Activation Energy For Creep Research Articles

Prediction of the Constitutive Equation for Uniaxial Creep of a Power-Law Material through Instrumented Microindentation Testing and Modeling

Indentation creep tests and finite element simulations were performed on a model material to show that the constitutive equation for conventional uniaxial creep can be derived using the instrumented indentation testing technique. When the indentation pressure and the indentation creep rate are maintained at constant values of ps and _inðsÞ, 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 displacement of the indenter while maintaining geometrical self-similarity. These findings indicate that a pseudo-steady deformation state 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 � · r equals ps=3. The equivalent plastic strain rate _ ¾r at this point is found to be _inðsÞ=3:6 in the case when the stress exponent for creep is 3. The stress exponent and the activation energy for creep extracted from the results of Al­5.3mol%Mg solidsolution alloy indentation tests are in close agreement with those of tensile creep tests reported in the literature. In addition, the values for � · r and _r 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 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, and furthermore the constitutive equation for tensile creep can be predicted with sufficient accuracy through indentation creep test results. [doi:10.2320/matertrans.M2013370]

Creep deformation behavior of Sn–Zn solder alloys

Creep properties of three Sn–Zn solder alloys (Sn–9Zn, Sn–20Zn, and Sn–25Zn, wt%) were studied using the impression creep technique. Microstructural characteristics were examined using a scanning electron microscope. The alloys exhibited stress exponents of about 5.0. The activation energy for creep was calculated to be ~50–75 kJ/mol with a mean value of 66.3 kJ/mol. The likely creep mechanism was identified to be the low temperature viscous glide of dislocations.

Creep and Creep-rupture Behavior of 2124-T851 Aluminum Alloy

AbstractHigh temperature creep and useful creep life behavior of Al-Cu-Mg (2124-T851 aluminum) alloy was investigated by conducting constant stress uniaxial tensile creep tests at different temperatures (473–563 K) and at stresses ranging from 80 to 200 MPa. It was found that the stress and temperature dependence of minimum creep rate could be successfully described by the power-law creep equation. The power-law stress exponent, n = 5.2 and the activation energy for secondary creep, Q = 164 kJ mol−1, which is close to that observed for self diffusion of aluminum (~140 kJ mol−1). The observed values of n and Q suggest that the secondary creep of 2124-T851 aluminum alloy is governed by the lattice diffusion controlled dislocation climb process. A Monkman-Grant type relationship between minimum creep rate and time for reaching 1.5% creep strain is proposed and could be employed for predicting the useful creep life of 2124-T851 aluminum alloy.

Impression creep behavior of Al–1.9%Ni–1.6%Mn–1%Mg alloy

In the present study, microstructure and creep behavior of an Al–1.9%Ni–1.6%Mn–1%Mg alloy were studied at temperature ranging from 493 to 513K and under stresses between 420 and 530MPa. The creep test was carried out by impression creep technique in which a flat ended cylindrical indenter was impressed on the specimens. The results showed that microstructure of the alloy is composed of primary α(Al) phase covered by a mantle of α(Al)+Ni3Al intermetallic compound. Mn segregated into AlxMnyNiz or Al6Mn phases distributed inside the matrix phase. It was found that the stress exponent, n, decreases from 5.2 to 3.6 with increasing temperature. Creep activation energies between 115kJ/mol and 151kJ/mol were estimated for the alloy and it decreases with rising stress. According to the stress exponent and creep activation energies, the lattice and pipe diffusion- climb controlled dislocation creep were the dominant creep mechanism.

Effect of DC current on tensile creep of pure tin

The tensile creep of tin was performed in a temperature range of 323–423K and under the tensile stress of 1.93–13.89MPa. During the constant-load tensile creep test, a direct electric current of electric current density in the range from 0 to 3.78kA/cm2 was passed through the tin specimen, which introduced electrical–thermal–mechanical interaction. A quasi-steady state creep deformation was observed under the simultaneous action of electrical current and tensile stress. The minimum creep rate increased with the increase in temperature, tensile stress and electrical current density. For the same tensile stress and the same chamber temperature, the minimum creep rate increased linearly with the square of the electric current density. A power-law relation was used to describe the stress dependence of the minimum creep rate for the tensile creep of tin. The passage of an electric current of high current density caused the rise of local temperature through the release of Joule heat and introduced the momentum exchange between high-speed mobile electrons and lattice atoms, which resulted in the increase of local grain rotation and grain boundary sliding. The electric current density had no significant effect on the stress exponent and activation energy of the tensile creep of tin for the experimental conditions.

Tensile creep of directionally solidified NiAl–9Mo in situ composites

Well-aligned Mo fiber-reinforced NiAl in situ composites were produced by specially controlled directional solidification. The creep behavior parallel to the growth direction was studied in static tensile tests at temperatures between 900°C and 1200°C. A steady-state creep rate of 10−6s−1 was measured at 1100°C under an initial applied tensile stress of 150MPa. Compared to binary NiAl and previously investigated NiAl–Mo eutectics with irregularly oriented Mo fibers, this value demonstrates a remarkably improved creep resistance in NiAl–Mo with well-aligned unidirectional Mo fibers. A high-resolution transmission electron microscope investigation of the NiAl/Mo interface revealed a clean semi-coherent boundary between NiAl and Mo, which enabled an effective load transfer from the NiAl matrix to the Mo fibers, and thus leads to the remarkably increased creep strength. The stress exponent, n, was found to be between 3.5 and 5, dependent on temperature. The activation energy for creep, Qc, was measured to be 291±19kJmol–1, which is close to the value for self-diffusion in binary NiAl. Transmission electron microscopy observations substantiated that creep occurred by dislocation climb in the NiAl matrix. The Mo fiber was found to behave in a quasi-rigid manner during creep. A creep model for fiber-reinforced metal matrix composites was applied for an in-depth understanding of the mechanical behavior of the individual components and their contribution to the creep strength of the composite.

Fabrication of Hybrid Surface Composite through Friction Stir Processing and Its Impression Creep Behaviour

Al-Ni in situ surface composites were fabricated by friction stir processing method. Friction stir processing produced a composite with nickel and NiAl3 as reinforcement particles in aluminium matrix. The particles were fine and were in the submicrometer size range. The separation distance between the particles was very small. Impression creep experiments were conducted on the samples both at friction stir zone and base material zone at various temperatures. Steady state creep rates were estimated, and activation energy for creep was calculated. It is observed that the friction stir zone offered a higher creep resistance compared to the base metal zone. Higher creep resistance is attributed to the dissolution of nickel atoms into aluminium matrix and the presence of fine nickel particles and NiAl3 precipitates. The measured activation energy indicated that the associated creep mechanism is the dislocation creep in the temperature range of 30–150°C, both in friction stir zone and base metal zone. At higher temperatures (150–180°C) the diffusion creep mechanism is suggested.

Basic creep models for 25Cr20NiNbN austenitic stainless steels

Basic models for solid solution and precipitation hardening during creep are presented for the austenitic stainless steels 25Cr20NiNbN (TP310HNbN, HR3C, DMV310N). The solid solution hardening is a result of the formation of Cottrell clouds of solutes around the dislocations. In addition to slowing down the creep, the solutes increase the activation energy for creep. The increase in activation energy corresponds to the maximum binding energy between the solutes and the dislocations. The formation of fine niobium nitrides during service enhances the creep strength. It is found that the nitrides have an exponential size distribution. In the modelling the critical event is the time it takes for a dislocation to climb over a particle. The creep models can accurately describe the observed time and temperature dependence of the creep rupture strength.

Flexural creep of zirconium diboride–silicon carbide up to 2200°C in minutes with non-contact electromagnetic testing

Abstract Flexural creep of ZrB 2 –30 vol% SiC ultra high temperature ceramic composite was studied at 1700–2200 °C and 20–50 MPa using the novel method of electromagnetic Lorentz force loading of electrically heated specimens. Experiments were conducted in air and in non-oxidizing atmospheres. The apparent activation energy for creep was 344 ± 35 kJ/mol for non-oxidizing conditions. The stress exponent was 1.4 ± 0.4. The creep rate was slightly higher in air due to a decrease in the size of the load bearing substrate because of oxidation. There was no evidence of electric field effects. Creep experiments could be performed up to 2200 °C very quickly, with experiments conducted in a few minutes.

High-temperature mechanical behavior of Al-Cu matrix composites containing diboride particles

Abstract An aluminum-copper matrix composite reinforced with aluminum diboride particles was studied at high temperature via thermomechanometry experiments. The matrix contained 2 wt% Cu, whereas the amount of boron forming AlB2 ranged from 0 to 4 wt%, i.e., 0 to 8.31 vol% of diboride particles. In the first segment of the research, we demonstrated that larger amounts of AlB2 particles raised the composite hardness even at 300°C. To assess the material creep behavior, another set of specimens were tested under 1 N compression at 400°C and 500°C for 12 h. Higher levels of AlB2 allowed the composites to withstand compression creep deformations at those temperatures. By using existing creep models developed for metal matrix composites we were able to determine that viscous slip deformation was the dominant deformation mechanism for the temperatures and stress levels used in our experiments. Additionally, the computed creep activation energy for these aluminum matrix composites were found comparable to the energies reported for other similar materials, for instance, Al/SiCp composites.

Investigation on the impression creep properties of a cast Mg–6Al–1Zn magnesium alloy

In the present study, the creep behavior of a cast Mg–6Al–1Zn (AZ61) magnesium alloy was investigated using impression creep technique at the temperature range of 423–495K under shear modulus normalized stress (σimp/G) between 0.02 and 0.0425. Based on the obtained results in the applied stress range, the creep behavior of the alloy was divided into two low and high stress regimes. Stress exponent varied in the range of 4–6 and 11–12 at the low and high stress regimes, respectively. Also, creep activation energy (Q) varied in the range of 113–199kJ/mol and 170–188kJ/mol at the low and high stress regimes, respectively. The activation energy values obtained at the low stress regime were between activation energy for magnesium lattice self diffusion (135kJ/mol) and dislocation pipe diffusion (92kJ/mol). Considering the obtained stress exponents and creep activation energies, it can be stated that the climb controlled dislocation creep and power law breakdown are the dominant creep mechanisms at the low and high stress regimes, respectively.

Enhanced superconducting performance of melt quenched Bi2Sr2CaCu2O8 (Bi-2212) superconductor

We scrutinize the enhanced superconducting performance of melt quench Bismuth based Bi2Sr2CaCu2O8 (Bi-2212) superconductor. The superconducting properties of melt quenched Bi-2212 (Bi2212-MQ) sample are compared with non-melted Bi2212-NM and Bi1.4Pb0.6Sr2Ca2Cu3O10 (Bi-2223). Crystal structure and morphology of the samples are studied using X-ray diffraction and Scanning Electron Microscopy (SEM) techniques. The high field (14 T) magneto-transport and DC/AC magnetic susceptibility techniques are extensively used to study the superconducting properties of the investigated samples. The superconducting critical temperature (Tc) and upper critical field (Hc2) as well as thermally activated flux flow (TAFF) activation energy are estimated from the magneto-resistive [R(T)H] measurements. Both DC magnetization and amplitude dependent AC susceptibility measurements are used to determine the field and temperature dependence of critical current density (Jc) for studied samples. On the other hand, the frequency dependent AC susceptibility is used for estimating flux creep activation energy. It is found that melt quenching significantly enhances the superconducting properties of granular Bi-2212 superconductor. The results are interpreted in terms of better alignment and inter-connectivity of the grains along with reduction of grain boundaries for Bi2212-MQ sample.

Microstructure and Creep Property of DZ125 Nickel-Based Superalloy

By means of creep property measurement and microstructure observation, an investigation has been made into the creep behaviors of DZ125 superalloy at high temperature and low stress. Results showed that the superalloy under the applied stress of 137MPa at 1293 K displayed a better creep resistance, and the apparent creep activation energy of the alloy during steady state creep was measured to be Q = 325.57 kJ/mol. The various microstructures were displayed in different regions of the sample, thereinto, the rafted γ phase was uniformly distributed in the regions far away from the fracture, but the twisted and coarser rafted γ phase appeared in the region near the fracture. The deformation mechanism of the alloy during steady state creep was the dislocations climbing over the rafted γ phase. In the later stage of creep, significant amount of dislocations shearing into the rafted γ-phase promoted the initiation and propagation of the cracks along the boundaries up to the occurrence of fracture, which was though to the fracture mechanism of the alloy during creep.

Creep behavior of Mg–9Gd–1Y–0.5Zr (wt.%) alloy piston by squeeze casting

Abstract The Mg–9Gd–1Y–0.5Zr (wt.%) alloy piston was produced by squeeze casting and further heat treated to peak-aged state. The tensile-creep behavior of the specimens taken from the piston top was investigated at temperatures of 200–300 °C (0.54–0.66Tm) and applied stresses of 50–120 MPa. The creep resistance decreases with increasing temperature and applied stress. When tested at 200 °C and 50 MPa, the specimen has a steady-state creep rate of 0.4 × 10− 9 s− 1 and creep strain of 0.014% after 100 h. The creep stress exponent and the activation energy are 1.66–4.47 and 115.9–174.2 kJ/mol, respectively, implying that dislocation creep is the rate-controlling mechanism. At 50–80 MPa the activation energy for creep indicates that dislocation climb is the creep mechanism. At 120 MPa the relatively high activation energy may be associated with the operation of cross slip. After test, the thermal stable β phase appears and its volume fraction increases with testing temperature. The steady-state creep rate and rupture time are modeled by the original and modified Monkman–Grant relationships. The microcracks and cavities preferentially nucleate at the grain boundaries, especially at triple points. Brittle rupture with few plastic tearing ridges is the dominant character of the creep fracture.

Creep Properties and Deformation Mechanisms of a FGH95 Ni-based Superalloy

By means of full heat treatment, microstructure observation, lattice parameters determination, and the measurement of creep curves, an investigation has been conducted into the microstructure and creep mechanisms of FGH95 Ni-based superalloy. Results show that after the alloy is hot isostatically pressed, coarse γ′ phase discontinuously distributes along the previous particle boundaries. After solution treatment at high temperature and aging, the grain size has no obvious change, and the amount of coarse γ′ phase decreases, and a high volume fraction of fine γ′ phase dispersedly precipitates in the γ matrix. Moreover, the granular carbides are found to be precipitated along grain boundaries, which can hinder the grain boundaries’ sliding and enhance the creep resistance of the alloy. By x-ray diffraction analysis, it is indicated that the lattice misfit between the γ and γ′ phases decreases in the alloy after full heat treatment. In the ranges of experimental temperatures and applied stresses, the creep activation energy of the alloy is measured to be 630.4 kJ/mol. During creep, the deformation mechanisms of the alloy are that dislocations slip in the γ matrix or shear into the γ′ phase. Thereinto, the creep dislocations move over the γ′ phase by the Orowan mechanism, and the \( \left\langle { 1 10 } \right\rangle \) super-dislocation shearing into the γ′ phase can be decomposed to form the configuration of (1/3) \( \left\langle { 1 12 } \right\rangle \) super-Shockleys’ partials and the stacking fault.

Effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy

This paper examines the effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy. The processed material with an ultrafine-grained (UFG) structure (d ≈ 150 nm) was prepared by multiaxial forging. Uniaxial constant stress compression and constant load tensile creep tests were performed at 648–698 K and at stresses ranging between 300 and 600 MPa on the UFG processed alloy and, for comparison purposes, on its coarse-grained (CG) state. The values of the stress exponents of the minimum creep rate n and creep activation energy Q c were determined. Creep behaviour was also investigated by nanoindentation method at room temperature under constant load. The microstructure was examined by transmission electron microscopy and scanning electron microscope equipped with an electron back scatter diffraction unit. The results of the uniaxial creep tests showed that the minimum creep rates of the UFG specimens are significantly higher in comparison with those of the CG state. However, the differences in the minimum creep rates of both states of alloy strongly decrease with increasing values of applied stress. The CG alloy exhibits better creep resistance than the UFG one over the stress range used; the minimum creep rate for the UFG alloy is about one to two orders of magnitude higher than that of the CG alloy. The indentation creep tests showed that annealing had little effect on the creep behaviour in UFG Ti alloy at room temperature.

Dislocation creep of polycrystalline dolomite

The field of dislocation creep and rheological parameters for coarse-grained (d=240μm) natural dolomite has been determined through experiments performed at temperatures of 700–1000°C, effective pressures of 300–900MPa and strain rates of 10−4/s to 10−7/s. At low strain (<7%), dolomite aggregates deform homogeneously and define a power law between strain rate and differential stress with a stress exponent of 3.0+/−0.1, but at higher strains, through-going, fine-grained (<10μm) shear zones develop in the dolomite aggregates concomitant with strain weakening. Recrystallization is limited at low strain and microstructures observed in the low strain samples include undulatory extinction, twins, grain boundary bulging, limited recrystallization along twins and fluid inclusion trails. These same microstructures are present outside of the narrow, through-going shear zones in high strain samples; however, within the shear zones the grain size is small (<10μm) with some larger porphyroclasts (20–50μm). Shear zones nucleate at fine-grained zones formed at twin boundaries, twin–twin intersections and fluid inclusion trails and is likely due to a switch in deformation mechanism due to the large strength contrast between the fine-grained zones deforming by diffusion creep and the coarse-grained protolith. The activation energy (Q) for creep of coarse-grained dolomite at low strain is 145kJ/mol. In contrast to other activation energies for dislocation and diffusion creep of minerals, Q for dislocation creep of dolomite is considerably less than that for diffusion creep (248kJ/mol). The results of this study indicate that coarse-grained dolomite will initially deform by dislocation creep at natural strain rates and temperatures between 200 and 550°C, but due to limited recovery mechanisms, fine-grained shear zones will nucleate and diffusion creep may control the rheology of these fine-grained shear zones in nature at temperatures above ~300°C.

Effects of Sn additions on the microstructure and impression creep behavior of AZ91 magnesium alloy

The microstructural evolution and creep behavior of a cast AZ91 alloy with 0.5, 1, and 2wt% Sn additions were examined by impression tests in the temperature range 423–523K under constant punching stresses in the range 150–650MPa. The results showed that, for all loads and temperatures, the AZ91–2Sn alloy had the lowest creep rates and, thus, the highest creep resistance among all materials tested. This is attributed to the formation of Mg2Sn, reduction in the volume fraction of the eutectic β-Mg17Al12 phase, and solid solution hardening effects of Al and Sn in the Mg matrix. The respective stress exponents and activation energies of 5.0–6.9 and 110–150kJ/mol were similar for all alloys studied. The decreasing trend of creep activation energy with stress suggests that two parallel mechanisms of lattice and pipe diffusion-controlled dislocation climb are competing. The former is the controlling mechanism at low stresses, while the latter prevails at high stresses.

High‐Temperature Creep Behavior of Dense SiOC‐Based Ceramic Nanocomposites: Microstructural and Phase Composition Effects

In this work, dense monolithic polymer‐derived ceramic nanocomposites (SiOC, SiZrOC, and SiHfOC) were synthesized via hot‐pressing techniques and were evaluated with respect to their compression creep behavior at temperatures beyond 1000°C. The creep rates, stress exponents as well as activation energies were determined. The high‐temperature creep in all materials has been shown to rely on viscous flow. In the quaternary materials (i.e., SiZrOC and SiHfOC), higher creep rates and activation energies were determined as compared to those of monolithic SiOC. The increase in the creep rates upon modification of SiOC with Zr/Hf relies on the significant decrease in the volume fraction of segregated carbon; whereas the increase of the activation energies corresponds to an increase of the size of the silica nanodomains upon Zr/Hf modification. Within this context, a model is proposed, which correlates the phase composition as well as network architecture of the investigated samples with their creep behavior and agrees well with the experimentally determined data.

Diffusional creep in Cu–Fe solid solutions

Abstract The activation energy of diffusional creep in copper–iron solid solutions was studied. Experiments were performed on foil samples in an atmosphere of dry hydrogen in a temperature range from about 1170–1340 K. The foils had a thickness of 1.8 × 10 −5 m and a length of about 0.15 m. The stress range was from 0 to 300 kPa. The method of the measurements was developed in our previous work and it was based on measuring the time dependence of stress on a sample. The viscosity of the solid solutions was higher than that for pure copper. It was shown that there is a gap on temperature dependences of viscosity. The temperature of the gap was about 1295 K. Our previous experiments on the Cu–Co system showed the same feature in the activation energies. As previously shown, two processes are required for describing the diffusional creep. They are the volume diffusion and generation/annihilation of the vacancies. The processes occur in sequence. We assume that only second type processes could lead to the appearance of the gap on the temperature dependence of viscosity in solid solutions.