Activation Energy For Creep Research Articles

Influence of heat treatment regimes on microstructure and creep properties of FGH95 nickel base superalloy

Abstract By means of creep curves measurement and microstructure observation, an investigation has been made into the influence of heat treatment regimes on microstructure and creep properties of FGH95 nickel base superalloy. Results show that, after the alloy is solution treated and cooled in oil bath, coarser γ′ precipitates are distributed in the previous particle boundaries (PPB) regions in which appears the γ′-free phase zone. No coarser γ′ phase is detected in the alloy cooled in molten salt, the grain size of which increases slightly and fine γ′ phase is dispersedly distributed in the grains, in addition, some (Nb, Ti)C particles are precipitated along the boundaries. In the ranges of the experimental stresses and temperatures, the alloy cooled in molten salt possesses a better creep resistance and longer creep life, the creep activation energy of the alloy is measured to be Q = 590.3 kJ/mol. During creep, the deformed mechanism of the alloy cooled in oil bath is that the slipping dislocations with double orientations are activated in the matrix, and some finer carbide particles are precipitated along the slipping traces. Moreover, the wider PPB regions are broken into finer grains due to the severe plastic deformation. However, not only dislocations tangle but also stacking fault appears in the alloy cooled in molten salt. The fact that the carbide particles precipitate along the boundary may effectively restrain the movement of dislocation slipping, which is one important factor for the alloy possessing better creep resistance and longer creep life.

Stress Exponent of Minimum Creep Rate and Activation Energy of Creep for Oxide Dispersion-strengthened Nickel-based Superalloy MA754

The stress exponent of the minimum creep rate, n, and the activation energy of creep, Qc, were obtained for the oxide dispersion-strengthened nickel-based superalloy MA754 by conducting creep tests at 1223–1273 K in the stress range of 130–190 MPa. The values of n and Qc in MA754 were determined to be 26 and 962 kJ/mol, respectively. The causes of these high values were determined by measuring the internal stress, σi, using the strain dip test. The ratio of σi to applied stress, σa, was very high at lower stresses, while at higher stresses, σi reached its saturated value with increasing stress. In this stress range, the ratio of σi to σa decreased drastically with increasing stress, which reflected the large increase in creep rate. Such a large increase in creep rate with increasing applied stress led to an unpredictably higher value of n. With increasing temperature, the saturated σi decreased, which resulted in a relatively large creep rate. The larger creep rate at high temperatures led to a large value of Qc. In addition, the dislocation density, ρ, of the interrupted creep specimens at the time of the minimum creep rate increased with increasing stress and reached the saturated value. This change in ρ with stress must be reflected in a large change in n and Qc through a change in σi.

押込み試験とモデリングによるベキ乗則材料の単軸クリープに関する構成式の予測

Indentation creep tests and finite element simulations were performed on a model material to show that a constitutive equation for conventional uniaxial creep can be derived using the instrumented indentation testing technique. When the indentation pressure and the indentation creep rate maintain constant values of ps and εin(s), respectively, the contours of the equivalent stress and the equivalent plastic strain rate in the region beneath the conical indenter expand according to the increase in the indenter displacement while maintaining the geometrical self-similarity. These findings indicate that a pseudo-steady state deformation takes place around the indenter tip. The representative point exhibiting the creep behavior within the limited region, which actually determines the indenter velocity, is defined as the location where the equivalent stress σr is equal to ps/3. The equivalent plastic strain rate εr at this point is found to be εin(s)/3.6 in the case that the creep stress exponent is 3. The stress exponent and the activation energy for creep extracted from the results of Al-5.3 mol%Mg solid-solution alloy indentation tests are in close agreement with those of tensile creep tests reported in the literature. In addition, the values for σr and εr agree well with the values for the applied stress and the corresponding creep rate in tensile creep tests at the same temperature. The above results show not only that the creep characteristics of advanced materials, which are often available in minute quantities or as small-volume specimens, can be obtained from carefully designed indentation creep tests, but also that the constitutive equation for tensile creep can be predicted with sufficient precision through indentation creep test results.

Temperature dependent stiffness and visco-elastic behaviour of lipid coated microbubbles using atomic force microscopy

The compression stiffness of a phospholipid microbubble was determined using force-spectroscopy as a function of temperature. The stiffness was found to decrease by approximately a factor of three from ∼0.08 N m−1, at 10 °C, down to ∼0.03 N m−1 at 37 °C. This temperature dependence indicates that the surface tension of lipid coating is the dominant contribution to the microbubble stiffness. The time-dependent material properties, e.g. creep, increased non-linearly with temperature, showing a factor of two increase in creep-displacement, from ∼24 nm, at 10 °C, to 50 nm, at 37 °C. The standard linear solid model was used to extract the visco-elastic parameters and their determination at different temperatures allowed the first determination of the activation energy for creep, for a microbubble, to be determined.

Grain boundary phase transformation in Cu–Co solid solutions

The present work examined the creep behavior of copper based solid solutions with cobalt at temperatures between 980°C and 1080°C and stresses lower than 0.2MPa. The samples were made from 18μm foil and were formed into cylinders. After the pre-annealing at 1000°C during about 30h the samples had a parquet structure. The experiments were performed in the hydrogen atmosphere. New equipment was designed for these measurements.The activation energy of pure copper creep was close to the activation energy of copper volume self-diffusion. Cu–Co solid solution creep rate was always lower than that of pure copper. It was shown that the creep activation energy in the relatively low temperature region was higher than in the high temperature region. The same behavior was typical for all studied solid solutions. The transition temperature was about 1030°C.It was proposed that such creep behavior was connected with grain boundary phase transformation and explanation was made in terms of interface controlled diffusional creep.

Creep and microstructure in powder metallurgy 15 vol.% SiCp–2009 Al composite

The creep behavior and microstructure of powder metallurgy (PM) 15 vol.% silicon particulate-reinforced 2009 aluminum alloy (SiCp–2009 Al composite) and its matrix PM 2009 Al were investigated over six orders of magnitude of strain rate and at temperatures in the range 618–678 K. The results show that the creep behavior of PM 15% SiCp–2009 Al composite resembles that of PM 2009 Al with regard to (a) the variations in both the apparent stress exponent and the apparent activation energy for creep due to applied stress, (b) the value of the true stress exponent, (c) the value of the true activation energy for creep, (d) the interpretation of creep in terms of a threshold stress, and (e) the temperature dependence of threshold stress. This resemblance implies that deformation in the matrix governs deformation in the composite. Analysis of the creep data in terms of creep rate against an effective stress shows that the creep behaviors of the composite and unreinforced alloy are consistent with the operation of viscous glide creep at low stresses. A comparison between the creep data of the composite and those of the unreinforced matrix revealed that the composite exhibited more creep-resistant characteristics than its matrix over the entire range of applied stresses.

Creep Behavior of Bi-Containing Lead-Free Solder Alloys

The creep behavior of Sn-3.0Ag-0.5Cu (SAC305), Sn-3.4Ag-1.0Cu-3.3Bi (SAC-Bi), and Sn-3.4Ag-4.8Bi (SnAg-Bi, all wt.%) was studied in constant-stress creep tests from room temperature to 125°C. The alloys were tested in two microstructural conditions. As-cast alloys had a composite eutectic-primary Sn structure, while in aged alloys the eutectic regions were replaced by a continuous Sn matrix with coarsened intermetallic (Cu6Sn5 and Ag3Sn) particles. After aging, Bi in SAC-Bi and SnAg-Bi was found as precipitates at grain boundaries and grain interiors. The creep resistance of of-cast SAC305 was higher than that of as-cast Bi-containing alloys, but after aging the SAC305 had the lowest creep resistance. The creep strain rates in SAC-Bi and SnAg-Bi were much less affected by aging. The apparent activation energy for creep was also changed more for SAC305 than for the other two alloys. The creep behavior of SAC-Bi and SnAg-Bi can be understood by considering the solubility of Bi in Sn. The difference in creep behavior between as-cast and aged SAC-Bi is greatly reduced when room-temperature test results are excluded from analysis. This suggests that the strongest influence on creep in these alloys is due to Bi solute interaction with moving dislocations during deformation.

Creep characteristics and microstructure in nano-particle strengthened AA6082

Abstract Creep characteristics of nano-particle strengthened AA6082 (Al–Mg–Si) alloy via precipitation hardening were investigated over five orders of magnitude of strain rates. The experimental data showed that the apparent creep behavior of AA6082 differed from that of pure Al with regard to two primary features: the apparent stress exponent, n a , is about 7 and the apparent activation energy for creep (206 kJ/mol) is higher than that for self-diffusion (142 kJ/mol). An examination of the data revealed a threshold stress whose temperature dependence is much stronger than that attributable to the shear modulus. Evidence based on transmission electron microscopy (TEM) investigation indicated that the origin of the threshold stresses was related to the interaction of dislocations with second phase particles, which were identified as Mg 2 Si. By incorporating the threshold stress into creep analysis, it was demonstrated that the true creep behavior of AA6068 was consistent with that ascribable to high-temperature climb and that the alloy was more creep resistant than Al.

Creep Failure and Prevention in Aluminum Wound Distribution Transformers

This paper presents an experimental study on electrical properties and creep behavior of thermo-mechanically processed 6060 aluminum alloy wire to assess its suitability as a winding conductor in place of electric conductor grade aluminum wire in distribution transformers. Electrical resistivity and the stress exponent and activation energy for creep were measured for both thermo-mechanically processed 6060 aluminum alloy and electric conductor grade aluminum wire. An algorithm is given for estimate the life and switching capability of the distribution transformers considering creep. The results of this study can be used as a guide to select the creep resistant winding material for designing and producing reliable distribution transformers and is valuable especially in power deficient areas, poorly designed and haphazardly expanded power distribution networks where repeatedly energized aluminum wound distribution transformers may fail due to creep in the high voltage winding conductors.

Effects of Nb and W additions on high-temperature creep properties of ferritic stainless steels for solid oxide fuel cell interconnect

Tensile and creep properties of a newly developed ferritic stainless steel (Crofer 22 H) with additions of Nb and W for planar solid oxide fuel cell (pSOFC) interconnect are investigated at 25–800 °C. These properties are compared with those of another ferritic stainless steel (Crofer 22 APU) without Nb or W. A significantly improved tensile and creep strength of Crofer 22 H over Crofer 22 APU for pSOFC interconnect is found and attributed to a precipitation strengthening effect of the Laves phase. According to the creep stress exponent, activation energy, and microstructural observations, the superiority of Crofer 22 H over Crofer 22 APU in creep resistance at 650–800 °C involves a power-law dislocation creep mechanism interacting with an in situ precipitation strengthening mechanism. The relation between creep rupture time and applied stress for Crofer 22 H is well described by a simple power law. Larson–Miller relationship is also applied and shows good results in correlating the creep rupture time with applied stress and temperature for the Crofer 22 H steel. A significant coarsening of the Laves phase is responsible for the reduced improvement of creep resistance in Crofer 22 H at the low-stress, long-term region of 800 °C.

Creep properties and effect factors of hot continuous rolled Ti–6Al–4V alloy

Abstract By means of heat treatment, measurement of creep properties and contract analysis of dislocations configuration, creep behaviors and effect factors of hot continuous rolled (HCR) Ti–6Al–4V alloy are investigated. Results show that, the microstructure of HCR alloy after solution treatment at temperature lower than β phase transus point consists of the super-saturation equiaxial α phase and “basket weave” structure. The quantity of “basket weave” structure increases with solution temperature, and the fully “basket weave” structure may be obtained after solution treatment at 1000 °C. Compared to the alloy solution treated at 940 °C, the alloy treated at 1000 °C has lower strain rate and longer creep lifetime under the conditions of applied stress of 575 MPa at 420 °C. In the range of the applied stress and temperature, the creep activation energy of the alloy is calculated to be Qa = 249.8 kJ/mol. Deformation mechanism of HCR alloy during creep is double orientations slips of dislocations activated within α phase with HCP structure, while deformation feature of the alloy solution treated at 940 °C is the wave-like 〈a + c〉 dislocations activated on the pyramidal planes in the α phase. Solution treated at 1000 °C, deformation mechanism of the alloy is that the multiple slips of (1/2)〈1 1 1〉 dislocations are activated within β phase with BCC structure, in which V-rich β phase with high volume fraction may enhance creep resistance, which is thought to be the main reason of the alloy possessing the low strain rate and longer creep lifetime.

Coupling the atmosphere with interior dynamics: Implications for the resurfacing of Venus

We calculated 2D and 3D mantle convection models for Venus using digitized atmosphere temperatures from the model of Bullock and Grinspoon (Bullock, M.A., Grinspoon, D.H. [2001]. Icarus 150, 19–37) to study the interaction between interior dynamics and atmosphere thermal evolution. The coupling between atmosphere and interior occurs through mantle degassing and the effect of varying concentrations of the greenhouse gas H 2O on the surface temperature. Exospheric loss of hydrogen to space is accounted for as a H 2O sink. The surface temperature enters the mantle convection model as a boundary condition. Our results suggest a self-consistent feedback mechanism between the interior and the atmosphere resulting in spatial–temporal surface renewal. Greenhouse warming of the atmosphere results in an increase in the surface temperature. Whenever the surface temperature reaches a critical value, the viscosity difference across the lithosphere becomes smaller than about 10 5 and the surface becomes locally mobile. The critical surface temperature depends on the activation energy for mantle creep, the stress exponent in the non-Newtonian mantle rheology law, and the mantle temperature. Surface renewal together with surface lava flow may explain why the surface of Venus is young on average, i.e. not older than a few hundred million years. The mobilization of the near-surface lithosphere increases the rate of heat removal from the mantle and thereby the interior cooling rate. The enhanced cooling results in a reduction of the water outgassing rates. As a consequence of decreasing water concentrations in the atmosphere, the surface temperature decreases. Our model calculations suggest that Venus should have been geologically active until recently. This is in agreement with several lines of observational evidence from thermal emissivity measurements and crater distribution analyses.

Creep Behavior of 9% Ni Alloy Steel at Elevated Temperatures

Little design data is available for the creep life prediction of 9% Ni alloy in elevated temperatures. Therefore, in this study, a series of creep tests under 16 combined conditions with 4 kinds of stresses and 4 temperatures was performed to obtain creep design and life prediction data for 9% Ni alloy, with the following results. The stress exponents decreased as the test temperature increased. The creep activation energy gradually decreased as the stresses became larger. The Larson-Miller parameter (LMP) constant for this alloy was estimated to be about 2.

Synthesis creep behavior of Sn63Pb37 under the applied stress and electric current

Synthesis creep test of Sn63Pb37 alloy was conducted under the combination of tensile stress and direct electric current. Results indicate that the steady-state creep strain rate of Sn63Pb37 alloy increases with increasing the electric current and changes more drastically at low stress level than that at high stress level. The stress exponent in power law decreases linearly with increasing the electric current, while the creep activation energy increases with the square of electric current. To illuminate the electromechanical interaction in creep behavior, a modified Arrhenius type model involving the current effect was proposed.

On the determination of vortex creep parameters in superconductors using standardmagnetization relaxation data

The relaxation of the irreversible magnetic momentm(t) inYBa2Cu3O7 (YBCO) films was investigated as a function of temperatureT and the externalmagnetic field H along the c axis applied in zero-field cooling conditions, for the determination ofvortex creep parameters. The data analysis was performed using theT and current density dependence of the normalized vortex creep activation energy, or by the fit of them(t) data with the well known interpolation formula in the framework of the generalvortex creep equation. It was found that (i) even for specimens with strongstatic pinning the characteristic pinning energy remains small in the low-T range, where the vortex creep appearing in standard magnetizationmeasurements is elastic, (ii) the observed strong increase of the timescale for creep and the decrease of the creep exponent with increasingH beyond the theoretical predictions can be attributed to the crossoverelastic-creep–plastic-creep generated by the macroscopic currents inducedin the specimen during magnetization measurements, and (iii) the creepparameters extracted with the interpolation formula are highly affected by them(t) data registeredat short t, which may lead to unphysical creep parameter values.

High Temperature Creep Behavior of Zn-1.0Cu-0.2Ti Alloy

In the present work, Zn-1.0Cu-0.2Ti alloy was prepared by melt casting and extruding processes. High temperature creep property of the alloy was determined using electronic creep relaxation testing machine. Microstructures of the alloy before and after creep test were observed and its high temperature creep mechanism was discussed. The results show that the steady-state creep rate of the alloy increases with temperature and stress. The logarithm of steady-state creep rate (ln) shows a linearity relationship with the logarithm of the stress (lnσ) and reciprocal of temperature (1/T). The stress exponent and apparent activation energy for creep have been determined to be 5.10 and 83.7 kJ/mol, separately. The predominant mechanism is mainly self-diffusional creep. The second phases on the grain boundary can block the slip of grain boundary and dislocation motion which can improve creep resistance of the alloy.

Microstructures and Creep Behaviour of Mg-4Al Alloy Containing Sr and Ca

Microstructures and creep property of the Mg-4Al based alloy with addition of 2% Sr and 1%Ca were investigated. The as-cast microstructures of the present alloy consist of dendritic α-Mg and two major intermetallics: lamellar eutectic C14-Mg2Ca and bulky type Mg-Al-Sr ternary phase. These intermetallics mainly distribute along grain or cell boundaries and form an almost continuous network. The alloy studied shows an excellent creep resistance under the experimental conditions. This is primarily attributed to formation of the thermostable intermetallics with addition of Sr and Ca to Mg-Al based alloy. The values of stress exponent, n, and creep activation energy, Q, imply that both dislocaiton motion and grain boundary sliding contribute to the creep deformation.

Dependence of dislocation creep of dunite on oxygen fugacity: Implications for viscosity variations in Earth's mantle

[1] Significant variations in flow behavior are known to exist both vertically and laterally in Earth's upper mantle. The sources of such variation may be thermal, compositional, or reflect differences in the chemical activity of components, such as oxygen, silica, and water. We report on the effects of oxygen fugacity on dislocation creep of dunite, with a view to understanding potential strength heterogeneity in the mantle. Although room pressure experiments on single crystals of olivine have shown a clear dependence of creep rate on oxygen fugacity, no prior deformation study of polycrystalline olivine-rich rocks has demonstrated such a dependency under high-pressure conditions. In this study we performed a series of dry creep experiments on a natural dunite under carefully controlled thermochemical conditions, including oxygen fugacity. The samples, cored from coarse-grained Aheim dunite with a grain size of ∼0.9 mm, were deformed under triaxial compression at oxygen fugacities fixed by either the iron/wustite or the nickel/nickel oxide solid state buffers, temperatures between 1150° and 1277°C, and differential stresses up to 300 MPa. The results of a global fit to all experimental data indicate a power law dependence of creep rate on oxygen fugacity, with an oxygen fugacity exponent of m = 0.20 ± 0.01, n = 3.6 ± 0.1, A = 102.6±0.3 s−1 MPa−3.6 Pa−0.2, and an activation energy for creep of 449 ± 7 kJ/mol. This activation energy is significantly less than the commonly used value of 535 kJ/mol because the earlier experiments made no corrections for the effects of oxygen fugacity. When applied to planetary interiors, an increase in oxygen fugacity by a factor of ∼103.5, from the iron/wustite to the fayalite-magnetite-quartz buffers, will result in a factor of ∼5 decrease in viscosity.

Characterization of the Ni-8YSZ Cermet Creep and Its Impact on the Cell 'Redox' Tolerance

Creep properties of a commercial Ni-8YSZ cermet have been investigated at the SOFC operating temperatures. Experiments have shown that Ni-8YSZ cermet exhibits substantial creep strain rates. The creep activation energy and the parameters of the Norton's law have been determined from the experimental data. Impact of such behavior on cell 'redox' tolerance has been discussed for both planar SOFC configurations. Considering unused Anode Supported Cells (ASC), oxidation tests have shown that the electrolyte cracking arises as soon as ~65% of Ni is transformed into NiO. A mechanical analysis, which takes into account the cermet creep, suggests that this threshold could be lowered after a long term operation. In the case of the Electrolyte Supported Cell (ECS), the electrochemical degradation has been recorder upon several full 'redox' cycles. This degradation is found to be well correlated with the viscoplastic deformation of the cermet.

Life prediction of a Ni-base superalloy

Rene 80 samples were creep–rupture tested in air between 1144 and 1255 K at various stress levels. The mean stress exponent, n, and the mean activation energy for creep were calculated from the experimental results. The accelerated creep life of the alloy was evaluated by using iso-stress parametric equations and Monkman–Grant method.