Showing papers on "Creep published in 1998"

Microstructure and deformation of two-phase γ-titanium aluminides

Abstract: During the past decade considerable research efforts have been directed towards achieving balanced engineering properties of two-phase γ-titanium aluminide alloys for future applications as structural materials. For optimization of mechanical properties such as yield and creep strengths, tensile ductility and fracture resistance, a basic understanding of the temperature dependent micromechanisms of plasticity and fracture, and their interplay with various microstructural constituents is required. In this review article, the current knowledge on dislocation types and slip systems, the development of deformation substructures, factors controlling the mobility and multiplication of dislocations, interface related plasticity, solid solution and precipitate strengthening mechanisms as well as microscopic aspects of creep and fracture will be addressed. These topics will be related to specific microstructures and associated engineering properties.

High-temperature deformation of dry diabase with application to tectonics on Venus

Abstract: We have performed an experimental study to quantify the high-temperature creep behavior of natural diabase rocks under dry deformation conditions Samples of both Maryland diabase and Columbia diabase were investigated to measure the effects of temperature, oxygen fugacity, and plagioclase-to-pyroxene ratio on creep strength Flow laws determined for creep of these diabases were characterized by an activation energy of Q = 485 +/- 30 kJ/mol and a stress exponent of n = 47 +/- 06, indicative of deformation dominated by dislocation creep processes Although n and Q are the same for the two rocks within experimental error, the Maryland diabase, which has the lower plagioclase content, is significantly stronger than the Columbia diabase Thus the modal abundance of the various minerals plays an important role in defining rock strength Within the s ample-to-sample variation, no clear influence of oxygen fugacity on creep strength could be discerned for either rock The dry creep strengths of both rocks are significantly greater than values previously measured on diabase under "as-received" or wet conditions Application of these results to the present conditions in the lithosphere on Venus predicts a high viscosity crust with strong dynamic coupling between mantle convection and crustal deformation, consistent with measurements of topography and gravity for that planet

Amorphous Silicoboron Carbonitride Ceramic with Very High Viscosity at Temperatures above 1500°C

Abstract: Recently, the viscosity of a predominantly amorphous silicon carbonitride (Si1.7C1.0±0.1N1.5) alloy with an apparent glass-transition temperature (Tg) of 1400°–1500°C was studied. In this study, the creep behavior of silicoboron carbonitride (Si2B1.0C3.4N2.3), which seems to have a Tg value of >1700°C, was examined. Both materials exhibited a three-stage creep behavior. In stage I, the creep rate declined, because of densification. In stage II, the strain rate approaches a steady state. In stage III, it resumes a declining strain rate, which ultimately decreased below the measurement limit of the system. At 1550°C in stage II, the viscosity of silicoboron carbonitride was six orders of magnitude higher than that of fused silica. Among the Si-C-N ceramics, only chemical-vapor-deposited and reaction-bonded silicon carbides seem to have greater creep resistance than the silicoboron carbonitrides at temperatures >1550°C.

Time effects in rock mechanics

Abstract: Experimental Foundations. Results and Background. Deformation Mechanisms for Creep. Rheological Constitutive Equations for Rocks. General Constitutive Equation. Damage and Creep Failure. Mining and Petroleum Engineering Problems. Closure and Failure of Vertical Caverns and Boreholes. Creep, Closure and Damage of Horizontal Tunnels. Creep, Damage and Failure Around Rectangular-Like Galleries or Caverns. References.

On dynamic recrystallization during solid state flow: Effects of stress and temperature

Abstract: A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state flow may represent a balance between grain size reduction and grain growth processes occurring directly in the boundary between the dislocation and diffusion creep fields. Accordingly, the recrystallized grain size (D) and flow stress (σ) at steady state will be related by the equation delineating the field boundary, which in general is temperature dependent. Creep experiments on a metallic rock analogue, Magnox, yielded D=10 1.12 exp[29.3/RT]σ 1.2:3 and demonstrated that D (μm) decreases with increasing σ (MPa) and increasing temperature (T) in a manner which is in agreement with the field boundary hypothesis. If the model applies to rocks, the widely accepted idea that dynamic recrystallization can lead to major rheological weakening in the Earth may not hold. Moreover, empirical D-σ relations, used in paleo-piezometry, will need to be modified to account for temperature effects.

Sintering and Creep Behavior of Plasma-Sprayed Zirconia and Hafnia Based Thermal Barrier Coatings

Abstract: The sintering and creep of plasma-sprayed ceramic thermal barrier coatings under high temperature conditions are complex phenomena. Changes in thermomechanical and thermophysical properties and in the stress response of these coating systems as a result of the sintering and creep processes are detrimental to coating thermal fatigue resistance and performance. In this paper, the sintering characteristics of ZrO2-8wt%y2O3, ZrO2-25wt%CeO2-2.5wt%Y2O3, ZrO2-6w%NiO- 9wt%Y2O3, ZrO2-6wt%Sc2O3-2wt%y2O3 and HfO2-27wt%y2O3 coating materials were investigated using dilatometry. It was found that the HfO2-Y2O3 and baseline ZrO2-Y2O3 exhibited the best sintering resistance, while the NiO-doped ZrO2-Y2O3 showed the highest shrinkage strain rates during the tests. Higher shrinkage strain rates of the coating materials were also observed when the specimens were tested in Ar+5%H2 as compared to in air. This phenomenon was attributed to an enhanced metal cation interstitial diffusion mechanism under the reducing conditions. It is proposed that increased chemical stability of coating materials will improve the material sintering resistance.

Low-frequency shear attenuation in polycrystalline olivine: Grain boundary diffusion and the physical significance of the Andrade model for viscoelastic rheology

Abstract: The high-temperature (1200–1285°C) torsional dynamic attenuation (10−3–100 Hz) and unidirectional creep behavior of a fine, uniform grain sized (d ≈ 3 μm) olivine (∼Fo92) aggregate have been measured. In all cases, the material is found to be mechanically linear (i.e., γ(t), γ ∝ σxy1), indicating that diffusional processes dominate the deformation kinetics in these experiments. The creep response displays a large decelerating transient in the strain rate leading to a nominally constant “steady state.” The attenuation behavior displays a band in QG−1 that is moderately dependent on frequency (QG−1 ≈ f−0.35) and temperature with −1.5

Creep behavior and deformation microstructures of Mg–Y alloys at 550 K

Abstract: Creep behavior and deformation substructures of Mg–Y binary alloys containing 0.2–2.4 mol% Y have been investigated at 550 K under 50–200 MPa. Creep strength is remarkably improved by a small addition of yttrium as compared with aluminum and manganese. Two stress regions are recognized based on the stress dependence of the minimum creep rate. The lower-stress region, where the stress exponent n is about 5, corresponds to the H region (class M behavior) commonly observed in other solid solution alloys. The concentration dependence of solid-solution Mg–Y alloys is similar to that of Mg–Al solid solution alloys: the apparent concentration exponent m is about −2. The effect of the yttrium concentration increases when the concentration exceeds 1.6 mol%. Transmission electron microscopy reveals that the activation of the non-basal slip systems is enhanced and dynamic precipitation occurs during creep deformation in a concentrated Mg–Y alloy. The high creep resistance can be correlated with two mechanisms, i.e. forest dislocation-hardening (work hardening) and dynamic precipitation of fine pseudo-equilibrium β′ phase on the dislocation lines during creep.

Modeling Oxidation Induced Stresses in Thermal Barrier Coatings

Abstract: Oxidation of the bond coat of a thermal barrier coating (TBC) is an important contributing factor to spallation of the ceramic top coat during service. However, the mechanisms by which oxidation facilitates TBC failure are poorly understood. It is also unclear how oxidation may interact with other potential degradation mechanisms. A finite element model was developed to evaluate stresses induced by thermal cycling of a typical plasma sprayed TBC system. It was found that oxidation of the bond coat had a strong effect on ceramic layer stresses and that oxidation induced stresses were influenced by other factors, such as bond coat creep, top coat creep and bond coat roughness.

Creep vs. stretch: a review of the viscoelastic properties of skin.

Abstract: Possessing viscous and elastic rheological properties, skin is viscoelastic. Mechanical creep, defined as the elongation of skin with a constant load over time beyond intrinsic extensibility, has been described as the vehicle harnessed for wound closure with presuturing, intraoperative tissue expansion, skin-stretching devices, and skin retraction with undermining. Resulting from the generation of new tissue due to a chronic stretching force, biological creep plays a role in conventional tissue expansion.

New, creep-resistant, low melting point solders with ultrafine oxide dispersions

Abstract: Nano-sized, nonreacting, noncoarsening oxide dispersoids have been incorporated into solder alloys to create a new, improved solder structure with an ultrafine grain size of ∼200–500 nm. The new solders exhibit significantly enhanced creep resistance combined with increased strength. The well-known thermal instability problem with ultrafine-grained structure appears to have been overcome in these solder alloys and the microstructure was seen to be quite stable upon high temperature exposure (e.g. 120°C). This is attributed to the presence of very fine dispersoid particles which impede grain boundary sliding and dislocation movement. The dispersions are seen to have a profound effect on the mechanical deformation characteristics of the solders with respect to creep. As much as three orders of magnitude reduction in the steady state creep rate has been achieved. The new solders also exhibit improved ductility under high strain rate deformation and improved strength (4–5 times higher tensile strength) at low strain rates. It is demonstrated that with a dispersion of TiO2 particles, the Pb-Sn eutectic solder with a melting point of 183°C can be made more creep-resistant than the 80Au-20Sn eutectic solder with a much higher melting point of 278°C. The new creep-resistant solders can be useful for optical and optoelectronic packaging in which dimensional stability of the assembled structure is essential.

Damage Localization of Conventional Creep Damage Models and Proposition of a New Model for Creep Damage Analysis

Abstract: After examining the ill-natured stress sensitivity of the conventional creep damage equations of Kachanov-Rabotnov, it was shown that this stress sensitivity is one of the most essential causes of damage localization and mesh-dependence in local approach of creep fracture analyses. Then, in order to avoid this difficulty, alternative constitutive and evolution equations of creep and creep-damage are proposed from the view point of micromechanics. Finally, the proposed equations of creep and creep-damage are applied to the creep crack growth analysis of a perforated plate under tension. Significant improvement in damage localization and the mesh-dependence of the numerical results are ascertained.

High-temperature Creep Resistance in Rare-earth-doped, Fine-grained Al2O3

Abstract: High-temperature creep in undoped Al2O3 and La2O3- or Y2O3- or Lu2O3-doped Al2O3 with a grain size of about 1 µm is examined in uniaxial compression testing at temperatures between 1150 and 1350 °C. The high-temperature creep resistance in Al2O3 is highly improved by the rare-earth oxide doping in the level of 0.045 mol %, and the creep rate is suppressed in the order La2O3

Creep contributes to the fatigue behavior of bovine trabecular bone.

Abstract: Repetitive, low-intensity loading from normal daily activities can generate fatigue damage in trabecular bone, a potential cause of spontaneous fractures of the hip and spine. Finite element models of trabecular bone (Guo et al., 1994) suggest that both creep and slow crack growth contribute to fatigue failure. In an effort to characterize these damage mechanisms experimentally, we conducted fatigue and creep tests on 85 waisted specimens of trabecular bone obtained from 76 bovine proximal tibiae. All applied stresses were normalized by the previously measured specimen modulus. Fatigue tests were conducted at room temperature; creep tests were conducted at 4, 15, 25, 37, 45, and 53 degrees C in a custom-designed apparatus. The fatigue behavior was characterized by decreasing modulus and increasing hysteresis prior to failure. The hysteresis loops progressively displaced along the strain axis, indicating that creep was also involved in the fatigue process. The creep behavior was characterized by the three classical stages of decreasing, constant, and increasing creep rates. Strong and highly significant power-law relationships were found between cycles-to-failure, time-to-failure, steady-state creep rate, and the applied loads. Creep analyses of the fatigue hysteresis loops also generated strong and highly significant power law relationships for time-to-failure and steady-state creep rate. Lastly, the products of creep rate and time-to-failure were constant for both the fatigue and creep tests and were equal to the measured failure strains, suggesting that creep plays a fundamental role in the fatigue behavior of trabecular bone. Additional analysis of the fatigue strain data suggests that creep and slow crack growth are not separate processes that dominate at high and low loads, respectively, but are present throughout all stages of fatigue.

Experimental Study of Drained Creep Behavior of Sand

Abstract: The experimental techniques pertaining to accurate measurement of creep in sand are explained in detail. Triaxial compression and proportional loading tests were performed in a triaxial setup for which special procedures were developed to maintain constant temperature, constant confining pressure, and constant axial load, whereas the axial and volumetric deformations were measured using two types of mechanical measurement systems, both free of zero drift. Special attention was paid to the avoidance of air and water leakage through the membrane in the long-term tests. The experiments show that the nature of creep strains is similar to that of plastic strains. They may be predicted from the framework provided by the hardening plasticity theory. In particular, the potential surface determined for the prediction of plastic strains may also be used for the prediction of time-dependent creep strains. From the experiments it also appears that the yield surface and the plastic potential surface move out together, and the point at which to evaluate inelastic strain increment directions is at the current location of the yield surface and the accompanying plastic potential surface.

Microstructural development during high temperature creep of 9% Cr steel

Abstract: The modified 9% Cr steel Type P91 is one of the materials presently employed in power plant pipework components. The detailed microstructural analysis of a trial melt produced by Vitkovice Steel, Ostrava is reported in the present work. The microstructural evolution during creep at 873 K was investigated by means of transmission electron microscopy and computer image analysis. Two main microstructural elements of tempered martensite ferritic steels, namely subgrains and secondary phase particles, are studied quantitatively. The influence of stress free ageing and the influence of stress under creep conditions on particle coarsening and subgrain growth is determined.

Competition Between Stress Generation and Relaxation During Oxidation of an Fe± Cr± Al± Y Alloy

Abstract: The residual biaxial stress inAl2O3 scales formed duringoxidation of Fe-22.0%Cr-4.8%Al-0.3%Y in the temperaturerange 1000-1300°C have been measured usingCr3+ luminescence piezospectroscopy. Thestress measurements together with substantialdimensional changes of the specimens show that oxidationproduces high compressive growth stresses in theAl2O3 scale in addition tothermal-mismatch stresses that arise during cooling from the oxidationtemperature. The magnitude of the growth stress isdetermined by two opposing processes: stress generation,associated with lateral growth of the scale, and stress relaxation in both the metal and oxide. Thescale lateralgrowth strain continuously increasesconcurrently with the scale thickening. Creep in theoxide is a major relaxation process when the scale isthin, while metal deformation becomes significantduring the later stages of oxidation at highertemperatures.

The creep and thermal stability characteristics of a unidirectionally solidified Al2O3/YAG eutectic composite

Abstract: Compressive creep characteristics at 1773, 1873, and 1973 K, oxidation resistance over 1000 h at a temperature of 1973 K in ambient air, and the thermal stability characteristics at 1973 K in ambient air of a unidirectionally solidified Al2O3/YAG eutectic composite were evaluated. At a test temperature of 1873 K and a strain rate of 10−4/s, the compressive creep strength of a eutectic composite manufactured by the unidirectional solidification method is approximately 13 times higher than that of a sintered composite with the same chemical composition. The insite eutectic composite also showed greater thermal stability, with no change in mass after an exposure of 1000 hours at 1973 K in ambient air. The superior high-temperature characteristics are closely related to such factors as (1) the in-situ eutectic composite having a microstructure, in which single crystal Al2O3 and single crystal YAG are three-dimensionally and continuously connected and finely entangled without grain boundaries and (2) no amorphous phase is formed at the interface between the Al2O3 and the YAG phases.

Evolution of microstructure in a modified 9Cr–1Mo steel during short term creep

Abstract: An investigation of the effect of creep exposure on the microstructure of a 9Cr–1Mo alloy for steam tubing was performed. The samples were machined from a tube, austenised at 1323 K for 15 min and air cooled to room temperature, followed by tempering at 1023 K for 1 h. Creep tests were performed at 848, 873, 898 and 923 K for different loading conditions. The conventional power law was used to describe the minimum creep rate dependence on applied stress; the stress exponent was found to increase when temperature decreased. Transmission electron microscopy (TEM) of the crept samples showed that during creep both subgrain and particle size increased; the statistical analysis of the dimensions of the precipitates revealed a bimodal distribution of particles that coarsen during creep exposure at testing temperatures. A linear dependence of subgrain size on the inverse of the modulus compensated stress was used to describe the softening of the dislocation substructure. A similar relationship was found to be also valid for particle carbides.

Orientation dependence of dislocation structures and deformation mechanisms in creep deformed CMSX-4 single crystals

Abstract: Transmission electron microscopy investigations have been carried out in single crystal creep specimens of the nickel-based superalloy CMSX-4, deformed to different stages of their creep lives The microstructural background of creep anisotropy was observed at 1123 and 1253 K At the lower temperature, the pronounced creep anisotropy can be attributed to the superposition of coherency and external stresses, leading to different dislocation configurations At the higher temperature, the creep anisotropy for [001] and [011] load axes is reduced by γ′ rafting in these orientations

Molecular-Dynamics Simulation of Grain-Boundary Diffusion Creep

Abstract: Molecular-dynamics (MD) simulations are used, for the first time, to study grain-boundary diffusion creep of a model polycrystalline silicon microstructure. Our fully dense model microstructures, with a grain size of up to 7.5 nm, were grown by MD simulations of a melt into which small, randomly oriented crystalline seeds were inserted. In order to prevent grain growth and thus to enable steady-state diffusion creep to be observed on a time scale accessible to MD simulations (of typically 10-9s), our input microstructures were tailored to (i) have a uniform grain shape and a uniform grain size of nm dimensions and (ii) contain only high-energy grain boundaries which are known to exhibit rather fast, liquid-like self-diffusion. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state diffusion creep that is homogenous (i.e., involving no grain sliding), with a strain rate that agrees quantitatively with that given by the Coble-creep formula.