Apparent Activation Energy For Creep Research Articles

Role of threshold stress in creep of IN740H, a γ′-lean Ni-based superalloy

IN740H, a Ni-based superalloy comprising a low-volume fraction of γ′, is a candidate material to be used over long periods at high temperatures and stresses, as conditions prevalent in advanced ultra-supercritical (AUSC) power plants. In this study, IN740H has been tested at temperatures above 700 °C to gain mechanistic insights into its high-temperature creep behavior. The material showed classic signatures of the presence of threshold stress, marked by observation of a high apparent creep stress exponent, n, (e.g., 10 at 750 °C) and a high apparent activation energy for creep, Qc (e.g., ∼545 kJ/mol), along with a rapid increase in n at lower stresses. Accounting for the threshold stress led to a decrease in the values of n and Qc to 4 and 280 kJ/mol, respectively. Furthermore, the transmission electron microscopy and atomic scale compositional analysis reveal the pinning of dislocations at γ-γ′ interface and segregation of Co, Cr and Mo atoms in the regions of γ-γ′ interface rich in dislocations. The above combination of n and Qc and the observation of dislocation pinning at the γ-γ′ interface indicate the dislocation climb over γ′ as the dominant creep mechanism in this γ′-lean Ni-based superalloy, with the detachment of dislocations from the γ-γ′ interface, augmented by the segregation of Co, Cr and Mo, as the mechanism responsible for the realization of the threshold stress. This work, thus, provides a new impetus to research on the long-term structural integrity of γ′-lean Ni-based superalloys exposed to extreme service conditions.

The synergistic effects of nano-sized α-Al(Mn,Cr,Fe)Si and Al–Si–Zr dispersoids on the creep behavior Al–Si–Mg–Cu (Mn, Cr, Zr) diesel engine alloy

The influence of Mn, Cr and Zr on the creep behavior of an Al–Si–Mg–Cu alloy intended for Diesel engine applications was studied. Two alloys, base alloy Al–7Si-0.3Mg-0.5Cu (MG1) and Al–7Si-0.3Mg-0.5Cu-0.15Mn-0.25Cr-0.15Zr (MG2-1) were creep tested at 300–350 °C and 28 MPa in the T7 heat-treated and soaked (100 h at 300 °C) condition. The apparent activation energy for creep for MG1 was 184 kJ mol−1 and for the MG2-1 was 234 kJ mol−1. The two alloys exhibited differing rate controlling creep mechanisms. The minimum creep rates of the MG2-1 alloy were 3–5 times lower than the MG1 alloy at 300 °C and 325 °C, respectively and only 1.3 times lower at 350 °C. The TEM investigation indicated creep deformation in the MG2-1 alloy was due to thermally activated restoration processes where grain boundary migration experienced Zener pinning by coherent and ∼5 nm αMG2-1-Al(Mn,Cr,Fe)Si. In the MG1 alloy, the creep mechanism was identified as climb over nano-sized (∼50 nm) non-coherent α-Al(Mn,Fe) dispersoids. EDS analysis showed that semi coherent Q’ phase (near equilibrium version of Q phase) existed in the MG1 alloy that had a nucleating role for nano-sized non-coherent α-Al(Mn,Fe) dispersoids.

Creep deformation behavior of the Ni–Fe-based GH984G alloy

The creep deformation behavior and microstructure evolution of the Ni–Fe-based GH984G alloy were investigated at temperatures of 700 °C–775 °C, with the stress ranging from 150 MPa to 400 MPa. The results demonstrate that the apparent stress exponent and apparent creep activation energy are 6.53 ± 0.74 and 546.7 ± 6.8 kJ/mol, respectively. The creep activation energy is higher than that of lattice self-diffusion, which is related to the interaction between dislocation and γ′ precipitates and can be corrected by threshold stress. The threshold stresses are 109.1 MPa, 51.5 MPa and 24.8 MPa at 700 °C, 750 °C and 775 °C, respectively. The true stress exponent is 4.95 ± 0.14 and the true creep activation energy is determined to be 288.3 ± 4.3 kJ/mol. The microstructure analysis demonstrates that the climbing of a/2 <110> perfect dislocation is the primary mechanism of creep at 700 °C. The dislocation climbing and a/6 <112> partial dislocation shearing into γ′ are both significant factors during creep deformation at 750 °C. The creep damage tolerance of 6.4 ± 1.1 indicates that microstructural degradation, including γ′ and M23C6 coarsening, is the primary creep damage mechanism for the GH984G alloy. The deformation mechanism map for GH984G alloy is established and the creep mechanism within the investigated conditions locates in the power-law creep domain controlled by dislocation climbing.

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.

WITHDRAWN: Influence of processing routes on the creep deformation and damage evolution of Zr-2.5Nb alloy

Abstract Tensile creep behavior of the Zr-2.5Nb alloy tubes having outer diameters of 90 mm and 110 mm, processed by extrusion and forging, respectively has been studied in the temperature range of 350-450°C under the stress range of 82-319 MPa. The creep curves have shown that the minimum creep rate and time to rupture changes with the applied stress and temperature. The apparent activation energy of creep ( Q C ) values, have been determined by using the power law for 90 mm and 110 mm tube samples are found to be ∼ 231 kJ/mol and ∼ 264 kJ/mol respectively, which is significantly higher than that for lattice self-diffusion of Zr (113 kJ/mol). The microstructural examination of the creep-ruptured samples through transmission electron microscopy (TEM) has shown evidence for multiplication of dislocations, along with their interaction with the β-(Nb, Zr) precipitates in the α-Zr matrix, which is mainly contributed to origin of threshold stress during creep. By incorporating the threshold stress data in the modified power-law relation, the true activation energy of creep ( Q t ) values from the samples of 90 mm and 110 mm tubes are found as 118 kJ/mol and 132 kJ/mol, respectively, which are close to that for lattice self-diffusion in Zr. The obtained true stress exponent ( n t ) values of 3.6 and 4.9 and formation of dislocations sub-grains in the power-law regime of steady state creep, which suggests that the dislocation climb is the rate-controlling mechanism. Further, creep data of the alloy are validated by the Monkman-Grant and Modified Monkman-Grant relationship, and time to rupture effectively predicted using the Larson-Miller Parameter. The creep damage tolerance factors are in the range of 1.5-2.5 for the tubes suggest cavity growth mechanism. Additionally, the comparison of creep data with the Ashby-Dyson maps indicates early formation of cavities due to creep.

The effect of Y and Nd additions on the microstructure and creep behavior of AZ80

The effect of yttrium (0.2Y wt.%) and yttrium + neodyminum (0.1Y+0.1Nd wt.%) additions on the long-term (>1200 h) tensile creep and microstructural evolution of AZ80 was investigated at temperatures ranging between 100 °C and 150 °C and stresses ranging between 50 MPa and 100 MPa. The alloying resulted in both the formation of Y- and/or Nd containing particles, and lower tensile strengths and elongation-to-failure. The minimum creep rate (ε˙min) increased with increasing applied creep temperature and stress. Y addition resulted in greater creep-resistance compared with that for (Y + Nd) additions. The ε˙min of AZ80 + 0.2Y was approximately half that of AZ80 at 50 MPa at 150 °C. The apparent creep activation energy for each alloy ranged between 73 and 93 kJ/mol, and pipe-diffusion-assisted dislocation creep was suggested to be the dominant secondary stage creep deformation mechanism. Discontinuous precipitation (DP) occurred during the creep exposure but was impeded by precipitate-containing twins. The DP was less concentrated in the Y and (Y + Nd) containing alloys, which exhibited lower Vickers hardness values than AZ80. Grain boundary and twin boundary cavitation/cracking were prevalent during creep. Minor Y or (Y + Nd) addition, resulted in less oxidation and extended the intermediate-temperature creep life.

Creep behavior of high-pressure die-cast AlSi10MnMg aluminum alloy

High-pressure die-cast (HPDC) AlSi10MnMg (AA365) has been widely used in the manufacturing of automotive components because it has excellent castability, is lightweight, and possesses good mechanical strength. In particular, this alloy must be capable of withstanding long-term exposure to high temperatures in order for these automobile parts to be useable. To investigate the viscoelastic behavior of HPDC AA365 with the aim of improving the long-term structural performance at high temperatures, we performed a creep test in the temperature range of 373–573 K with a constant applied stress of 64–217 MPa. A micro-hardness test of the HPDC AA365 alloy, performed after the creep test, yielded hardness values of 64.0 HV and 49.9 HV for the gauge and grip sections, respectively. The observed grain-size differences between the sections may have led to this difference in micro-hardness. Moreover, both values of micro-hardness were relatively low compared with the value of 93.5 HV for HPDC AA365 before the creep test. This was because of recovery and recrystallization. The creep tests also showed that the minimum creep rate increased with increasing applied stress for each temperature, causing a decrease in the time to failure. Based on the true stress exponent (n) on a scale of 1 to 90, various creep mechanisms for HPDC AA365 were revealed, including the Harper-Dorn, dislocation, and power-law breakdown mechanisms. The specific creep mechanism depended on temperature and applied stress. Furthermore, owing to the presence of the threshold stress, the apparent creep activation energy (259.08 kJ/mol) was substantially higher than that of aluminum (Al) self-diffusion (Qd = 143 kJ/mol). The existence of the threshold stress can also be demonstrated by a high n value that does not correspond to the n of pure Al under certain conditions. Failure in these alloys can be caused by void coalescence, cracked-brittle Si particles, and intermetallic phases that served to initiate and propagate the crack. Our results demonstrated that the fracture mechanism changed from transgranular to intergranular fracture in response to the applied stress and temperature. Our findings can improve the applications of high-pressure die-cast aluminum alloys.

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

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

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

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

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

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

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

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

Creep performance and damage mechanism for Allvac 718Plus superalloy

The creep behavior, creep damage and creep life of 718Plus alloy were investigated at temperatures ranging from 650 to 750 °C in air. The fracture surface, microstructure evolution and deformation mechanisms were studied by optical, scanning and transmission electron microscope (SEM and TEM). The results show that creep curves of 718Plus alloy consist of primary stage and tertiary stage without steady-state region. The Monkman-Grant relationship of 718Plus alloy can be described by tr = 9.32E-2/(ε̇min)0.75 and apparent creep activation energy of 718Plus alloy is about 572.7 kJ/mol. Further investigation shows creep damage of alloy at 650 °C can be mainly ascribed to the effect of grain boundary sliding (GBS) and microtwinning. The alloy shows transgranular fracture at high stress, while it presents ductile fracture at low stress due to the nucleation, growth and linkage of creep voids. At 750 °C, the alloy failed in the form of ductile fracture mainly due to the microstructure instability and dislocation glide. At higher stress of 700 °C, the effect of GBS is mainly responsible for the creep damage of 718Plus alloy, while GBS, dislocation glide and microstructure instability work together and lead to the creep damage of alloys at lower stress of 700 °C. The fracture mechanism map and fracture models for 718Plus alloy were constructed. Remaining life assessment of 718Plus alloys was also evaluated by Larson-Miller parameter method.

Ultrahigh-temperature tensile creep of TiC-reinforced Mo-Si-B-based alloy

In this study, the ultrahigh-temperature tensile creep behaviour of a TiC-reinforced Mo-Si-B-based alloy was investigated in the temperature range of 1400–1600 °C at constant true stress. The tests were performed in a stress range of 100–300 MPa for 400 h under vacuum, and creep rupture data were rationalized with Larson-Miller and Monkman-Grant plots. Interestingly, the MoSiBTiC alloy displayed excellent creep strength with relatively reasonable creep parameters in the ultrahigh-temperature range: a rupture time of ~400 h at 1400 °C under 137 MPa with a stress exponent (n) of 3 and an apparent activation energy of creep (Qapp) of 550 kJ/mol. The increasing rupture strains with decreasing stresses (up to 70%) and moderate strain-rate oscillations in the creep curves suggest that two mechanisms contribute to the creep: phase boundary sliding between the hard T2 and (Ti,Mo)C phases and the Moss phase, and dynamic recovery and recrystallization in Moss, observed with orientation imaging scanning electron microscopy. The results presented here represent the first full analysis of creep for the MoSiBTiC alloy in an ultrahigh-temperature range. They indicate that the high-temperature mechanical properties of this material under vacuum are promising.

Thermomechanical properties of the Garden Gulch Member of the Green River Formation

Extensive well logging and laboratory rock mechanics experiments gathered within the context of in-situ retorting are used to derive an anisotropic thermomechanical model for deformation of oil shale from the Garden Gulch Member of the Green River Formation in the Piceance Basin of Colorado. Elastic moduli determined from sonic velocities are shown to be consistent with but about twice as high as those from static measurements at small strain. Porosity correlates inversely with organic content, and the relative contributions of kerogen, minerals, and porosity to sonic velocities are determined from a combination of new and literature data. Young’s modulus of kerogen derived by extrapolation to high kerogen volume fraction is consistent with measurements from previous workers. The temperature dependence and anisotropy of the triaxial tests can be described fairly well by Reuss and Voigt averages for a laminar material. The intrinsic (end-member) static Young’s moduli of the kerogen and mineral components are 1 and 7 GPa, respectively, with thermal softening of each described by apparent activation energies of 17 and 9 kJ/mol, respectively. Isostatic compression creep tests are used to further constrain the temperature dependence of the plastic deformation viscosity. The apparent activation energy for creep is about 35 kJ/mol. The thermochemical properties of the clay-rich Garden Gulch Member are compared to those of the dolomite-rich Parachute Creek Member. At low organic content, the Young’s modulus of the Garden Gulch Member is about four times lower, and the properties of the two members approach each other for high organic content.

Compression creep of copper under electric current studied by a spark plasma sintering (SPS) apparatus

A spark plasma sintering (SPS) apparatus was successfully employed to perform uniaxial compressive creep tests on pure copper under a stress of 30MPa in the 400–600°C temperature range. By utilizing two different configurations, the creep experiments were conducted without or with a low-density pulsed direct electric current (~ 6–7A/mm2) passing through the samples. It was found that under the influence of the applied electric current, the creep rate increased significantly, while the extent of the effect diminished with temperature. The apparent activation energy for creep with applied current decreased from 110 to 66kJ/mol. This was attributed to the effect of current on the thermally-activated process and dislocations motion. No distinct evidence that the electric current affects the microstructure was observed.

Transmission electron microscopy study of the microstructural evolution during high-temperature and low-stress (011) [11] shear creep deformation of the superalloy single crystal LEK 94

Abstract

High-temperature deformation and fracture mechanisms of an advanced heat resistant Fe-Cr-Ni alloy

In this work creep-strain test ranging from 180 to 240MPa at 973K, 998K, and 1023K were conducted to investigate the high temperature deformation and fracture mechanisms for Sanicro 25 alloy. The relationship between minimum creep rate and applied stress followed Norton's law. Apparent stress exponent values of 8, 6.5, and 5 were obtained at 973K, 998K, and 1073K, respectively, with an apparent creep activation energy value of 437.3–606.1kJ/mole illustrating that the rate-controlled creep occur in Sanicro 25 during creep. The threshold stresses were 129.5, 111.5, and 82.0MPa at 973K, 998K, and 1023K, respectively. The true creep activation energy and the true stress exponent were found to be 271.6kJ/mole and 3, respectively. The presence of nanoscale MX and Cu precipitates were observed and theoretical calculations of threshold stresses confirmed that shearing of the nano-Cu precipitates occurred at 973K, while the dislocations climbed up MX precipitates at 973–1023K. The apparent stress exponent obtained for creep rupture shows that similar mechanisms operate in creep deformation and rupture behavior for Sanicro 25. A damage tolerance factor of ~5 in the alloy indicates that the microstructural degradation such as coarsening of precipitate and subgrain structure is the dominant creep damaging mechanism in the alloy.

Creep Properties of the As-Cast Al-A319 Alloy: T4 and T7 Heat Treatment Effects

In this work, the creep behavior of a commercial Al-A319 alloy was investigated in the temperature range of 413 K to 533 K (140 °C to 260 °C). Tensile creep specimens in the as-cast condition and after heat treating by solid solution (T4) and by aging (T7) were tested in a stress range varying from 60 to 170 MPa. It was found that steady-state creep strain rate was significantly low in the T7 condition when compared with either the T4 or as-cast alloy conditions. As a result, the time to failure behavior considerably increased. The experimentally determined creep exponents measured from the stress–strain curves were 4 for the as-cast alloy, 7.5 in the solid solution, and 9.5 after aging. In particular, after solid solution a grain substructure was found to develop which indicated that creep in a constant subgrain structure was active, thus accounting for the n exponent of 7.5. In the aged condition, a stress threshold is considered to account for the power law creep exponent n of 9.5. Moreover, It was found that the creep activation energy values were rather similar for the alloys in the as-cast (134 kJ/mol) and T4 (146 kJ/mol) conditions. These values are close to the one corresponding to pure Al self-diffusion (143 kJ/mol). In the aged alloy, the apparent creep activation energy (202 kJ/mol) exceeded that corresponding to Al self-diffusion. This deviation in activation energy is attributed to the effect of temperature on the alloy elastic modulus. Microstructural observations using transmission electron microscopy provided further support for the various dislocation-microstructure interactions exhibited by the alloy under the investigated creep conditions and implemented heat treatments.

Tensile creep behavior of Sn–Ag–Cu–Ni multicomponent lead-free solder alloy

In the process of electronic packaging, the dissolution of under bump metallizations, such as Cu and Ni, into liquid solder occurs during soldering, which can change the original solder to a multicomponent one. Under the trend of miniaturization, it is quite necessary to evaluate the properties of multicomponent solder with excessive Cu and Ni compositions. In this study, the tensile creep behavior of Sn–3.5Ag–2.0Cu–0.5Ni multicomponent lead-free solder alloy is investigated at three temperatures, i.e., 303, 348 and 393 K. The steady-rate creep rates are obtained in the range of 10−4–10−8 s−1, when the normalized stress, σ/E, is in the range of 10−4–10−3. Based on the Dorn equation, the apparent stress exponent (n a), threshold stress (σ th), and activation energy of creep (Q C) are calculated at the three temperatures. It is found that the Sn–3.5Ag–2.0Cu–0.5Ni solder alloy shows a better creep performance than pure tin and eutectic Sn–3.5Ag solder due to the strengthening effect of Ag3Sn and (Cu,Ni)6Sn5 IMC precipitations. The true stress exponent for creep is identified to be 7, indicating that the creep behave is controlled by the dislocation-pipe diffusion in the tin matrix.

On the role of Re in the stress and temperature dependence of creep of Ni-base single crystal superalloys

In the present study we investigate the creep behavior of a Ni-base single crystal superalloy. We evaluate the stress and temperature dependence of the minimum creep rate, which shows a power law type of stress dependence (characterized by a stress exponent n) and an exponential type of temperature dependence (characterized by an apparent activation energy Qapp). Under conditions of high temperature (1323K) and low stress (160MPa) creep, n and Qapp are determined as 5.3 and 529kJ/mol, respectively. For lower temperatures (1123K) and higher stresses (600MPa) the stress exponent n is higher (8.5) while the apparent activation energy of creep is lower (382kJ/mol). We show that there is a general trend: stress exponents n increase with increasing stress and decreasing temperature, while higher apparent activation energies are observed for lower stresses and higher temperatures. We use density functional theory (DFT) to calculate the activation energy of diffusion for Re in a binary Ni–Re alloy with low Re-concentrations. The resulting energy is almost a factor 2 smaller than the apparent activation energy of creep. We explain why it is not straightforward to rationalize the temperature dependence of creep merely on the basis of the diffusion of one alloying element. We show that the evolution of the microstructure also must be considered.