ε, was found to obey the relation:

where σ = applied tensile stress, k = the Boltzmann constant, T = absolute temperature, n = 3.8 to 4.5, and A = ll × 10⁻² (MN/m²)^-4 s^-1. Dislocation substructures developed during creep were studied by transmission electron microscopy and etch pitting techniques. At 1400°C, the dislocation density, ρ , at 0.10 tensile creep strain depended on applied stress as ρασ ^2.1. Numerous dislocation loops and long straight dislocations were present, but subboundaries were seldom observed. The results are discussed in terms of two possible operative creep mechanisms: (1) a recovery process based on annealing out of dislocation dipoles and loops, and (2) dislocation glide limited by atmospheres of charged defects surrounding dislocations.     

Effect of Creep Damage on the Tensile Creep Behavior of a Siliconized Silicon Carbide

The tensile creep behavior of a siliconized silicon carbide was investigated in air, under applied stresses of 103 to 172 MPa for the temperature range of 1100° to 1200°C. At 1100°C, the steady-state stress exponent for creep was approximately 4 under applied stresses less than the threshold for creep damage (132 MPa). At applied stresses greater than the threshold stress for creep damage, the stress exponent increased to approximately 10. The activation energy for steady-state creep at 103 MPa was approximately 175 kJ/mol for the temperature range of 1100° to 1200°C. Under applied stresses of 137 and 172 MPa, the activation energy for creep increased to 210 and 350 kJ/mol, respectively, for the same temperature range. Creep deformation in the siliconized silicon carbide below the threshold stress for creep damage was determined to be controlled by dislocation processes in the silicon phase. At applied stresses above the threshold stress for creep damage, creep damage enhanced the rate of deformation, resulting in an increased stress exponent and activation energy for creep. The contribution of creep damage to the deformation process was shown to increase the stress exponent from 4 to 10.     

Creep of Hardened Portland Cement Paste Under Cyclic Temperature

Sealed cement paste specimens (water/cement ratio 0.5), loaded at 28 days to a constant stress/strength ratio of 0.2, were subjected to cyclic temperatures of 90° to 23°F (32.2° to —5°C) and their creep behavior was studied in relation to the amplitude, frequency, and mean value of the cyclic temperature. Temperature cycling increased creep above that which occurred at the mean temperature and this creep may be greater than that which would occur at the highest temperature. Assuming that creep is a rate-process phenomenon, it is possible to predict the creep under one temperature regime, given the creep under another and the temperature histories of both. Also, the ratio (creep under cyclic temperature/creep at the mean constant temperature) bears a hyperbolic cosine relation to the temperature amplitude; thus, if all else is equal, a greater amplitude gives a greater creep. Within the limits of the experiment, the greater the period of the cyclic temperature the greater the creep. The increase in creep above that at mean temperature appears to be greater when the mean temperature is in the range where evaporable water freezes. Irrecoverable creep appears to increase with the amplitude of temperature as well as with temperature per se. The influence of variable temperature on creep is explained in terms of the superimposition of hygrothermal diffusion on stress-induced moisture diffusion in the creeping specimen, as well as the influence of temperature on the diffusion coefficient of the diffusing mass.     

Creep Deformation of 0° Sapphire

Creep deformation of 0° sapphire was studied between 1600°and 1800°C at stresses up to 114 MN/m². Microscopical evidence (dislocation structures observed by transmission electron microscopy (TEM) and by etch pits) suggested that Nabarro climb was the predominant deformation mechanism. Both the experimental creep rates and stress exponents were in good agreement with those predicted by this model. Although non-basal dislocations with a ½〈101〉 Burgers vector were present, the good creep resistance of 0° sapphire was attributed to the difficulty of activating pyramidal slip.     

Creep Behavior of a Model Refractory System MgO-CaMgSiO4

The effects of the presence of a silicate boundary phase on the high-temperature creep behavior of a model refractory system MgO-CaMgSiO₄ (monticellite, CMS) were studied at 1200° to 1450°C. A change in the dominant mechanism of deformation was determined with increasing temperature and decreasing applied stress. It was concluded that, at 1200°C, deformation is controlled by a dislocation mechanism in the MgO framework, whereas at higher temperatures creep is the result of simultaneous mechanisms but dominated by viscous deformation of the silicate boundary region.     

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.     

Dislocation Structures in Creep-Deformed Polycrystalline MgO

Secondary creep of polycrystalline MgO with grain sizes of 100 and 190 μm was investigated at 1300° to 1460°C under compressive loads of 2.5 to 5.5 kgf/mm². The dependence of creep rate on load follows a power law with an exponent of 3.2±0.3. The process is thermally activated, with an activation energy of 76 ± 12 kcal/mol. The creep rate is independent of grain size. The dislocation structure was investigated by transmission electron microscopy. The total dislocation density follows the relation, σ= bG √ρ, commonly found for metals. The dislocations form a 3-dimensional network in which many dislocation segments lie in their slip or climb planes. On the basis of this structure, a model is proposed in which glide is the principal cause of deformation but the rate-limiting process, i.e. annealing of the network, is diffusion-controlled. Theoretical estimates and experimental results agree within 1 order of magnitude.     

High-Temperature Creep and Kinetic Demixing in (Co,Mg)O

The creep behavior of fine-grained (Co_0.5Mg_0.5)O and (Co_0.25Mg_0.75)O has been characterized as part of an investigation of kinetic demixing in solid-solution oxides due to a nonhydrostatic stress. (i) For low stresses and small grain sizes, the dominant deformation mechanism for both compositions is diffusional creep limited by the transport of oxygen along grain boundaries. The oxygen grain-boundary diffusivity, D _o ^b is independent of oxygen partial pressure. The values of ω D _o ^b , where ω is the grain-boundary width, that have been determined from the steady-state diffusional creep rates are given by ω D _o ^b =4.7×10⁻⁸ exp[-230 (kJ/mol)/ RT ] (cm³/s) for (Co_0.5Mg_0.5)O in the range 950° to 1200°C and ω D _o ^b =7.4 × 10⁻⁸ exp[-263 (kJ/mol)/ RT ] (cm³/s) for (Co_0.25Mg_0.75)O in the range 1100° to 1250°C. Since oxygen diffusion controls the rate of diffusional creep, kinetic demixing is not observed in deformed samples of either composition. (ii) For high stresses and large grain sizes, the dominant deformation mechanism in both cases is dislocation-climb-controlled creep, where the rate of dislocation climb is controlled by oxygen lattice diffusion. Based on the positive dependence of creep rate on oxygen partial pressure, it is concluded that oxygen diffuses through the lattice by an interstitial mechanism.     

Compressive Creep of Al2O3 Single Crystals

Aluminum oxide single crystals deformed by dislocation glide and deformation twinning during compressive creep at 1400° to 1700°C. The activation energy for basal slip was a function of the applied stress and agreed with activation energies previously measured by observation of yielding phenomena. The overcoming of a large Peierls-Nabarro stress is the most probable rate-controlling mechanism. Rhombohedral twinning, a significant deformation mode in creep, depends on surface damage for nucleation. The activation energy for rhombohedral twin growth, a function of the applied stress, is substantially lower than that for basal slip. When basal slip and rhombohedral twinning occur concurrently, creep by basal slip results, but the presence of twins can substantially reduce the creep rate.     

Creep of CaO-Stabilized ZrO2

The compressive creep of 18 mol% CaO-stabilized ZrO₂ was studied at 1200° to 1400°C and 500 to 4000 psi. The specimens were polycrystalline with grain diameters from 7 to 29 μm. The activation energy for creep is 94 kcal/mol, and the creep rates are linearly proportional to the stress and to the inverse of the grain size. These results lead to the conclusion that creep in 18 mol% CaO-stabilized ZrO₂ may be controlled by cation diffusion associated with grain-boundary sliding.     

Compressive Creep of SiC-Whisker-Reinforced Al2O3

Compressive creep of SiC-whisker-reinforced Al₂O₃ composites (0, 5, 15, and 25 wt% SiC) was measured in the temperature range of 1300° to 1500°C in air and argon. The creep resistance increased with increasing whisker concentration. The results indicated that the whiskers degraded in air, increasing strain rates compared to those in argon. Stress exponents between 1.0 and 2.0 and an activation energy of 620 ± 100 kJ/mol were measured. Transmission electron microscopy observations indicated that cavitation was minimal and that the deformed composites had the same dislocation structure as did the as-received samples.     

Determination of Pipe Diffusion Coefficients in Undoped and Magnesia-Doped Sapphire (α-Al2O3): A Study Based on Annihilation of Dislocation Dipoles

The breakup of dislocation dipoles in plastically deformed samples of undoped and 30-ppm-MgO-doped sapphire (α-Al₂O₃) was monitored using conventional TEM techniques. Dislocation dipoles break up into prismatic dislocation loops in a sequential process during annealing; i.e., dislocation loops are pinched off at the end of a dislocation dipole. This pinch-off process is primarily controlled by pipe diffusion, and pipe diffusion coefficients at temperatures between 1300° and 1500°C were estimated by monitoring the kinetics of the dipole breakup process. We determined D _P^U= 8.1(_–4.3^+9.1) × 10^–3 exp [–(4.5 ± 1.3 eV )/ kT )] m²/s for the undoped material. The pipe diffusion kinetics for the MgO-doped crystal was determined at 1250° and 1300°C and was about 6 times higher than for undoped sapphire. Finally, climb dissociation of the dislocations constituting the perfect dipoles in sapphire is common; annihilation of one set of partials can result in the formation of faulted dipoles, which can pinch off to form faulted dislocation loops. D _P^U for faulted dipoles in the undoped material was determined at 1300° and 1350°C, and was about 4–10 times higher than for perfect dipoles.     

Atmospheric Effects on Compressive Creep of SiC-Whisker-Reinforced Alumina

A SiC-whisker-reinforced alumina composite was crept in compression at 1200° to 1400°C in an air ambient and in nitrogen. The data were described by a power-law-type constitutive relation. The measured value of the stress exponent was n = 1 at 1200°C and n = 3 at 1300° and 1400°C in both ambients. TEM observations were correlated with the measured creep response to determine active deformation mechanisms. Values of n = 1 were associated with diffusional creep and unaccommodated grain-boundary sliding, while values of n = 3 were associated with increased microstructural damage in the form of cavities. Experiments conducted in circulated air resulted in higher creep rates than comparable experiments in nitrogen. The accelerated creep rates were caused by the thermal oxidation of SiC and the resultant formation of a vitreous phase along composite interfaces. The glassy phase facilitated cavitation, weakened interfaces, and enhanced boundary diffusion.     

Creep of Mixed-Oxide Fuel Pellets at High Stress

The rate of steady-state compressive creep in (U,Pu)O_2-ε was investigated in the power-law creep region (at a constant stress of 69 MN/m² between 1600° and 1500°C) as a function of the oxygen-to-metal (O/M) ratio (1.88 to 1.995). The creep rate over this range is independent of the starting material and decreases with increasing O/M ratio. The apparent activation energy for creep and the preexponential structure factor are sensitive functions of the O/M ratio, with approximately the same dependence on this ratio as these parameters measured at low stresses. These results imply that diffusion of the same defect species controls creep deformation in both the stress-assisted diffusion region (ε∞σ) and the dislocation-motion region (ε∞σ^4.4).