Activation Energy For Creep Research Articles

Effects of temperature and filler content on the creep behaviour of a polyurethane rubber

Both elastic stress-strain testing and (primary and secondary) creep testing have been carried out on unreinforced rubber and on two particulate-reinforced rubber composites. This has been done at three different temperatures for all three materials. The stress-strain curve of the rubber conforms well to that expected from classical rubber elasticity theory. When expressed as true stress–strain relationships, all three materials exhibit approximately linear plots, with the increases in stiffness on adding the particulate conforming to composite elasticity theory. The creep behaviour of all three materials can be captured well using a Miller-Norton formulation and the observed dependence on temperature has been used to estimate the activation energy for creep to be ~7 kJ mole-1. This is thought to indicate that the creep process does not involve rupture of covalent bonds (for the range of applied loads used), but is associated with physical processes such as molecular untangling. The fillers do enhance the creep resistance, to a degree that is quantitatively consistent with the expected load transfer from matrix to particulate.

Tensile creep and rupture behavior along with evolution of microstructure in a Zr-2.5Nb alloy

Tensile creep behavior of the Zr-2.5Nb alloy has been studied through tests under constant load (stress range ~ 137–371 MPa) between 275 and 375 °C. The minimum creep rate of the alloy is found to vary with applied stress and temperature following a power-law relation. The values of stress exponent (n) obtained in the range of 5.3–6.8 in the temperature interval of 300–375 °C; whereas it is calculated as 1.2 and 7.0 under low and high stresses, respectively at 275 °C. The apparent activation energy of creep (QC) and stress exponent have been determined by analyzing the experimental data. The obtained QC (~198.5 ± 10.7 kJ/mol) is found to be higher than the lattice self-diffusion activation energy of pure zirconium (113 kJ/mol). Using this value, stress exponent ~5.6 ± 0.23 is obtained by the temperature-compensated power law. Microstructural characterization by transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) analysis has confirmed coarsening of β-(Nb, Zr) precipitates with compositional changes during creep. The dislocation-precipitate interaction as evidenced by TEM observations is considered to be the origin of threshold stress, which decreases with increasing temperature. Considering the threshold stress, true activation energy of creep (Qt) and true stress exponent (nt) are found as ~160.4 ± 6.9 kJ/mol and ~4.8 ± 0.22, respectively. Analysis of creep data has confirmed the role of dislocation climb as the rate-controlling mechanism, along with validity of Monkman-Grant and modified Monkman-Grant relations. Scanning electron microscopy (SEM) of creep fracture surfaces has revealed evidence for prominent ductile fracture. Further, the obtained creep damage tolerance factor value of 1.9, indicating the predominance for cavitation during creep.

The creep behavior of Mg–9Al–1Si–1SiC composite at elevated temperature

Abstract The creep properties of as-cast Mg–9Al–1Si alloy and Mg–9Al–1Si–1SiC composite were compared. The results show that Mg–9Al–1Si–1SiC composite performs a better creep resistance than that of Mg–9Al–1Si alloy at constant temperature and stress (473 K,70 MPa). Besides, the creep behavior of Mg–9Al–1Si–1SiC composite at various temperature from 448 K to 498 K and under stresses of 70–90 MPa were systematically investigated. The Mg–9Al–1Si–1SiC composite exhibited a stress exponent from 5.5 to 6.9 and the creep activation energy fell within the range of 86–111 kJ/mol. The results showed that the creep mechanism of Mg–9Al–1Si–1SiC composite was mainly attributed to the effects of secondary phase strengthening mechanism and dislocation climb mechanism.

A comparative study on the effects of Gd, Y and La rare-earth elements on the microstructure and creep behavior of AZ81 Mg alloy

Impression creep tests were performed to study the effects of separate additions of 1 wt% Gd, Y, and La rare-earth (RE) elements on the microstructure and creep characteristics of a cast AZ81 Mg alloy. Constant punching stresses in the range 200–700 MPa were applied to the specimens at temperatures in the range of 423–523 K. The results revealed that the RE-containing alloys had lower creep rates in comparison with the base AZ81 alloy, the effect which was more significant in the AZ81+1La alloy. The enhanced creep resistance of the RE-containing alloys is a consequence of; (i) formation of Al2Gd and Al2Y particles with cubic morphologies, as well as Al11La3 particles with acicular-branched morphology, (ii) reduction in volume fraction of the β‒Mg17Al12 phase, and (iii) solid solution strengthening of Al in the Mg matrix. The studied alloys had stress exponents and activation energies in the range of 4.2–6.7 and 83–118 kJ/mol, respectively. The observed declining trend of creep-activation energy with increasing stress indicates the competition of two simultaneous mechanisms of lattice- and pipe-diffusion controlled dislocation climb. Dislocation climb controlled by dislocation pipe diffusion, and the climb of edge dislocations were found to be prevalent at high and low stresses, respectively.

Deformation Behavior and Cavitation of AA2017 at Elevated Temperatures

In this work, deformation behavior of AA2017-T4 at elevated temperatures was studied employing uni-axial tensile and creep experiments. Tensile tests were carried out at temperatures varying 150–500 °C under different strain rates then, a combination of neural network and dynamic material modeling was utilized to construct the processing maps. Furthermore, creep experiments were conducted to assess inelastic deformation behavior of the alloy at temperatures between 150 and 225 °C and stresses in the range of 150 to 230 MPa. Microstructural evaluations were carried out for determination of microstructural changes and formation of voids and cavities within the samples. The results showed that dynamic precipitation could occur during deformation at temperatures 175–225 °C leading to negative strain-rate sensitivity at true strains larger than 0.1. The main softening process was detected as dynamic recovery at temperatures higher than 250 °C however, dynamic recrystallization could also occur at low strain rates and temperatures higher than 400 °C. The activation energies for hot deformation were computed as 380.6 kJ mole−1 at 250–350 °C and it was reduced to 224.7 kJ mole−1 for the range of 350–500 °C. This showed the hard particle could significantly change rate of flow softening. The creep activation energy was determined as 169.5 kJ while the stress-exponent varied between 5.5 and 10.1 at temperatures between 150 and 225 °C indicating that dynamic recovery controlled by dislocation climb could be the governing creep mechanism.

A machine-learning-based surrogate model of Mars’ thermal evolution

SUMMARY Constraining initial conditions and parameters of mantle convection for a planet often requires running several hundred computationally expensive simulations in order to find those matching certain ‘observables’, such as crustal thickness, duration of volcanism, or radial contraction. A lower fidelity alternative is to use 1-D evolution models based on scaling laws that parametrize convective heat transfer. However, this approach is often limited in the amount of physics that scaling laws can accurately represent (e.g. temperature and pressure-dependent rheologies or mineralogical phase transitions can only be marginally simulated). We leverage neural networks to build a surrogate model that can predict the entire evolution (0–4.5 Gyr) of the 1-D temperature profile of a Mars-like planet for a wide range of values of five different parameters: reference viscosity, activation energy and activation volume of diffusion creep, enrichment factor of heat-producing elements in the crust and initial temperature of the mantle. The neural network we evaluate and present here has been trained from a subset of ∼10 000 evolution simulations of Mars ran on a 2-D quarter-cylindrical grid, from which we extracted laterally averaged 1-D temperature profiles. The temperature profiles predicted by this trained network match those of an unseen batch of 2-D simulations with an average accuracy of $99.7\, {\rm per~cent}$.

Influence of heat treatment on the structure evolution and creep deformation behavior of a precipitation hardened B2-(Ni, Fe)Al alloy

The effect of quenching and aging on structural evolution of the Ni27Fe26Al32Cr10Co5 alloy hardened by coherent α-Fe(Cr) precipitates was studied by scanning electron microscopy (SEM) and high-resolution electron microscopy (HRTEM). Heat treatment of the alloy was shown to optimize the morphology and size of hardening α precipitates and give rise to particles of the intermetallic σ-FeCr phase along the grain boundaries.The mechanical properties of the alloy were measured in the temperature range of 273–1573 K. Temperature dependences for the offset yield strength at different strain rates and the Young's modulus values were obtained. Quenching followed by age hardening increased the resistance to viscoplastic strain. The strain rate sensitivity coefficient was evaluated in the temperature range of 873–1573 K. The elastic limit and yield stress values reached under external tensile stress at 973 and 1073 K were determined.The analyzed B2-(Ni, Fe)Al-based alloy was subjected to deformation at stresses of 100, 150, 200, and 300 MPa in the temperature range of 773–1200 K. The steady-state creep strain rate was approximated by the Arrhenius creep equation (stress power being 8.3 and the effective creep activation energy being 335 kJ/mol). Threshold stresses equal to 94, 70, and 33 MPa were observed at 873, 973, and 1073 K, respectively. Calculations of critical stresses for the potential mechanisms controlling creep strain showed that the threshold stresses are caused by local and general dislocation climb over coherent α precipitates. The HRTEM studies of the dislocation substructure of the alloy after high-temperature creep testing confirmed that the creep strain mechanism was controlled by dislocation climb.

Effect of Ni content on the creep properties of Cu/Sn–0.3Ag–0.7Cu/Cu solder micro-joints

The tensile creep properties of line-type Cu/Sn–0.3Ag–0.7Cu(SAC0307)–xNi (x = 0, 0.02, 0.05 and 0.10 wt%)/Cu solder micro-joints were comparatively investigated via dynamic mechanical analysis. The effect of Ni content on the creep behaviour of the solder micro-joints was analysed and discussed. The creep behaviour of the joints at various temperatures and applied stresses was described by a power law. The results showed that the creep lifetime, creep activation energy and creep stress exponent of the Ni-containing solder micro-joints were greater than those of the Ni-free joint. Hence, adding Ni to the SAC0307 solder can enhance the creep resistance of the solder joints. The creep activation energy and the creep stress exponent of the solder micro-joints at 353–398 K and under a tensile stress of 8–15 MPa were 82.44–93.36 kJ/mol and 4.35–4.75, respectively. As the Ni content increased, the creep activation energy and the creep stress exponent first increased and then decreased. The Cu/SAC0307–0.05Ni/Cu solder micro-joint exhibited the strongest creep resistance among the tested joints. The creep deformation mechanisms of all solder micro-joints were mainly controlled by dislocation climbing, and the main creep failure mode was mixed brittle/ductile fracture.

Understanding the role of hydrogen on creep behaviour of Zircaloy-4 cladding tubes using nanoindentation

The present article investigates the influence of hydrogen concentration on the creep performance of cold-worked stress-relieved unirradiated Zircaloy-4 cladding tube using nanoindentation technique. The as-received Zircaloy-4 tube is hydrided to the concentrations of 600 ppm and 900 ppm using gaseous hydrogen charging method. Constant load indentation creep tests are performed for a dwell period of 600 s in the temperature range of 300°C-500 °C at 1000 μN, 2000 μN, and 3000 μN. The impact of hydrogen is evaluated in terms of steady state power law creep exponent and activation energy. The power law creep exponent decreases with increase in hydrogen concentration, however, it remains fairly constant with increase in temperature up to 500 °C. Moreover, activation energy too decreases significantly with increase in hydrogen concentration. The mean stress exponent and activation energy are found to be 3.58 and 28.67 kJ/mol, respectively, for as-received sample.

Structure related potential-upsurge during tensile creep of high entropy Al20Ce20La20Ni20Y20 metallic glass

In this paper, the dramatical increase in the apparent activation energy (defined as potential-upsurge) of tensile creep of a high entropy Al20Ce20La20Ni20Y20 metallic glass was discovered. The temperature referring to the occurrence of potential upsurge is around the onset relaxation temperature of as-spun specimen (110 °C (383 K)) measured by differential scanning calorimetry. Below 110 °C, the value of apparent activation energy is determined to be 3.9 kJ/mol, nevertheless, this value dramatically increases to 35.4 kJ/mol above 110 °C. Kelvin model was utilized to resolve the creep mechanism. The short-time relaxation behavior indicates a dramatical increase above 110 °C. The work reveals that the extension and retardation of smaller defects are responsible for the potential-upsurge in the apparent activation energy of tensile creep.

Creep of Heusler-Type Alloy Fe-25Al-25Co

Creep of an alloy based on the intermetallic compound Fe2AlCo was studied by compressive creep tests in the temperature range from 873 to 1073 K. The stress exponent n and the activation energy of creep Q were determined using the multivariable regression of the creep-rate data and their description by means of sinh equation (Garofalo equation). The evaluated stress exponents indicate that the dislocation climb controls creep deformation. The estimated apparent activation energies for creep are higher than the activation enthalpy for the diffusion of Fe in Fe3Al. This can be ascribed to the changes in crystal lattice and changing microstructure of the alloy.

Creep properties of high dense La9.33Si6O26 electrolyte for SOFCs

High density La9.33Si6O26 polycrystals were fabricated by conventional and spark plasma sintering starting from nanopowders synthesized by freeze-drying. The materials exhibit a homogeneous microstructure formed by equiaxed grains with average sizes of 1.1 μm and 0.2 μm-diameter depending on the sintering route. Compressive mechanical tests were performed in air at constant strain rate between 900 and 1300 °C. A gradual brittle-to-ductile transition was found with increasing temperature and/or decreasing strain rate. Grain boundary sliding is the main deformation mechanism in the ductile region, characterized by a stress exponent n = 1 for the conventional sintered (large-grained) material and n = 2 for the spark plasma sintered (fine-grained) material; in both cases, the activation energy for creep was 360 kJ/mol. Effective cation diffusivities have been derived from mechanical data by comparison with appropriate models. The creep properties of lanthanum silicates are reported here for the first time.

Formulation of Clay Refractory Bricks: Influence of the Nature of Chamotte and the Alumina Content in the Clay

Refractory materials from kaolinitic clays and clay chamotte or quartz were studied to increase the refractoriness under load at temperature above 1300°C. Two different clays mined in Burkina Faso were used and chamotte grains were obtained by preliminary firing a local clay. Fired materials at 1350-1400°C present a typical granular composite microstructure where large grains of chamotte or quartz are embedded in the clay matrix phase. Under load at high temperature, the behavior of material is influenced by the nature of the clay matrix phase that progressively melt at high temperature, the type of chamotte or quartz grains, the grain sizes of different phases and the sequence of the thermal transformations during firing. Kinetics of creep under a constant load were characterized against temperature and time. It gives the typical temperatures at fixed creep strains, that’s a well-recognized method for the refractoriness quantification. It’s shown that the kinetic of creep change with the variation of viscosity with temperature of the melted clay matrix phase, that’s related to both the chemical composition and the extend of the micro-composite nature of the heat transformed clays. Results also indicated that values of activation energy for creep are correlated to the refractoriness of materials.

Исследование механизмов уплотнения нанопорошков скуттерудитов CoSb-=SUB=-3-=/SUB=- в процессе активированного полем спекания

The densification parameters have been calculated for CoSb3 nanopowders treated within the field activated sintering. We have obtained the values of creep activation energy, strain rate sensitivity exponent and material parameter that characterize the creep process. It is concluded that densification of CoSb3 is due to the grain boundary sliding based creep. The simulation of the sintering process has been carried out using the finite element method; the change of sample sizes due to densification has been calculated using the creep model. The conformity between simulation and experimental results has been shown.

Tensile creep behavior of a die-cast Mg–6Al-0.2Mn–2Ca-0.3Si (wt.%) alloy

Tensile creep property of a die-cast AMXS6020 (Mg–6Al-0.2Mn–2Ca-0.3Si, wt.%) alloy has been investigated. The as-die-cast AMXS6020 alloy consists of α-Mg, Al2Ca and (Mg, Al)2Ca eutectics at grain boundaries. MgCaSi and Al8Mn5 intermetallic particles also exist both inside α-Mg grains and along the grain boundaries. Tensile creep tests, which were carried out at temperatures from 100 to 225 °C and stress range of 30–75 MPa for 100 h, show that the AMXS6020 alloy has excellent creep resistance as compared to commercial Mg alloys. When the temperature is increased from 100 °C to 180 °C or higher, the creep activation energy, Q, changes from 73 to 191 kJ/mol under the applied stress of 50 MPa. The creep results and TEM analysis of the specimens crept at 150 and 200 °C under the applied stress of 50 MPa suggest that the creep is controlled by grain boundary sliding at the temperature below 180 °C, while the creep is controlled by cross slip to non-basal planes at the temperature over 180 °C.

Influence of aging and HIP treatment on the structure and properties of NiAl-based turbine blades manufactured by laser powder bed fusion

The parameters and strategy of laser powder bed fusion (LPBF) applied in the manufacture of turbine rotor blade models using an innovative Ni41Al41Cr12Co6 alloy were optimised. The effect of exposure to heat treatment by aging and hot isostatic pressing (HIP) on the evolution of structural phase transformations was studied by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR TEM). HIP eliminates the structural anisotropy and facilitates the segregation of 0.02–1.5 μm coherent α-Cr precipitates oriented along the NiAl <011> direction. Nanoparticles of the Hf and Heusler phase Ni2AlHf were predominantly redistributed from the boundaries equiaxial with the NiAl grains, which increased the elevated-temperature strength of the alloy. The mechanical properties were studied in the temperature range of 923–1273 K. The temperature-force dependence of the steady-state creep rate, activation energy for creep, and mechanisms of plastic strain accumulation were determined. The temperature dependences of the Young's modulus and offset yield stress were plotted. It was found that the post-treatment of the aged LPBF- built blades by HIP had a positive influence on the resistance to plastic deformation. The high-temperature strength of the rotor blade material fabricated by LPBF + aging + HIP was higher than that of the as-HIP and as-LPBF + aging material by 85 K. The temperature-force operating conditions (over 100,000 h) at a plastic strain less than 1 % were identified.

Creep Rupture Assessment of New Heat-Resistant Sanicro 25 Steel Using Different Life Prediction Approaches

The paper deals with creep life and creep strength estimations of Sanicro 25 steel through various continuum damage models and modified extrapolation methods. Four different continuum damage models, i.e., modified K-R model, Dyson model, W-T model and modified W-T model considering the stress-dependent failure strain, were incorporated into numerical simulations using user subroutines so as to analyze the creep strain behavior and the creep rupture time. In the case of 700 °C, the predicted creep rupture lifetimes determined from modified K-R model, Dyson model, W-T model and modified W-T model all matched with the experimental data, where the modified W-T model provided the best prediction. However, at 750 °C, only the predictions using W-T model and modified K-R model matched with the experimental data. The Dyson model and modified W-T model underestimated the creep rupture lifetime. Besides the creep damage approaches, a modified extrapolation method developed by Wilshire (based on the stress-dependent creep activation energy) was also tested and verified to make predictions of creep lifetime under various stress conditions more easily than the creep damage models. A general comparison of the analyses made with different life prediction methods in Sanicro 25 steel was presented in this paper.

Stable second phase: The key to high-temperature creep performance of particle reinforced aluminum matrix composite

High-temperature creep behavior of 45vol%-SiCp/2024Al was studied at 250–350 °C under 40–140 MPa. True stress exponent and activation energy were measured 8 and 227.7 kJ/mol, respectively. The feature substructure of SiCp/2024Al was the second phase Al2Cu dispersed at the interface. The evolution of microstructure and the reason for its high creep activation energy were both analyzed and discussed. θ′ phase and S′ phase underwent different evolution processes during creep. θ′ phase gradually transformed to θ phase and remain stable from the steady-state creep until creep rupture. High content of the interface facilitated the dispersion of Al2Cu. The relationship between true stress exponent and interface content as well as the key to high true stress exponent of high volume fraction particle reinforced aluminum matrix composite were also discussed. This research provided new perspectives and guidance for designing and fabricating composites with superior high-temperature creep properties.

Predictions of long-term creep life for the family of 9–12 wt% Cr martensitic steels

Two contemporary power-law creep methodologies for predicting long-term creep life from short-term lab-based experiments were extended to 48 different alloy compositions in the family of 9–12 wt% Cr steels. The study highlights the limitations that the data imposes on the applicability of the two methods for predicting lifetimes, while also assessing the assumptions made by both methods. It was found that a dependency exists between the creep activation energy and the test conditions when activation energy was calculated using a broader range of normalized stress states (σ/σTS) than were previously explored. This contradicts the assumptions of constant activation energy made by both contemporary methods. This raises questions about both the physical interpretation of activation energy and the reliability of the extrapolation of lifetime predictions made. The paper highlights that the modified power-laws are empirical tools and illustrates the importance of study protocol design for data collection, when short-term experiments are limited to a maximum of 6,000 h. In addition, the effects of alloying additions and tempering temperature on the creep mechanism were explored.

Creep Failure and Damage Mechanism of Inconel 718 Alloy at 800–900 °C

The creep behavior and damage mechanisms of 718 alloys were investigated at 800–900 °C in air. The fracture morphology and microstructure evolution were observed by optical, scanning and transmission electron microscope. Besides, the creep damage tolerance (λ) and creep strain evolution curve were also calculated. The results showed that the creep curves of 718 alloys at 800 or 850 °C consisted of primary and tertiary stages, while the steady-state region became apparent at 900 °C. The apparent creep activation energy of 718 alloy was in the range from 446.3 to 491.8 kJ/mol. The alloy presented ductile fracture at 800 °C due to the nucleation, growth and linkage of creep voids. However, the failure of alloys at 850 or 900 °C presented necking to a point due to the microstructure degradation. Further investigations showed the softening of materials and the loss of mechanical performance could be mainly attributed to the coarsening or decrease of strengthening precipitates. Above 850 °C, it was found that γ′ phases would dissolve into matrix and stress promoted the re-dissolution of γ′ phases or led to the break of δ phases. Moreover, the creep strain evolution curves indicated that 718 alloys kept a relative stable state at 800–900 °C when the strain fraction was below 1.