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.     

Low-Stress Creep of Single-Crystalline Calcium Oxide

The creep behavior of single-crystalline CaO compressed along the 〈100〉 direction has been investigated over the temperature range 1350° to 1450°C at stresses between 5 to 15 MPa. Creep strains greater than 0.10 were required to achieve a steady-state creep rate. Power-law creep was exhibited over the entire stress range with a stress exponent equal to 4.4 and an apparent activation energy for creep of 345 kJ·mol⁻¹.     

Flexural Stress Rupture and Creep of Selected Commercial Silicon Nitrides

Four commercial Si₃N₄ compositions were compared with regard to flexural stress rupture and creep in ambient air as functions of temperature from 1100° to 1400°C and stress from 200 to 350 MPa. One Si₃N₄, SN252, was found to be more resistant to time-dependent deformation in both stepped-temperature stress rupture tests and creep tests than a very similar Si₃N₄ composition and two other dissimilar Si₃N₄ compositions. Materials were compared on the bases of percent final strains, creep rates, and posttest microscope examinations. The latter revealed tensile face transverse cracking, and slow crack growth. The superior behavior of the SN252 Si₃N₄ was related to its microstructure.     

Tensile Creep of Alumina-Silicon Carbide "Nanocomposites"

The tensile creep behavior of an (Al₂O₃-SiC) nanocomposite that contains 5 vol% of 0.15 μm SiC particles is examined in air under constant-load conditions. For a stress level of 100 MPa and in the temperature range of 1200°–1300°C, the SiC reduces the creep rate of Al2O3 by 2–3 orders of magnitude. In contrast to Al₂O₃, the nanocomposite exhibits no primary or secondary stages, with only tertiary creep being observed. Microstructural examination reveals extensive cavitation that is associated with SiC particles that are located at the Al₂O₃ grain boundaries. Failure of the nanocomposite occurs via growth of subcritical cracks that are nucleated preferentially at the gauge corners. A modified test procedure enables creep lifetimes to be estimated and compared with creep rupture data. Several possible roles of the SiC particles are considered, including (i) chemical alteration of the Al₂O₃ grain boundaries, (ii) retarded diffusion along the Al₂O₃-SiC interface, and (iii) inhibition of the accommodation process (either grain-boundary sliding or grain-boundary migration).     

Creep Mechanism of Polycrystalline Yttrium Aluminum Garnet

The high-temperature deformation behavior of a fine-grained polycrystalline yttrium aluminum garnet (YAG) was studied in the temperature range of 1400° to 1610°C using constant strain rate compression tests under strain rates ranging from 10⁻⁵/s to 10⁻³/s. The stress exponent of the creep rate, the activation energy in comparison with that for single-crystal YAG, and the grain size dependence suggest that Nabarro–Herring creep rate limited by the bulk diffusion of one of the cations (Y or Al) is the operative mechanism.     

Low-Temperature Creep of Nanocrystalline Titanium(IV) Oxide

Nanocrystalline TiO₂ with densities higher than 99% of rutile has been deformed in compression without fracture at temperatures between 600° and 800°C. The total strains exceed 0.6 at strain rates as high as 10⁻³ s⁻¹. The original average grain size of 40 nm increases during the creep deformation to final values in the range of 120 to 1000 nm depending on the temperature and total deformation. The stress exponent of the strain rate, n , is approximately 3 and the grain size dependence is d ^{− q} with q in the range of 1 to 1.5. It is concluded that the creep deformation occurs by an interface reaction controlled mechanism.     

Creep Deformation in an Alumina–Silicon Carbide Composite Produced via a Directed Metal Oxidation Process

Flexural creep studies were conducted in a commercially available alumina matrix composite reinforced with SiC particulates (SiC_p) and aluminum metal at temperatures from 1200° to 1300°C under selected stress levels in air. The alumina composite (5 to 10 μm alumina grain size) containing 48 vol% SiC particulates and 13 vol% aluminum alloy was fabricated via a directed metal oxidation process (DIMOX(tm))† and had an external 15 μm oxide coating. Creep results indicated that the DIMOX Al₂O₃–SiC_p composite exhibited creep rates that were comparable to alumina composites reinforced with 10 vol% (8 (μm grain size) and 50 vol% (1.5 μm grain size) SiC whiskers under the employed test conditions. The DIMOX Al₂O₃–SiC_p composite exhibited a stress exponent of 2 at 1200°C and a higher exponent value (2.6) at ≥ 1260°C, which is associated with the enhanced creep cavitation. The creep mechanism in the DIMOX alumina composite was attributed to grain boundary sliding accommodated by diffusional processes. Creep damage observed in the DIMOX Al₂O₃-SiC_p composite resulted from the cavitation at alumina two-grain facets and multiple-grain junctions where aluminum alloy was present.     

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.     

Flexural Creep Deformation of ZrB2/SiC Ceramics in Oxidizing Atmosphere

Flexural creep of ZrB₂/0–50 vol% SiC ceramics was characterized in oxidizing atmosphere as a function of temperature (1200°–1500°C), stress (30–180 MPa), and SiC particle size (2 and 10 μm). Creep behavior showed strong dependence on SiC content and particle size, temperature and stress. The rate of creep increased with increasing SiC content, temperature, and stress and with decreasing SiC particle size, especially, at temperatures above 1300°C. The activation energy of creep showed linear dependence on the SiC content increasing from about 130 to 511 kJ/mol for ceramics containing 0 and 50 vol% 2-μm SiC, respectively. The stress exponent was about 2 for ZrB₂ containing 50 vol% SiC regardless of SiC particle size, which is an indication that the leading mechanism of creep for this composition is sliding of grain boundaries. Compared with that, the stress exponent is about 1 for ZrB₂ containing 0–25vol% SiC, which is an indication that diffusional creep has a significant contribution to the mechanism of creep for these compositions. Cracking and grain shifting were observed on the tensile side of the samples containing 25 and 50 vol% SiC. Cracks propagate through the SiC phase confirming the assumption that grain-boundary sliding of the SiC grains is the controlling creep mechanism in the ceramics containing 50 vol% SiC. The presence of stress, both compressive and tensile, in the samples enhanced oxidation.     

Compressive Creep Performance of SrFeO3

Compressive creep performance of strontium-deficient and strontium-excess SrFeO_3-δ materials has been investigated in the temperature range 800°–1000°C and in the stress range 2.5–25 MPa. The absolute densities of the strontium-deficient and strontium-excess materials are 4.99 and 5.25 g/cm³, respectively, which corresponds to porosities of ∼2% and 5%, respectively. Both materials contain secondary phases because of the cation nonstoichiometry. The creep rate is faster for the strontium-deficient material than the strontium-excess material. The stress exponent is approximately unity, and the activation energy is 260± 30 kJ/mol for both materials. The results can be explained by a cation diffusion mechanism. The present findings are discussed in relation to previous sintering data and the possible application of these materials as oxygen-permeable membranes.     

Tensile Creep and Creep Rupture Behavior of Monolithic and SiC-Whisker-Reinforced Silicon Nitride Ceramics

The tensile creep and creep rupture behavior of silicon nitride was investigated at 1200° to 1350°C using hotpressed materials with and without SiC whiskers. Stable steady-state creep was observed under low applied stresses at 1200°C. Accelerated creep regimes, which were absent below 1300°C, were identified above that temperature. The appearance of accelerated creep at the higher temperatures is attributable to formation of microcracks throughout a specimen. The whisker-reinforced material exhibited better creep resistance than the monolith at 1200°C; however, the superiority disappeared above 1300°C. Considerably high values, 3 to 5, were obtained for the creep exponent in the overall temperature range. The exponent tended to decrease with decreasing applied stress at 1200°C. The primary creep mechanism was considered cavitationenhanced creep. Specimen lifetimes followed the Monkman–Grant relationship except for fractures with large accelerated creep regimes. The creep rupture behavior is discussed in association with cavity formation and crack coalescence.     

Creep and Microstructural Evolution at High Temperature of Liquid-Phase-Sintered Silicon Carbide

The compressive creep characteristics at 1625°C of liquid-phase-sintered silicon carbide ceramics containing 5 and 15 wt% of crystalline Y₃Al₅O₁₂ (YAG) as the secondary phase were studied. In the two cases, strains between 10% and 15% were reached without failure. The creep behavior was characterized by a stress exponent n ≈2, and the proportion of secondary phase was related to the creep resistance of the materials. The microstructural evolution during creep consisted firstly in the re-distribution of the secondary phase, probably as a consequence of its viscous flow at the creep conditions, and secondly an extensive nucleation and growth of cavities, which was more important for the highest YAG content. The latter reflects the carbothermal reduction that the secondary phase undergoes during creep.     

Experimental Assessment of Plasticity of Nanocrystalline 1.7 mol% Yttria Tetragonal Zirconia Polycrystals

High temperature mechanical behavior of nanocrystalline 1.7 mol% (3 wt%) yttria tetragonal zirconia polycrystals (nc-YTZP) was characterized by compression creep tests. The hot isostatically pressed nc-YTZP with mean grain size of 120 nm was subjected to grain growth to obtain grain sizes in the range of 120–310 nm. Direct measurements of the creep parameters were performed in the temperature range 1150°–1300°C and stress range 5–400 MPa. The strain rates at 1150°C ranged between 2 × 10⁻⁷ and 9 × 10⁻⁵ s⁻¹ when increasing the stress from 15 to 400 MPa. Values of the stress exponent, n =2.0±0.3, and the activation energy, Q =630±40 kJ/mol, were obtained for all test conditions. A value of the grain size exponent, p =1.5±0.3, was obtained at 1150°C in the stress range studied. Detailed microstructural observations revealed the absence of glassy phase at the grain boundaries. The creep parameters were compared with those from the literature, and the results were discussed in terms of the model recently developed by the authors, with a reasonable agreement.     

Creep in Yttria- and Scandia-Stabilized Zirconia

Compression creep measurements at constant load on ZrO₂-6 mol% Sc₂O₃ (grain size ∼1 μm), ZrO₂-6 mol% Y₂O₃ (grain size ∼17 μm), and heat-treated ZrO₂-6 mol% Sc₂O₃ (grain size ∼2 μm) yield activation energies of 89, 86, and 74 kcal/mol, respectively. The creep rates are linearly proportional to the inverse square of the grain size of the material. A stress exponent, n , of 1.5 was found for the scandia-doped zirconia and two regimes, with n =1 and 6, were found for the yttria-doped zirconia. These data, supported by metallographic evidence, are interpreted as showing that n =1 is associated with cation diffusion control of creep, n =6 with local propagation of inter-crystalline cracks, and n =1.5 with a transition region.     

Superplastic Deformation in Fine-Grained YBa2Cu3O7−x

The superplastic behavior of YBa₂Cu₃O_{7− x} ceramic superconductors was studied. Large compressive deformation over 100% strain was measured in the temperature range of 775°–875°C, with a strain rate of 1 × 10⁻⁵ to 1 × 10⁻³/s, and a grain size of 0.5–1.4 μm. The nature of the deformation was investigated in terms of three deformation parameters: the stress exponent ( n ), the grain size exponent ( p ), and the activation energy ( Q ). The measured values of these parameters were n = 2 ± 0.3, p = 2.7 ± 0.7, and Q = 745 ± 100 kJ/mol. With the aid of the deformation map, the deformation mechanism was identified as grain boundary sliding accommodated by grain boundary diffusion. The conclusion is consistent with the microstructural observations made by SEM and TEM: the invariance of equiaxed grain shape, the absence of significant dislocation activity, no grain boundary second phases, and no significant texture development.     

Thermal/Strain History Effects on Creep of Refractory Concrete

A temperature and stress cycling technique was developed to examine the effects of thermal/strain history on creep of refractory concrete. Creep of a 90+% Al₂O₃ refractory concrete and a high-purity calcium aluminate cement was investigated under stresses of 3.4 to 20.7 MPa at 500° to 1200°C. It was found that on initial heat-up of a cured specimen of either the concrete or cement, the ΔH_c∼130 to 170 kJ/mol. After subsequent cycling of the temperature, ΔH_C ∼620 to 720 kJ/mol. The stress exponent for the initial application of stress was <1. For subsequent cycling of the stress n >2.5. It was determined from these and other results that multiple "deformation" processes are acting simultaneously on initial heat-up of the form:
These processes include crystallization, phase changes, sintering, stress-aided sintering, and steady-state creep. Mechanisms of creep and failure are discussed.     

Compressive Creep and Creep Failure of 8Y2O3/3Al2O3-Doped Hot-Pressed Silicon Nitride

The compressive creep properties of hot-pressed Si₃N₄8Y₂O₃—3Al₂O₃ (wt%) have been investigated in the temperature range of 1543–1603 K in air. The stress exponent, n , of the power creep law was determined to be 1.5, and the activation energy was determined to be 650 kJ/mol. Transmission electron microscopy observations showed that grain-boundary sliding occurred with cavitation formation in the grain-boundary glassy phase. The quasi-steady-state creep results were consistent with that of the diffusion-controlled solution—diffusion—precipitation creep mechanism, and the distinguished failure mechanism was cavitation creep damage controlled by the viscosity of the boundary glassy phase. The compressive creep failure time, obtained at 1573 K, in the stress range of 175–300 MPa, followed the MonkmanGrant relation, indicating that cavity growth was mainly controlled by the creep response of the material.     

Creep of 6H α-Silicon Carbide Single Crystals

The compressive creep behavior of single-crystal 6H α-SiC was measured for orientations parallel to and at 45° to [0001]. Deformation of the 45° orientation was dominated by basal slip. Steady-state creep rates above 10^-7/s were measured at temperatures as low as 800°C. An activation energy of 277 kJ/mol and a stress exponent of 3.32 were determined. Creep testing with applied stresses parallel to [0001] was performed at 1650°C to 1850°C, yielding a stress exponent and activation energy of 4.93 and 180 kJ/mol, respectively. The occurrence of basal slip in the [0001] specimens suggested that significant off-axis stresses were present during testing.     

Creep Behavior and Grain-Boundary Sliding in Polycrystalline A12O3

The creep properties of polycrystalline A1₂O₃ (grain size 14 to 65 μm) were examined under compressive stresses of between 4,000 and 18,000 psi (27.6 and 124 MPa) in the range 1600° to 1700°C. Two distinct types of behavior were observed. The creep rate of medium-grained specimens (14 to 30 μm) could be described by ασ^1.2 / d² where σ is the applied stress and d is the grain size. These results are consistent with the Nabarro-Herring creep mechanism. For the coarse-grained (65 μm) specimens, the creep rate was related to the stress by ασ^2.6. This behavior was not related to cracking; instead, a dislocation mechanism was thought to be rate-controlling. Considerable evidence for grain-boundary sliding was seen, and measurements showed that grain-boundary sliding contributed between 46 and 77% of the total strain in the 3 medium-grained specimens examined and between 38 and 50% in the 3 coarsegrained specimens examined.     

Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride

The creep behavior of a commercial grade of Si₃N₄ was studied at 1350° and 1400°C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 ± 22.9 kJ/mol, and compression, 489.2 ± 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple-junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ∼18% of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution–precipitation of Si₃N₄. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution–precipitation creep of Si₃N₄ did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.