Assessment of intrinsic creep resistance evolution based on the results of constant load creep tests

In the present study a method to assess the intrinsic creep resistance evolution by using data of constant load creep tests is proposed. The investigation has been performed on the austenitic steels X6 CrNi 18 11 and X8 CrNiMoNb 16 16. To develop the constitutive equation describing the intrinsic creep resistance evolution a simple structural mechanical model has been used. The applied model is based on a modification of the Levy-Mises equation for plasticity to consider creep time effects. The proposed model has been verified experimentally. The experimental creep resistance evolution curves have been derived with the aid of strain transient dip test performed in transient, steady state and accelerated creep stage.     

Effect of carbide precipitation on the creep behavior of Alloy 800HT in the Temperature Range 700 ° to 900 °

The creep behavior of alloy 800HT was studied at 700 °, 800 °, and 900 ° under stresses ranging from 30 to 170 MPa. Samples that were tested in the as-quenched condition after solution treatment exhibited longer creep life than those that were overaged before testing. This difference in creep life was found to increase at lower creep stresses at a given temperature. This phenomenon is attributed to the precipitation of M₂₃C₆carbides during the early stages of creep, which strengthen the material by exerting threshold stresses on moving dislocations and thereby reducing the creep rate. A model is developed to describe the influence of carbide precipitation during creep on the behavior of the material under different creep temperatures and stresses. Comparison with the experimental results shows that the model gives accurate predictions of the creep behavior of the material in the range of stresses and temperatures used in the present study. In addition to its predictive value, the model is useful in understanding the factors that affect the creep behavior of materials when precipitation of hard phases is taking place during creep. The strengthening effect of particle precipitation during creep, as represented by the value of the threshold stress, is shown to be a complex function of the supersaturation of the matrix, the applied creep stress, and the test temperature.     

Evolution of fiber fragmentation in a short-fiber-reinforced metal-matrix model composite during tensile creep deformation—An acoustic emission study

The objective of this work is to obtain deeper insight into the damage evolution occurring during creep in short-fiber-reinforced metal-matrix composites. Uniaxial tensile creep experiments were performed on a model composite with a lead (Pb) matrix. This system was chosen because it allowed the performance of all creep tests at room temperature, thus facilitating the detection of fiber fragmentation by acoustic emission measurements. By this experimental approach, for the first time, quantitative information about the spatial and temporal evolution of microfractures in creeping metal-matrix composite of this kind was obtained. The acoustic emission results show that fiber fragmentation sets in early in the creep life and continues to operate up to macroscopic failure, thus affecting the creep behavior in all stages including the steady-state regime. During the whole creep process, the fracture sites are homogeneously distributed in the specimen volume. These findings largely support the micromechanical damage model proposed by Dlouhy and co-workers, in which the creep process in short-fiber-reinforced metal-matrix composites is described as an interplay of work hardening and recovery in the matrix as well as fragmentation of the fibers.     

Assessment of creep and rupture behavior of 2.25Cr-1Mo steel — A strain based approach and its limitation

Strain based approach for reliable assessment of creep behavior is employed for creep data generated for different microstructural conditions as well as obtained from literature for 2.25Cr-1Mo steel. The influence of thermal ageing and pre-strain on the shape of creep curves has been studied. Analysis of data revealed that irrespective of initial microstructures, thermal ageing increases the tendency to soften but not the pre-strain. Softening due to carbide coarsening thus appears to be the dominant mechanism of creep. Based on this, a strain based approach for creep life assessment was developed. A fairly accurate prediction of creep life up to 5% creep is achieved using eight materials constants extracted from the creep curves of steel having similar initial microstructure. The limitation and reliability of the approach used to assess creep behavior of service-exposed steels have been discussed.     

Time-Dependent Reliability of PSC Box-Girder Bridge Considering Creep, Shrinkage, and Corrosion

Bridge performance undergoes time-varying changes when exposed to aggressive environments. While much work has been done on bridge reliability under corrosion, little is known about the effects of creep and shrinkage on the reliability of concrete bridges. In this paper, the CEB-FIP model for creep and shrinkage is applied by using finite-element (FE) analysis in conjunction with probabilistic considerations. Verification is made by comparing the analytical findings with existing test data. More specifically, a time-dependent reliability assessment is made for a composite prestressed concrete (PSC) box-girder bridge exposed to a chloride environment. This realized via an advanced probabilistic FE method. The postcracking behavior of the thin-walled box girder is described using composite degenerated shell elements, and importance sampling is used to improve the efficiency of the reliability analyses. It is shown that concrete creep and shrinkage dominate during the early stages of bridge structure deterioration. This is accompanied by a decrease in reliability owing to the combined action of creep, shrinkage, and corrosion. The reliability indexes for the serviceability and the tendon yielding limit state fall below the target levels prior to the expected service life. Therefore, early maintenance and/or repair measures are required.     

Continuum damage mechanics of constrained intergranular cavitation

A three-dimensional micromechanical model for polycrystalline materials undergoing constrained intergranular cavitation is proposed. The model combines advantages of phenomenological and micromechanical approaches, and can be calibrated from standard uniaxial creep data. When applied to creep analysis of two ferritic steels, the model accurately predicts the creep life under multiaxial stresses as well as the amount of creep damage.     

Measurement and modeling of creep in open-cell NiAl foams

Nickel-rich NiAl foams, consisting of open cells with hollow struts and exhibiting two relative densities (5.0 and 6.6 pct) and average cell sizes (1.27 and 0.85 mm), were creep tested between 800 °C and 1100 °C under compressive stresses between 0.10 and 1.50 MPa. For stresses lower than 0.50 MPa, the foams exhibit secondary creep with power-law behavior characterized by creep exponents and activation energies close to those of bulk, nickel-rich NiAl. A three-dimensional (3-D) finite-element model (FEM) was implemented for a cell consisting of hollow struts on a cubic lattice, which predicts creep rates in reasonable agreement with the experimental data. Based on these numerical results, a simplified analytical model is proposed, whereby struts parallel to the applied stress deform by creep in a purely compressive mode, while perpendicular struts prevent buckling but provide no direct load-bearing capacity. This simple model produces results that are very close to the predictions of the complex numerical model and in good agreement with the experimental data. By contrast, an existing model considering creep bending of struts within the foam predicts strain rates that are too high by approximately two orders of magnitude.     

Prediction of steady state creep behavior of two phase composites

A simple continuum mechanics-based model has been developed to predict the steady state creep rates of composites containing coarse and rigid reinforcements from the matrix creep behavior. The model has been derived on the basis of a unit cell, representative of the composite microstructure, which is idealized to a pattern of periodic, cubic inclusions distributed uniformly in a continuous creeping matrix. Comparisons of the predicted creep rates are made with the experimental data of a number of two phase systems as well as transversely loaded continuous fiber reinforced composites. A good agreement between the predicted and measured creep rates is seen for most of the systems. However, for some composites, the calculations overestimate the creep rates significantly at intermediate volume fractions, typically, 0.3–0.4. It is suggested that factors such as the differences between the microstructure of the matrix in the composite and that of the monolithic matrix could be responsible for the differences in the predicted and experimentally measured creep behavior. Finally, an assessment of the predictions of the model proposed in this study with rigorous, self-consistent calculations as well as finite element simulations has been made.     

Part II. The creep behavior of Ti-Al-Nb O+bcc orthorhombic alloys

The intermediate-temperature (650 °C to 760 °C) creep behavior of orthorhombic (O)+bcc alloys containing 50 at. pct Ti was studied. Ti-25A1-25Nb, Ti-23Al-27Nb, and Ti-12Al-38Nb ingots were processed and heat treated to obtain a wide variety of microstructures. Creep deformation mechanisms and the effects of grain size, phase volume fraction, tension vs compression and aging on creep rates were examined. Unaged microstructures, which transformed during the creep experiments, exhibited larger primary creep strains than transformed microstructures, which were crept after long-term aging. The deformation observations and calculated creep exponents and activation energies suggested that separate creep mechanisms, dependent on the applied stress level, were dominating the secondary creep behavior. Coble creep characteristics, including relatively low activation energies and dislocation densities as well as stress exponents close to unity, were exhibited at low applied stresses. Experiments on fiducially marked specimens indicated that grain-boundary sliding was occurring for intermediate applied stresses. In this regime, the minimum creep rates were proportional to the applied stress squared and inversely proportional to the grain size. Dislocation-controlled creep characteristics, including stress exponents of greater than or equal to 3.5 and relatively high activation energies and dislocation densities, were exhibited at high stresses. Overall, the minimum creep rates were dependent on microstructure and stress. Within the low-to-intermediate stress regimes, subtransus processed and heat-treated microstructures, which contained much finer grain sizes than supertransus microstructures, exhibited the poorest creep resistance. The influence of grain size was not as significant within the high-stress regime. It is shown that for low-to-intermediate stress levels, grain size is the dominant microstructural feature influencing the creep behavior of O+bcc alloys.     

Elevated temperature creep-rupture behavior of the single crystal nickel-base superalloy NASAIR 100

The creep and rupture behavior of [001] oriented single crystals of the nickel-base superalloy NASAIR 100 was investigated at temperatures of 925 and 1000 °C. In the stress and temperature ranges studied, the steady state creep rate, time to failure, time to the onset of secondary creep, and the time to the onset of tertiary creep all exhibited power law dependencies on the applied stress. The creep rate exponents for this alloy were between seven and eight, and the modulus-corrected activation energy for creep was approximately 350 kjoule/mole, which was comparable to the measured activa-tion energy for Ostwald ripening of the γ′ precipitates. Oriented γ′ coarsening to form lamellae perpendicular to the applied stress was very prominent during creep. At 1000 °C, the formation of a continuous γ-γ′ lamellar structure was completed during the primary creep stage. Shear through the γ-γ ' interface is considered to be the rate limiting step in the deformation process. Gradual thickening of the lamellae appeared to be the cause of the onset of tertiary creep. At 925 °C, the fully developed lamellar structure was not achieved until the secondary or tertiary creep stages. At this temperature, the γ-γ′ lamellar structure did not appear to be as beneficial for creep resistance as at the higher temperature.     

Plastic deformation of aluminum under repeated loading

The plastic deformation behavior of high purity (99.999 pct) polycrystalline and single crystal aluminum under repeated stressing was investigated by studying the creep behavior. The creep behavior under repeated stressing (cyclic creep) was compared with the static creep behavior at identical peak stresses. The influence of such experimental variables as the applied stress, the amplitude of cyclic stress, the test temperature and the static creep rate prior to stress cycling were systematically examined. The most important experimental observation in this study was that the cycling of the creep stress could either enhance or retard the creep deformation, depending upon the combination of the experimental variables. The experimental variable that had the most significant influence on the cyclic creep behavior was the applied stress; the enhancement of the creep rate was observed above a threshold stress, while the cyclic stress retarded the creep deformation at lower stresses. The threshold stress was found to depend sensitively on temperature. The implications of the threshold stress were examined by an analysis of the work-hardening behavior.     

Plastic deformation of aluminum under repeated loading

The plastic deformation behavior of high purity (99.999 pct) polycrystalline and single crystal aluminum under repeated stressing was investigated by studying the creep behavior. The creep behavior under repeated stressing (cyclic creep) was compared with the static creep behavior at identical peak stresses. The influence of such experimental variables as the applied stress, the amplitude of cyclic stress, the test temperature and the static creep rate prior to stress cycling were systematically examined. The most important experimental observation in this study was that the cycling of the creep stress could either enhance or retard the creep deformation, depending upon the combination of the experimental variables. The experimental variable that had the most significant influence on the cyclic creep behavior was the applied stress; the enhancement of the creep rate was observed above a threshold stress, while the cyclic stress retarded the creep deformation at lower stresses. The threshold stress was found to depend sensitively on temperature. The implications of the threshold stress were examined by an analysis of the work-hardening behavior.     

High-temperature creep in an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification

The creep behavior of an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification is investigated at three temperatures ranging from 623 to 723 K. The measured minimum creep strain rates cover seven orders of magnitude. The creep behavior is associated with the true threshold stress, decreasing with increasing temperature more strongly than the shear modulus of aluminum. The minimum creep strain rate is controlled by the lattice diffusion in the alloy matrix, and the true stress exponent is close to 5. The apparent activation energy of creep depends strongly on both applied stress and temperature and is generally much higher than the activation enthalpy of lattice self-diffusion in aluminum. Also, the apparent stress exponent of minimum creep strain rate depends on applied stress as well as on temperature and is generally much higher than the true stress exponent. This behavior of both the apparent activation energy and apparent stress exponent is accounted for by the strong temperature dependence of the threshold stress-to-shear modulus ratio. The true threshold creep behavior of the alloy is interpreted in terms of athermal detachment of dislocations from fine incoherent Al₁₂(Fe, V)₃Si phase particles, admitting a temperature dependence of the relaxation factor characterizing the strength of the attractive dislocation/particle interaction.     

Interpretation of high-temperature creep of type 304 stainless steel

The elevated-temperature creep behavior of Type 304 stainless steel is examined in terms of the measured effective and internal stresses. Results show that the mean effective stress is related to the applied stress by a power law of the form σ^* = α(σ)^β, where the constants α and β are functions of temperature. The dependence of creep rate on applied stress follows a power law, and the stress exponent is dependent on temperature. The latter behavior arises from the variation in the mean effective stress with applied stress and temperature. The creep rates are also described as a function of effective stress. The dislocation velocity-stress exponent obtained from stresschange tests is higher than the effective stress exponent evaluated from creep data. The dependence of creep rate on temperature at various values of effective stress yields a total activation energy of approximately the same magnitude as self-diffusion.     

Investigations on intercrystalline creep crack growth of the austenitic steels X 6 CrNi 18 11 and X 8 CrNiMoNb 16 16

The aim of the investigations was to get information about the behaviour of intercrystalline creep cracks during creep deformation from the evaluation of metallographic micrographs and finite element calculations. Two austenitic steels, X 8 CrNiMoNb 16 16 and X 6 CrNi 18 11, were investigated by carrying out creep tests at a temperature of 973 K with varying nominal stresses leading to times of failure from 1 to 3 783 h, as well as to certain creep strains in the tertiary creep stage. These creep tests were followed by analyzing metallographic micrographs of the creep tested specimens and by carrying out finite element calculations, which simulated creep crack growth. It was found that intercrystalline creep cracks in austenitic steel grow independently from momentary crack length but dependent on applied nominal stress and time. Finite element calculations show that local creep strain is a criterion for intercrystalline creep crack growth, contrary to stress-dependent fracture mechanical results.     

Features and effect factors of creep of single-crystal nickel-base superalloys

The creep behavior of two single-crystal nickel-base superalloys with [001] orientation has been studied by measuring the creep curves, internal friction stress of dislocation motion, transmission electron microscopy (TEM) observation and energy-dispersive X-ray analysis (EDAX) composition analysis. The results show that over the stress and temperature range, there are different creep activation energies, time exponents, and effective stress exponents in two alloys at different creep stages. The size and volume fraction of the γ′ phase in the tantalum-free alloy is obviously decreased with the elevated temperature. This results in the decrease of the internal friction stress during steady-state creep. Higher levels of tungsten in the alloy result in a smaller strain value and lower directional-coarsening rate during primary creep. During steady-state creep, the primary reason for the better creep resistance of the other alloy is that it contains more Al and also Ta, which maintains a high volume fraction of γ′ phase. The dislocation climb over the γ′ rafts is the major deformation mechanisms during steady-state tensile creep. The fact that the strain rate is decreased with the increase of the size and volume fraction of the γ′ rafts may be described by a modified constitutive equation that is based on a model of the rate of dislocation motion.     

Analysis of the creep behavior of silicon carbide whisker reinforced 2124 Al(T4)

The effect of stress and temperature on the steady state creep rate of SiC_w/2124 Al (T4) has been determined. The stress exponent for steady state creep of the composite is shown to increase from a value of 8.4 at 177 °C to a value of 21 at 288 °C. The activation energy for creep was determined to be 277 kJ/mol for testing in the temperature range from 149 to 204 °C and 431 kJ/mol for testing from 274 to 302 °C. These values are much greater than that for self-diffusion in aluminum. Such a severe temperature and stress dependence of the steady state creep rate is characteristic of precipitation and oxide dispersion strengthened nickel-base superalloys, where the creep behavior is explained by the particle strengthening contribution being a significant fraction of the applied creep stress. In contrast, the estimated particle strengthening for the composite is much less than the applied creep stresses. Alternate strengthening mechanisms are proposed to account for the observed creep behavior of the composite material, including the effect of temperature on the measured values of the stress exponent and activation energy for creep.     

Grain boundary sliding in the presence of grain boundary precipitates during transient creep

A constitutive rate equation for grain boundary sliding (GBS), in the presence of grain boundary precipitates, is developed. Langdon’s GBS model is modified by incorporating physically de-fined back stresses opposing dislocation glide and climb and by modifying the grain size de-pendence of creep rate. The rate equation accurately predicts the stress dependence of minimum creep rate and change in activation energy occurring as a result of changing the grain boundary precipitate distribution in complex Ni-base superalloys. The rate equation, along with the math-ematical formulations for internal stresses, is used to derive a transient creep model, where the transient is regarded as the combination of primary and secondary stages of creep in constant load creep tests. The transient creep model predicts that the transient creep strain is dependent on stress and independent of test temperature. It is predicted that a true steady-state creep will only be observed after an infinitely long time. However, tertiary creep mechanisms are expected to intervene and lead to an acceleration in creep rate long before the onset of a true steady state. The model accurately predicts the strain vs time relationships for transient creep in IN738LC Ni-base superalloy, containing different grain boundary carbide distributions, over a range of temperatures.     

Elevated temperature mechanical properties and residual tensile properties of two cast superalloys and several nickel-base oxide dispersion strengthened alloys

The elevated temperature tensile, stress-rupture and creep properties and residual tensile properties after creep straining have been determined for two cast superalloys and several wrought Ni-16Cr-4Al-yttria oxide dispersion strengthened (ODS) alloys. The creep behavior of the ODS alloys is similar to that of previously studied ODS nickel alloys. In general, the longitudinal direction is stronger than the long transverse direction, and creep is at least partially due to a diffusional creep mechanism as dispersoid-free zones were observed after creep-rupture testing. The tensile properties of the nickel-base superalloy B-1900 and cobalt-base superalloy MAR-M509 are not degraded by prior elevated temperature creep straining (at least up to 1 pct) between 1144 and 1366 K. On the other hand, the room temperature tensile properties of ODS nickel-base alloys can be reduced by prior creep strains of 0.5 pct or less between 1144 and 1477 K, with the long transverse direction being more susceptible to degradation than the longitudinal direction.