Activation Energy For Creep Research Articles

Microstructural evolution and impression creep properties of a lead-based alloy PbSn16Sb16Cu2

Abstract The impression creep behavior of a lead-based PbSn16Sb16Cu2 alloy was studied at stresses in the range from 15 to 30 MPa and temperatures in the range from 333 to 393 K. XRD, SEM, and EDS techniques were used to analyze microstructural evolutions of the alloy before and after creep at different impression creep conditions. Results show that, in the range of experimental conditions, the calculated stress exponent and the creep activation energy of the alloy are 4.12 and 60.56 kJ mol−1, respectively. Grain boundary diffusion-dominated dislocation climbing is the main impression creep mechanism of PbSn16Sb16Cu2 alloy. Creep rate increases and creep resistance decreases with the increase of temperature and stress, respectively. Two reasons dominate the creep process: first, Sn is largely precipitated from the solid solution in the matrix, which weakens the overall strength of the matrix during the creep process; second, as temperature and stress increase, the atoms are vibrated more fiercely by thermal energy, which results in a softening of the matrix and SnSb phase.

Creep properties and microstructure evolution at 260–300 °C of AlSi10Mg manufactured via laser powder-bed fusion

The aging behavior and creep resistance of eutectic AlSi10Mg (Al–10Si-0.6Mg, wt.%) manufactured by laser powder bed fusion (L-PBF) are studied at 260 and 300 °C. The Si phase, which forms a fine interconnected network of 100–200 nm filaments in the as-printed alloy, coarsens into blocky, 2–3 μm particles after 1 month exposure at 260 and 300 °C, with hardness decreasing following a power-law with exponent n = 0.06–0.08. AlSi10Mg with ∼1 μm Si particles (achieved by aging at 300 °C for 96 h) exhibits creep resistance at 260 and 300 °C, for test durations of a few days, comparable to those of (i) cast alloys: hypereutectic Al–Si alloys with coarse Si particles and eutectic Al–Ce, Al–Ce–Ni with much finer, and more coarsening-resistant eutectics phases (e.g., Al11Ce3, Al3Ni); and (ii) L-PBF Al–Mg–Zr alloys. At 260 and 300 °C, our L-PBF AlSi10Mg shows a power-law creep behavior with high apparent stress exponents (na = 10–13, higher than n = 4.0 for Al–Mg) for strain rates between ∼10−8 and ∼10−4 s−1. Also, a high apparent creep activation energy Qa = 256 kJ/mol is measured between 200 and 320 °C at 45 MPa, compared to Q = 142 kJ/mol for Al. These high apparent stress exponent and activation energy are consistent with power-law dislocation creep with a threshold stress, originating from load-transfer from the creeping Al(Mg) matrix to non-creeping Si particles.

Sodium mechanics: effects of temperature, strain rate, and grain rotation and implications for sodium metal batteries

Sodium-metal batteries are an attractive energy storage technology, owing to the low cost and relative earth abundance of sodium. However, despite the importance of electro-chemo-mechanical phenomena in alkali-metal batteries, experiments on the mechanical properties of metallic Na are relatively scarce, especially in terms of its viscoplastic response. In this work, we measured the temperature- and rate-dependent deformation of sodium by performing tensile tests in inert gas environments. Digital image correlation measurements were performed to track the local strain and rotation fields. Temperature-dependent experiments were used to experimentally calibrate the activation energy for creep. Furthermore, local deformation measurements of sodium suggest that grain-size and crystallographic orientation effects may be important considerations for sodium-based batteries. Finally, the implications of creep on the performance of Na-metal batteries discussed, providing guidance for future modeling efforts.

The creep performance of a sand-cast Mg–2.8 Nd–0.8 Zn–0.5 Zr–0.3 Gd alloy at 241 to 262°C

AbstractSecondary creep rates of sand-cast Mg–2.8 Nd–0.8 Zn–0.5 Zr –0.3 Gd (wt.%) alloy at 242.5 ± 1.5 °C and 40 to 90 MPa applied tensile stress are shown to be a factor of ~ 24 smaller than reported earlier for chill-cast Mg–2.4 Nd–0.4 Zn–0.3Mn at 240 °C. The relative contributions of grain/cell size and of alloying in achieving this improved creep resistance are discussed. Additionally, preliminary results at 248 and 262 °C indicate an activation energy for creep somewhat larger than that for the earlier alloy at 150 to 240 °C and that for lattice self-diffusion in magnesium.

High-temperature compressive creep tests of U3Si2 with spark plasma sintering: Experiments and finite element modeling

This paper reports a systematic high-temperature creep behavior of dense U3Si2 pellets using a spark plasma sintering (SPS) apparatus at elevated temperatures and pressures under vacuum conditions. The stress exponent was subsequently derived to be 3.21 at 1173 K and 2.17 at 1223 K, respectively, indicating a grain boundary sliding creep mechanism. The creep activation energy was determined to be 203.6 ± 19.0 kJ/mol, which agrees well with the literature. Finite element modeling was performed using the creep parameters fitted from the strain-time plot. The results suggest an excellent match with the experimental data, confirming the validity of the experiments. Microstructure characterizations indicate that the main phase of the specimens after creep tests remains to be U3Si2, with a 4 μm thick layer of nano-sized particles induced from the diffusion between U3Si2 and alumina disc used to avoid electric current passing through the sample. The successful conduct of creep experiments demonstrates the great potential of SPS to perform high-temperature mechanical testing of nuclear fuels under vacuum conditions. The subsequent finite element modeling exhibits excellent capabilities for accurately predicting material performance in the creep tests and provides a practical tool in evaluating nuclear fuels’ performance for a much-extended time scale.

Further results on creep behaviour of sand-cast Mg–2.8Nd–0.8Zn–0.5Zr–0.3Gd alloy at 0.56 to 0.61T m under stresses 40 to 90 MPa

Abstract An earlier study of the creep behaviour of Elektron 21 alloy has been extended to 290 °C (0.61T m; T m: melting temperature). The combined results confirm a stress exponent of creep rate close to 6, but with an activation energy for creep of (310 ± 20) kJ/mol for 240 to 290 °C and 40 to 90 MPa. Possible mechanisms giving rise to this behaviour are discussed.

Исследование особенностей высокотемпературной деформации керамик из чистого карбида вольфрама с различным размером зерна

The mechanism of high-temperature creep deformation during compression tests of binderless tungsten carbide specimens with different initial particle size was studied. Tungsten carbide samples with high relative density (96.1 – 99.2 %) were obtained by high-speed spark plasma sintering (SPS) from nano-, submicron-, and micron-sized α-WC powders. The creep tests were carried out in two modes: isothermal soaking at different temperatures (1300 – 1375 °C) at a given stress, allowing to estimate the activation energy of creep, and tests by “stress jumps” at 1325 °C, allowing to estimate the value of the coefficient n in the creep equation. It is shown that the value of creep activation energy in ultrafine grained tungsten carbide with grain size ~ 0.15 µm sintered from plasma chemical nanopowders is ~ 31 kTm. This value is 1.5 – 2 times higher than the creep activation energy in fine-grained tungsten carbide samples obtained by SPS from submicron (~ 0.8 μm) and micron (~ 3 μm) industrial powders. It was found that the value of the coefficient n varies from 2.4 to 3.1, which corresponds to the case of motion of lattice dislocations in the field of uniformly located point obstacles. It has been suggested that one of the reasons for the increase in creep activation energy in tests of tungsten carbide is an increased volume fraction of W2C low carbide particles formed during high-speed sintering of plasma-chemical α-WC nanopowders with an increased concentration of adsorbed oxygen.

Deformation structure in Alloy-type creep of Cantor alloy

Compression creep tests were conducted in order to investigate the creep behavior of high entropy alloy Cantor at high temperatures. It was found that the creep behavior of Alloy-type was observed in low stress region and that of Metal-type was observed in high stress region. In addition, the activation energy of creep increased with increasing stress. This suggests that a kind of rate controlling diffusion atoms is changing depending on the stress. Microstructures after creep were observed using the Electron Back Scatter Diffraction method. The generation of dynamic recrystallized grains and formation of sub-boundaries were observed in high stress region. This is known as a typical deformation structure of Metal-type creep. In addition, a lot of low-angle grain boundaries were observed. These characteristic structures are similar to that observed in FCC metals in which stacking fault energy is relatively low.

Tensile and creep behavior of nickel nanowires containing volume defects: Insight into the deformation mechanisms and microstructural evolution using molecular dynamics simulations

In this paper, we report the tensile and creep behavior of nickel nanowire (NW) containing volume defects (voids) using molecular dynamics (MD) simulations. The role of void (diameter = 30 Å) on the tensile deformation behavior and mechanisms is investigated at varying strain rates (108 s−1 – 1010 s−1) and temperatures (300 K–1200 K). Similarly, the creep properties are investigated at constant stresses in the range of 2 GPa–5 GPa, and temperatures in the range of 1000 K–1200 K. Embedded atom method (EAM) potential is used to model the interactions between nickel atoms. The NW model size used is 100 Å (x-axis) × 1000 Å (y-axis) × 100 Å (z-axis) and comprises of ∼920,000 atoms. Tensile results show that the load-bearing capacity, elastic modulus of the NWs have decreased with an increase in the number of voids. The microstructural investigations show that void acts as a source of dislocations and cause plastic strain in the NW. However, at higher temperatures, free surface edges act as the source of dislocations. Creep test results show that the strain rates in the steady-state region increase with the applied stress and temperature. The deformation mechanism is by diffusional creep in the steady-state region, and the stress exponent ‘n’ varies from 0.8 to 2.77. Microstructural investigations show the formation of stacking faults and twins. Based on the activation energy for creep, we observe that multi-void NW is less creep resistant than single void nickel NW.

High temperature creep behavior of a low alloy Mn-Mo-Ni reactor pressure vessel steel

Creep behavior of a low alloy Mn-Mo-Ni reactor pressure vessel steel received in post weld heat treatment condition has been studied in the temperature range of 600–900 °C. The steel exhibited three stage creep behavior. The steady or secondary creep rates for different stress conditions for a particular temperature were determined using stress change tests. Temperature change tests were conducted to determine the steady state creep rates at different temperatures at a particular stress. The creep rates obtained from different tests were analyzed using the Norton's and Bird-Mukherjee-Dorn creep equations to determine the stress exponent (n) and the creep activation energy (Q). The values of n and Q obtained signify dislocation climb as the underlying creep mechanism in steel. Electron back scatter diffraction (EBSD) study indicated formation of fine subgrain structure in the crept sample.

Creep properties of advanced austenitic steel 709 determined through short experiments under in-situ neutron diffraction followed by TEM characterization

In this study, the creep mechanisms at play in Alloy 709 (Fe-20Cr-25Ni) are investigated by performing short-term creep-type tests under in-situ neutron diffraction experiments. Short tests are performed in the temperature range of 500 to 900 °C under constant load with a load ranging from 50 to 150 MPa. The creep exponent and activation energy are determined using the Bird-Mukherjee-Dorn relation and compared to that obtained from conventional longer creep tests from the literature. Scanning transmission electron microscopy (S/TEM) of the post-creep microstructure indicates that interaction of dislocations with precipitates are a dominant mechanism at play. Furthermore, local elemental mapping indicated chemical segregation at grain boundaries and formation of complex precipitates.

MCMC inversion of the transient and steady-state creep flow law parameters of dunite under dry and wet conditions

The rheology of the upper mantle impacts a variety of geodynamic processes, including postseismic deformation following great earthquakes and post-glacial rebound. The deformation of upper mantle rocks is controlled by the rheology of olivine, the most abundant upper mantle mineral. The mechanical properties of olivine at steady state are well constrained. However, the physical mechanism underlying transient creep, an evolutionary, hardening phase converging to steady state asymptotically, is still poorly understood. Here, we constrain a constitutive framework that captures transient creep and steady state creep consistently using the mechanical data from laboratory experiments on natural dunites containing at least 94% olivine under both hydrous and anhydrous conditions. The constitutive framework represents a Burgers assembly with a thermally activated nonlinear stress-versus-strain-rate relationship for the dashpots. Work hardening is obtained by the evolution of a state variable that represents internal stress. We determine the flow law parameters for dunites using a Markov chain Monte Carlo method. We find the activation energy 430pm 20 and 250pm 10 kJ/mol for dry and wet conditions, respectively, and the stress exponent 2.0pm 0.1 for both the dry and wet cases for transient creep, consistently lower than those of steady-state creep, suggesting a separate physical mechanism. For wet dunites in the grain-boundary sliding regime, the grain-size dependence is similar for transient creep and steady-state creep. The lower activation energy of transient creep could be due to a higher jog density of the corresponding soft-slip system. More experimental data are required to estimate the activation volume and water content exponent of transient creep. The constitutive relation used and its associated flow law parameters provide useful constraints for geodynamics applications.Graphical

Ultra-high creep resistant SiC ceramics prepared by rapid hot pressing

The high-temperature compression creep of additive-free β/α silicon carbide ceramics fabricated by rapid hot pressing (RHP) was investigated. The creep tests were accomplished in vacuum at temperature range 1500 °C–1750 °C and compressive loads of 200 MPa to 400 MPa. Under investigated condition the RHP ceramics possessed the lowest creep rate reported in the literature. The observed strain rates changed from 2.5 × 10−9 s−1 at 1500 °C and a lowest load of 275 MPa to 1.05 × 10−7 s−1 at 1750 °C and a highest load of 400 MPa. The average creep activation energy and the stress exponent remain essentially constant along the whole range of investigated parameters and were 315 ± 20 kJ∙mol−1, and 2.22 ± 0.17, respectively. The suggested creep mechanism involves GB sliding accommodated by GB diffusion and β−α SiC phase transformation.

Impression creep behavior of Babbitt alloy SnSb8Cu4

Abstract The creep behavior of the babbitt alloy SnSb8Cu4 was studied by impression under constant stress in the range of 15-30 MPa and at temperatures in the range of 333-393 K. OM, XRD, and SEM technologies were used to investigate the microstructural evolution of the material before and after creep. In conclusion, the stress exponent of SnSb8Cu4 in impression condition is 2.7. The creep activation energy is 46.7 kJ × mol-1, which reveals that the creep mechanism is based on dislocation glide controlled by dislocation pipe diffusion. With an increase in creep temperature and stress, the thermal vibration of the atom and atomic diffusion velocity increase. Moreover, Cu6Sn5 phase cannot effectively hinder dislocation glide and strengthen the material in a condition of high creep temperature and high stress, thus resulting in the decrease of creep resistance. After creep, the grains are elongated along the deformation flow direction both near the pressure punch edge and in the hemispherical zone beneath the pressure punch. The most severe deformation zone is near the pressure punch edge, whereas, no detectable deformation is found in other areas. It is concluded that impression creep is a localized phenomenon.

High temperature impression creep behavior and microstructures of wrought ZM21 magnesium alloy

Mg-Zn alloys are promising candidates for their application in automotive, electronics and aerospace applications. For their successful application, one of the performance parameters that needs to be evaluated is their creep behavior at elevated temperatures. Hence this paper evaluates the high temperature creep behavior of wrought ZM21 magnesium alloy by impression test The tests were performed under constant temperature and stress. A flat ended cylindrical punch was used to create impressions. The temperature was varied between 398 K and 598 K while the stresses were varied from 200 MPa to 500 MPa (normalized stress: 0.014 ≤ σimp/ G ≥ 0.032). A power-law creep deformation was assumed to calculate creep exponent and activation energy using the steady state minimum impression velocity obtained from impression tests. The creep behavior was analyzed with the help of impression creep curves and plastic deformation was analyzed with the help of micrographs. It was found that creep exponent varied between 4.5 and 6 and activation energy between 73.28 and 113.35 kJ/mol were obtained. From the study it was concluded that the creep mechanism involved was pipe-diffusion-controlled dislocation climb.

Mechanical behavior at elevated temperatures of an Al–Mn–Mg–Sc–Zr alloy manufactured by selective laser melting

A high temperature compression and compression creep study was undertaken performed on Al–Mn–Mg–Sc–Zr alloy produced by selective laser melting (SLM). High temperature compression tests in the temperature range from 150 to 300 °C were conducted for the as-fabricated (AF) and aged (HT) SLM specimens. The 0.2% proof stress (σ0.2) showed comparable values at different loading directions, which reveal an isotropic mechanical behavior. σ0.2 of the HT samples exceeded that of the AF at temperatures up to 200 °C. At 250 °C, σ0.2 of AF surpasses that of HT, due to the interaction of solid solution strengthening above 0.5 of the homologous temperature and the formation of new precipitates in this temperature range. For AF and HT σ0.2 becomes similar again at 300 °C. Creep tests were conducted at 250 °C in a stress range of 100–150 MPa. The Norton stress exponent was determined to (5.1 ± 0.2) indicating that creep is dislocation climb controlled. An activation energy for creep of (198 ± 11) kJ/mol was found. It is suggested that solid solution strengthening in conjunction with a dispersed L12 Al3Sc precipitates improved the creep resistance compared to conventionally cast-manufactured Al–Sc–Zr alloys.

Evolution of microstructure during tensile creep deformation of nickel-based disk superalloy

The aim of the present investigation is to study the tensile creep behavior of nickel-based disk superalloy, which is widely used in the manufacturing of turbine disks for the aero engines. Tensile creep tests were performed over a wide range of stresses (100–900 MPa) and temperatures (625–850 °C) and rupture time varies from 93.1 to 9473 h. Furthermore, stress rupture strength was predicted using the time-temperature parameter at different temperatures for longer durations. The stress dependence of minimum creep rate for the alloy is found to follow the power law. The stress exponent (n) decreased from 31.2 to 7.8 with increase in the test temperature from 625 °C to 700 °C. Subsequently, the rate-controlling mechanism of creep is identified as dislocation climb by adopting the threshold stress analysis in the temperature range of 625–700 °C. At higher temperature, the n values are drastically reduced to 2.9, 2.7 and 2.2 at 750, 800 and 850 °C, respectively, due to dislocation annihilation and dissolution of precipitates. This indicates that the rate-controlling mechanism of creep changes from viscous glide to grain boundary sliding (n = 2.2). The activation energy for creep (QC) has been determined by using the modified power law in the temperature range of 625–700 °C and is found to be 598 kJ/mol, whereas the obtained QC is also decreased significantly to 435 kJ/mol at higher temperature range (750–850 °C). The calculated QC values are found to be 52.5% and 34.7% higher than the activation energy for lattice self-diffusion of nickel (284 kJ/mol). Post creep microstructural examination using transmission electron microscopy (TEM) revealed extensive deformation in the microstructure is accommodated through the γ′ precipitates, formation of stacking faults and deformation twins within the larger γ′ precipitates. The major findings are well in strong agreement with the hardness characterization, where prominent increase in hardness within localized deformation is mainly due to extensive dislocation-interactions with the γ′ precipitates.

Creep of single-crystals of nickel-base γ-alloy at temperatures between 1150 °C and 1288 °C

A γ-analogue of the superalloy CMSX-4 that does not contain the strengthening γ′-phase and only consists of the γ-solid solution of nickel has been designed, solidified as single-crystals of different orientations, and tested under creep conditions in the temperature range between 1150 and 1288 °C. The tests have revealed a very high creep anisotropy of this alloy, as was previously found for CMSX-4 at supersolvus temperature of 1288 °C. This creep anisotropy could be explained by the dominance of 011111 octahedral slip. Furthermore, the analysis of the creep data has yielded a high value of the creep activation energy, Qc≈442 kJ/mol, which correlates with the high activation energy of Re diffusion in Ni. This supports the hypothesis that dislocation motion in the γ-matrix of Re-containing superalloys is controlled by the diffusion of the Re atoms segregating at the dislocation core. The Norton stress exponent n is close to 5, which is a typical value for pure metals and their alloys. The absence of γ′-reprecipitation after high-temperature creep tests facilitates microstructural investigations. It has been shown by EBSD that creep deformation results in an increasing misorientation of the existing low angle boundaries. In addition, according to TEM, new low angle boundaries appear due to reactions of the a/2011 mobile dislocations and knitting of new networks.

ɤ-TiAl alloy: revisiting tensile creep deformation behaviour and creep life at 832 °C

ABSTRACT Creep deformation in single-phase ɤ-TiAl alloys manufactured using different processing techniques has been an extensively studied topic owing to the high specific strength and excellent creep properties of these alloys at temperatures between 760 and 1000°C. In addition, these lightweight and creep-resistant alloys are being presently considered as replacements to the comparatively heavier Ni-based superalloys for application in the low-pressure turbine blades of the next-generation gas turbine engines. However, there is limited information on the tensile creep deformation behaviour and creep life of ɤ-TiAl alloys at 832°C where these alloys have been reported not to exhibit steady-state creep. To this end, the present work revisits the work on understanding the tensile creep deformation behaviour of wrought single-phase ɤ-TiAl alloy by Saha [1] and is aimed to develop an understanding of the tensile creep deformation behaviour at 832°C and the influence of creep activation energy on the creep life of wrought single-phase ɤ-TiAl alloy for stress levels of 69.4 and 103.4 MPa at 832°C using Monkman–Grant [2] approach.

Influence of discharge plasma modification on physical properties and resultant densification mechanism of spherical titanium powder

During powder sintering, it is widely accepted that the three unfavorable features, i.e., closed-pore hollow powder, satellite powder, and low excess free energy, can affect the densification behavior; however, their specific influences have previously never been determined. In this work, we report the efficient elimination of the three unfavorable features for atomized spherical titanium powder via discharge plasma modification and further clarify their influences on the densification behavior during spark plasma sintering. Our results show that the activation energy of power-law creep for the modified powder is two times lower than that of the atomized counterpart.