Higher Creep Rates Research Articles

Superplastic creep of AuSn eutectic solder alloy

The large-strain tensile creep behavior of Au80Sn20 wt.% eutectic was studied at temperatures between 125°C and 200°C. Constant load creep experiments were performed and a power law relationship was created from this data to predict creep as a function of stress and temperature with a best-fit stress exponent n=2.1 and activation energy for creep of Q=148.4kJ/mol. Results show that the favorable creep resistance of AuSn reduces considerably at temperatures above 125°C, having 1000× higher creep rate at 200°C than 125°C. Of particular note is the observation of superplasticity in AuSn, with failure strains reaching 740% at 200°C.

Creep behavior of nanocrystalline Au films as a function of temperature

Freestanding nanocrystalline Au films, subjected to nominally elastic loads at 25–110 °C, demonstrated high primary (10−7–10−4 s−1) and steady-state creep rates (10−8–10−5 s−1). The deformation mechanisms for creep were strongly temperature dependent: grain boundary sliding-based creep dominated at room temperature and 50 °C, while the contribution of dislocation-mediated creep increased at 80 and 110 °C. The effect of applied stress on primary and steady-state creep strain at different temperatures was captured well by a non-linear model that was based on the kinetics of thermal activation. Multi-cycle creep experiments showed that at room temperature virtually all the primary strain accumulated during each forward creep cycle was recovered upon complete unloading. As the contribution of dislocation-mediated creep increased with temperature, the ratio of strain recovery to primary strain accumulated during each cycle was reduced due to the accumulation of plastic strain at higher temperatures. Notably, at all temperatures, the steady-state creep rate decreased after the first creep cycle. Moreover, the entire creep response remained virtually unchanged in all subsequent cycles, which implies that the first creep cycle resulted in mechanical annealing. This conclusion was further supported by calculations of the activation entropy: A reduction in its magnitude between the first and all subsequent creep cycles at all temperatures pointed out to mechanical annealing of initial material defects during the first loading cycle. The negative values of the calculated activation entropy indicated that entropy changes due to annihilation of defects-dominated entropy changes associated with the generation of new defects. Finally, the activation entropy for steady-state creep was temperature insensitive, but increased with stress, which is consistent with an increase in defect generation at higher stresses.

Creep modelling of 316H stainless steel over a wide range of stress

Investigation of material creep behaviour in the diffusion controlled creep regime is often unfeasible because of the long duration associated with low stress levels. On the other side, extrapolation from higher creep rates usually provides inaccurate results because of the sharp change in the data trend as a result of the change in the governing deformation mechanism from dislocation to diffusion type controlled creep. Similarly, extrapolation based on creep models, which have been formulated and validated mainly for dislocation type creep (such power law creep with the creep exponent ranging from 6 to 9), underestimates the actual creep rate significantly. Recently, Bonora and Esposito (2010) developed a mechanism-based model (BE model) capable to account for deformation and damage mechanism occurring in creep. In this work the BE model was applied to AISI 316H stainless steel for which considerable creep data in both dislocation and diffusion temperature/stress controlled regime were available. Using the same data set, the predictive capabilities of several models were compared.

OMC/A286ろう付異材継手の高温強度評価

Strength evaluation for brazed joint between OMC and A286 at high temperature is needed because the combustion chamber of LE-9 engine for H3 rocket is planned to be manufactured by HIP (Hot Isostatic Pressing) brazing for realizing high reliability and low cost. This paper introduces the internal pressure burst test and creep test results of brazed joint specimen connecting OMC and A286, respectively a copper alloy and a nickel-based alloy composing the combustion chamber. Internal pressure burst test results are evaluated by the strength of stress singularity field in brazed joint. Creep test results are evaluated by the stress at fracture origin after stress redistribution due to high creep rate of brazed material.

The influence of Al elements on the structure and the creep behavior of AlxCoCrFeNi high entropy alloys

The microstructure, crystal structure, creep behavior and mechanism of AlxCoCrFeNi (x is in molar ratio) high entropy alloys were examined. The low Al content of Al0.15CoCrFeNi alloy had single FCC structure with columnar cell microstructure, while the high Al content of Al0.60CoCrFeNi alloy contained FCC+BCC duplex crystal structure with columnar dendrite microstructure. The creep property, evaluated by stress relaxation test, showed that the Al0.15CoCrFeNi alloy had a higher creep resistant property than that of the Al0.60CoCrFeNi alloy. The creep constitutive equation for the Al0.15CoCrFeNi alloy was established with an average stress exponent of 5.56 and an average activation energy of 385kJmol−1, while it was with a larger average stress exponent of 8.82 and a smaller average activation energy of 334kJmol−1 for the Al0.60CoCrFeNi alloy. The analysis of apparent activation volume reveals that both alloys were in a rate limiting mechanism of cross-slip. Then, the higher stacking fault energy of the Al0.60CoCrFeNi alloy was considered to result in its higher creep rate. Therefore, it was concluded that the increased Al element tended to reduce the creep resistant property of the AlxCoCrFeNi alloys by increasing the stacking fault energy.

Critical current density, vortex dynamics, and phase diagram of single-crystal FeSe

We present a comprehensive study of the vortex pinning and dynamics in a high-quality FeSe single crystal, which is free from doping introduced inhomogeneities and charged quasi-particle-scattering because of its innate superconductivity. Critical current density, $J_c$, is found to be almost isotropic, and reaches a value $\sim$ 3 $\times$ 10$^4$ A/cm$^2$ at 2 K (self-field) for both $H$ $\|$ $c$ and $ab$. The normalized magnetic relaxation rate $S$ (= $\mid$dln$M$/dln$t$$\mid$) shows a temperature insensitive plateau behavior in the intermediate temperature range with a relatively high creep rate ($S$ $\sim$ 0.02 under zero field), which is interpreted in the framework of the collective creep theory. A crossover from the elastic to plastic creep is observed, while the fish-tail effect is absent for both $H$ $\|$ $c$ and $ab$. Based on this observation, the origin of the fish-tail effect is also discussed. Combining the results of $J_c$ and $S$, vortex motion in FeSe single crystal is found to be dominated by sparse strong point-like pinning from nm-sized defects or imperfections. The weak collective pinning is also observed and proved in the form of large bundles. Besides, the vortex phase diagram of FeSe is also constructed and discussed.

Impression creep behavior of Zn–4Al–3Mg–xSn high-temperature lead-free solders

The microstructure and impression creep behavior of the high-temperature Zn–4Al–3Mg–xSn (x=0, 7, and 13wt.%) alloys were studied under constant punch stress in the range of 50–800MPa and at temperatures in the range of 345–473K. The results showed that for all loads and temperatures, addition of Sn to the base alloy results in higher creep rates, and thus lower creep resistances. The inferior creep behavior of the quaternary alloys is ascribed to the presence of the soft Sn that reduces the volume fraction of the fine α–η eutectic and eutectoid structures, and the hard MgZn2 and Mg2Zn11 second phase particles. Despite the deteriorating effects of Sn, the creep resistance of the quaternary alloys was still higher than those of the Zn–Sn and Pb–Sn high-temperature solders. The stress exponents of 4.0–7.9 and activation energy values of 51.6–92.6kJmol−1, are indicative of a dislocation climb controlled creep mechanism. Dislocation climb controlled by pipe diffusion is the rate controlling mechanism at low temperatures, whereas dislocation climb controlled by lattice diffusion is the dominant creep mechanism at high temperatures.

Effects of loading strain rate and stacking fault energy on nanoindentation creep behaviors of nanocrystalline Cu, Ni-20 wt.%Fe and Ni

Creep behaviors of nanocrystalline (NC) Cu, Ni-20(wt.%)Fe and Ni with comparable grain sizes were examined by nanoindentation tests. It was showed that the creep behaviors depend on loading strain rate and stacking fault energy (SFE). At high loading strain rate, the initial creep rates of three NC materials are far higher than those predicted by the models of the Coble creep and the grain boundary sliding and are the highest and lowest for NC Cu and NC Ni, respectively, while that of NC Ni–20Fe lies between them due to their different SFE. At low loading strain rate, the creep behaviors do not exhibit significant difference among three NC materials. Our analysis revealed that grain boundary dislocation sources can be activated at high loading strain rate and the emitted dislocations from grain boundaries can be effectively stored in the loading regime, but cannot at low loading strain rate. The very high initial creep rates of three NC materials are attributed to the rapid absorptions of the stored dislocations in the loading regime and the newly nucleated dislocations in the holding regime. The higher creep rate of NC Cu as compared with those of NC Ni–20Fe and NC Ni arises from the higher density of the stored dislocations caused by the enhanced interactions of dislocations with twins and stacking faults due to its lower SFE.

Program and Results of Reactor Tests of Mixed Nitride Fuel for Fast Reactors

In the Proryv project, mixed uranium-plutonium nitride fuel is to be used in the future nuclear power with fast reactors with sodium coolant (BN-1200) and with lead coolant (BREST). At present, experimental fuel elements and fuel assemblies with mixed nitride fuel are fabricated by means of a technology developed at the Bochvar All-Russia Research Institute for Inorganic Materials for testing in the BN-600 reactor and the MIR and BOR-60 research reactors. The program of reactor tests of the experimental fuel assemblies and some preliminary results of these tests are presented. World experience in research on mixed uranium-plutonium nitride fuel is limited to ~200 fuel elements, irradiated to maximum burnup ~18% h.a. at maximum linear power density 410–1300 W/cm [1, 2]. In our country, we have the results of the irradiation of ~1300 fuel elements with uranium nitride fuel in the BR-10 reactor [3]. Even though many properties of uranium nitride and mixed nitride fuel are close, the results of the research cannot fully serve as validation for the BREST and BN-600 fuel elements, for which the operating parameters determining their serviceability differ considerably from those of fuel elements with uranium nitride fuel. The results of the irradiation of four fuel elements with mixed nitride fuel to maximum burnup 12.1% h.a. in the BOR-60 reactor as part of a Russian–French experiment must also be taken into account [4]. Low swelling and gas-release and the corrosion damage to the cladding in this experiment were obtained for high-purity mixed nitride fuel fabricated from the initial metals with oxygen content <0.15 wt.% and carbon content <0.1 wt.% with pellet density 85%. The mixed nitride used in the experiment of [4] was characterized by elevated plutonium content 45 and 60 %, which could have promoted high creep rates and, correspondingly, lower stresses in the fuel-element cladding. For this reason, it is necessary to have information on its in-reactor behavior with the lower plutonium content provided in the fuel elements of the BREST and BN-1200 reactors. Analysis showed that it is necessary to obtain additional data on the pre-reactor and reactor properties of mixed uranium-plutonium nitride fuel. This requires reactor tests of BN-1200 and BREST type fuel elements in the BOR-60 and other research reactors in order to achieve statistically representative validation of the serviceability of mixed nitride fuel as part of full-scale experimental BN-600 fuel assemblies. A complex program of computational and experimental validation of using mixed uranium-plutonium nitride fuel in the BN-1200 and BREST-OD-300 reactors was developed as part of the Proryv project [5]. This program provides for pre-reactor investigations of the properties of fuel fabricated by the carbothermal synthesis technology and reactor tests of fuel elements in the BN-600 reactor and the MIR and BOR-60 research reactors [6]. The serviceability of fuel elements with

Micromechanical Simulation of Thermal Cyclic Behavior of ZrO2/Ti Functionally Graded Thermal Barrier Coatings

This study numerically investigates cyclic thermal shock behavior of ZrO2/Ti functionally graded thermal barrier coatings (FG TBCs) based on a nonlinear mean-field micromechanical approach, which takes into account the time-independent and dependent inelastic deformation, such as plasticity of metals, creep of metals and ceramics, and diffusional mass flow at the ceramic/metal interface. The fabrication processes for the FG TBCs have been also considered in the simulation. The effect of creep and compositional gradation patterns on micro-stress states in the FG TBCs during thermal cycling has been examined in terms of the amplitudes, ratios, maximum and mean values of thermal stresses. The compositional gradation patterns highly affect thermal stress states in case of high creep rates of ZrO2. In comparison with experimental data, maximum thermal stresses, amplitudes and ratios of thermal stresses can be effective parameters for design of such FG TBCs subject to cyclic thermal shock loadings.

Material Parameters for Creep Rupture of Austenitic Stainless Steel Foils

Creep rupture properties of austenitic stainless steel foil, 347SS, used in compact recuperators have been evaluated at 700 °C in the stress range of 54-221 MPa to establish the baseline behavior for its extended use. Creep curves of the foil show that the primary creep stage is brief and creep life is dominated by tertiary creep deformation with rupture lives in the range of 10-2000 h. Results are compared with properties of bulk specimens tested at 98 and 162 MPa. Thin foil 347SS specimens were found to have higher creep rates and higher rupture ductility than their bulk specimen counterparts. Power law relationship was obtained between the minimum creep rate and the applied stress with stress exponent value, n = 5.7. The value of the stress exponent is indicative of the rate-controlling deformation mechanism associated with dislocation creep. Nucleation of voids mainly occurred at second-phase particles (chromium-rich M23C6 carbides) that are present in the metal matrix by decohesion of the particle-matrix interface. The improvement in strength is attributed to the precipitation of fine niobium carbides in the matrix that act as obstacles to the movement of dislocations.

Magnetic relaxation and collective vortex creep in FeTe0.6Se0.4 single crystal

We study the vortex dynamics in a high-quality FeTe0.6Se0.4 single crystal by performing magnetization measurements of the screening current density Js and flux creep rate S. The temperature dependence of S shows a plateau in the intermediate-temperature region with a high creep rate ∼ 0.03, which is interpreted in the framework of the collective creep theory. A crossover from elastic to plastic creep is observed. The glassy exponent and barrier height for the flux creep are directly determined by the extended Maley's method. Js with flux creep, obtained from magnetic hysteresis loops, is successfully reproduced based on the collective creep analysis. We also approach the critical current density without flux creep by means of the generalized inversion scheme, which proves that the and pinning coexist in the FeTe0.6Se0.4 single crystal.

Small effect of water on upper-mantle rheology based on silicon self-diffusion coefficients

Water has been thought to affect the dynamical processes in the Earth's interior to a great extent. In particular, experimental deformation results suggest that even only a few tens of parts per million of water by weight enhances the creep rates in olivine by orders of magnitude. However, those deformation studies have limitations, such as considering only a limited range of water concentrations and very high stresses, which might affect the results. Rock deformation can also be understood as an effect of silicon self-diffusion, because the creep rates of minerals at temperatures as high as those in the Earth's interior are limited by self-diffusion of the slowest species. Here we experimentally determine the silicon self-diffusion coefficient DSi in forsterite at 8 GPa and 1,600 K to 1,800 K as a function of water content CH2O from less than 1 to about 800 parts per million of water by weight, yielding the relationship, DSi ≈ (CH2O)(1/3). This exponent is strikingly lower than that obtained by deformation experiments (1.2; ref. 7). The high nominal creep rates in the deformation studies under wet conditions may be caused by excess grain boundary water. We conclude that the effect of water on upper-mantle rheology is very small. Hence, the smooth motion of the Earth's tectonic plates cannot be caused by mineral hydration in the asthenosphere. Also, water cannot cause the viscosity minimum zone in the upper mantle. And finally, the dominant mechanism responsible for hotspot immobility cannot be water content differences between their source and surrounding regions.

A thermally activated model for room temperature creep in nanocrystalline Au films at intermediate stresses

Uniaxial tension creep experiments conducted on free-standing nanocrystalline Au films with 40 nm grain size showed high initial primary creep rates of 3.3 × 10−8–2.7 × 10−7 s−1, reaching steady-state creep rates of 5.5 × 10−9–1.1 × 10−8 s−1 after 5–6 h. The isochronous creep curves pointed to the need for a nonlinear creep model based on the kinetics of thermal activation, which captured well the effect of applied stress amplitude. The implications of this model on the long term stability of nanocrystalline films were explored numerically for periodic stress profiles.

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.

Microstructure and creep characteristics of Zn–3Cu–xAl ultra high-temperature lead-free solders

This study investigates the microstructure and impression creep behavior of the ultra high-temperature Zn–3Cu–4Al (ZCA34), Zn–3Cu–5Al (ZCA35), and Zn–3Cu–6Al (ZCA36) solder alloys under constant punch stress in the range of 70–800MPa and at temperatures in the range of 345–495K. The results showed that for all loads and temperatures, increasing Al content of the alloys results in higher creep rates, and thus lower creep resistances. This is attributed to the partial spheroidisation of the lamellar eutectic structure, and the four phase transformation α+ε→T′+η. The former may ease grain boundary sliding at high temperatures and the latter results in a lesser amount of the more creep resistant ε-phase. The stress exponents of 5.0–7.9 and activation energy values of 52.5–100.3kJmol−1, are indicative of a dislocation climb controlled creep mechanism. The observed increasing trend of creep activation energy with temperature means that two parallel mechanisms of lattice-diffusion-controlled and pipe-diffusion-controlled dislocation climb are competing. Dislocation climb controlled by pipe diffusion is the rate controlling mechanism at low temperatures, whereas dislocation climb controlled by lattice diffusion is the dominant creep mechanism at high temperatures.

A Revised Architecture of Primary Cell Walls Based on Biomechanical Changes Induced by Substrate-Specific Endoglucanases

Xyloglucan is widely believed to function as a tether between cellulose microfibrils in the primary cell wall, limiting cell enlargement by restricting the ability of microfibrils to separate laterally. To test the biomechanical predictions of this "tethered network" model, we assessed the ability of cucumber (Cucumis sativus) hypocotyl walls to undergo creep (long-term, irreversible extension) in response to three family-12 endo-β-1,4-glucanases that can specifically hydrolyze xyloglucan, cellulose, or both. Xyloglucan-specific endoglucanase (XEG from Aspergillus aculeatus) failed to induce cell wall creep, whereas an endoglucanase that hydrolyzes both xyloglucan and cellulose (Cel12A from Hypocrea jecorina) induced a high creep rate. A cellulose-specific endoglucanase (CEG from Aspergillus niger) did not cause cell wall creep, either by itself or in combination with XEG. Tests with additional enzymes, including a family-5 endoglucanase, confirmed the conclusion that to cause creep, endoglucanases must cut both xyloglucan and cellulose. Similar results were obtained with measurements of elastic and plastic compliance. Both XEG and Cel12A hydrolyzed xyloglucan in intact walls, but Cel12A could hydrolyze a minor xyloglucan compartment recalcitrant to XEG digestion. Xyloglucan involvement in these enzyme responses was confirmed by experiments with Arabidopsis (Arabidopsis thaliana) hypocotyls, where Cel12A induced creep in wild-type but not in xyloglucan-deficient (xxt1/xxt2) walls. Our results are incompatible with the common depiction of xyloglucan as a load-bearing tether spanning the 20- to 40-nm spacing between cellulose microfibrils, but they do implicate a minor xyloglucan component in wall mechanics. The structurally important xyloglucan may be located in limited regions of tight contact between microfibrils.

Comparative Study of Creep and Stress Relaxation Behavior for 7055 Aluminum Alloy

In order to study the similarities and dissimilarities between creep and stress relaxation behavior of age formed aluminum alloys, both creep ageing and stress relaxation ageing experiments have been conducted with plate shaped 7055 aluminum alloy specimens on the 100 KN tensile testing machine performed at 120 °C for 20 h, under different stress levels from 190.0 to 357.8 MPa. The experimental results show that similar variation trends for creep and stress relaxation behavior were observed. Both creep and stress relaxation curves can be divided into two stages. During the first stage, higher creep rate and stress relaxation rate occur, which increase with stress levels but decrease with ageing time. While during the second stage, both the creep rate and the stress relaxation rate reach its lowest value and keep constant. A set of unified creep ageing constitutive equations has been developed and calibrated from creep experimental data, which can be used to predict the creep strain under age forming conditions perfectly. But the experimental results from stress relaxation ageing tests cannot be predicted with the established creep ageing constitutive equations, which shows that there is not a one-to-one correspondence between creep and stress relaxation, creep deformation is the most important but not the only reason for stress relaxation under age forming condition.

Study of geometric stability and structural integrity of self-healing glass seal system used in solid oxide fuel cells

A self-healing glass seal has the potential to restore its mechanical properties upon being reheated to the solid oxide fuel cell (SOFC) stack operating temperature. Such a self-healing feature is desirable for achieving high seal reliability during thermal cycling. Self-healing glass is also characterized by its low mechanical stiffness and high creep rate at SOFC operating temperatures. Therefore, the geometric stability and structural integrity of the glass seal system are critical to its successful application in SOFCs. This paper describes studies of the geometric stability and structural integrity of the self-healing glass seal system and the influence of various interfacial conditions during the operating and cooling-down processes using finite element analyses. For this purpose, the test cell used in the leakage tests for compliant glass seals, conducted at Pacific Northwest National Laboratory (PNNL), was taken as the initial modeling geometry. The effect of the ceramic stopper on the geometric stability of the self-healing glass sealants was studied first. Two interfacial conditions of the ceramic stopper and glass seals, i.e., bonded (strong) or unbonded (weak), were considered. Then the influences of interfacial strengths at various interfaces, i.e., stopper/glass, stopper/PEN, as well as stopper/IC plate, on the geometric stability and reliability of glass during the operating and cooling processes were examined.

Temperature Aging, Compression Recovery, Creep, and Weathering of a Foam Silicone Sealant for Bridge Expansion Joints

Silicone foam was investigated as a sealant for small movement expansion joints in bridge decks. This paper presents results from the laboratory assessment of a model foam sealant subjected to thermal aging (exposure to high and low temperatures and temperature cycling), fatigue conditions, and outdoor weathering. Parallel tests were performed on a commercial solid-silicone bridge-joint sealant. Test results showed that the solid sealant recovered faster than the foam sealant after being subjected to prolonged compression at elevated temperature. When subjected to a constant tensile force, both the foam and solid sealants exhibited high initial creep rates (rate of elongation), but appeared to reach an equilibrium level of elongation at longer times. The foam sealant creeps at a slower rate and takes more time to get to the equilibrium elongation, whereas the solid sealant creeps much faster initially and reaches equilibrium faster. Thermal aging was found to have significant effects on the sealant modulus (increases with temperature); however, the effects on ultimate stress and strain were not apparent for both types of sealants. Temperature cycling between 24 and -29°C was observed to diminish the ultimate stress and strain of both the sealants by roughly 25%; however, no significant changes in modulus were found. Results from the tests on a limited number of outdoor-weathered sealant specimens showed that weathering appeared to produce an increase in sealant tensile and shear moduli (i.e., hardening effects because of weathering) and a decrease in ultimate strain. The weathered samples show tension loading-unloading behavior similar to the unaged samples. The tensile stress relaxation rate of the outdoor-weathered sealants could not be distinguished from those laboratory-cured (unweathered) counterparts.