Activation Energy For Creep Research Articles

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.

Tensile creep behaviors of Mg-10Gd-0.4Zr alloy in different heat treatment states

Mg-Gd alloys show good mechanical properties due to the strengthening β′ phases. However, the β′ phases are unstable and easily transform into other phases, which leads to the severe performance deterioration at elevated temperatures. In this paper, the tensile creep behaviors of Mg-10Gd-0.4Zr alloy in different heat treatment states were investigated at temperatures from 225 °C to 300 °C under stresses from 50 MPa to 80 MPa. The results showed that the creep behaviors of the solution treated (T4) and peak-aged (T6) alloys were dependent on the critical transition temperature. Microstructure evolution analysis revealed that the creep resistance of the T6 alloy was better than that of the T4 alloy below the critical transition temperature due to the existence of more β′ phases. With further increasing the creep temperature, the formation of β1 phase and the widening of precipitate-free zones (PFZs) in the T6 alloy significantly deteriorated the creep resistance, while the precipitated β′ phases strengthened the T4 alloy effectively. It resulted in that the T4 alloy exhibited better creep resistance above the critical transition temperature. The stress exponents (3.7–6.2) and creep activation energy (127 kJ mol−1) of T4 alloy indicated that the dominated creep mechanism was dislocation climb assisted by lattice self-diffusion of Mg atoms. The stress exponents (3.6–6.4) of T6 alloy were close to that of T4 alloy, while the creep activation energy changed from 94 kJ mol−1 to 192 kJ mol−1 with increasing creep temperature. Observation of dislocation configuration revealed that the creep mechanism of T6 alloy transformed into dislocation cross slip above the critical transition temperature.

Through-thickness heterogeneity in creep properties of friction stir welding 7B50-T7451 aluminum alloy thick plate joint: Experiments and modeling

Through-thickness heterogeneity in creep properties of 7B50-T7451 aluminum alloy Friction Stir Welding (FSW) joints was investigated. Creep tests for three slices of the FSW joint were conducted at the temperature of 150–200 °C and applied stress of 60–225 MPa. The theta projection method was used to predict creep curves and minimum creep rate. The results show that the minimum creep rate increases and creep rupture life decreases with the increase of creep temperature and applied stress. Creep properties of the FSW joint deteriorate along the thickness direction from the top to the bottom. The threshold stress of all three slices of the FSW joint decreases with the increase of creep temperature and even disappears at 200 °C for the bottom slice. Creep activation energy approaches the activation energy of the lattice self-diffusion of aluminum. The value of true stress exponent for different slices is approximately equal to three. The predominant creep mechanism of the FSW joint is dislocation viscous glide by lattice self-diffusion. What is more, a constitutive model is established based on the theta method to accurately describe creep behavior of different slices of the FSW joint.

Creep strengthening mechanisms of a superlattice γ′-strengthened Co–Al–W–Ta–Ti single crystal superalloy at steady-state conditions under compressive stresses

The strengthening mechanism of steady-state creep is a critical factor in developing advanced superalloys. In this study, the steady-state creep mechanisms of a superlattice γ′-strengthened Co–Al–W–Ta–Ti single crystal superalloy are investigated with the creep conditions of 750 °C/800 MPa, 900 °C/420 MPa and 1000 °C/137 MPa. Results suggest that the microstructures of steady-state creep samples are significantly different under the above three experimental conditions. A detailed TEM study shows that, Lomer-Cottrell locks and stacking faults interactions are responsible for the low creep rate at low temperature and high stress creep regime, dislocations cross-slip and the dislocation tangles contribute the main creep resistance at intermediate temperatures and moderate stress creep regime, interactions of stacking faults and dislocation networks are the strengthening mechanism for the high temperature and low stress creep regime. The steady-state creep rate of investigated alloy follows the classical power law equation, and the creep activation energy calculated by the power law equation is 378.46 kJ/mol. The results of this study provide insights into the effects of temperature and stress on steady-state creep mechanisms, and high activation energy makes the γ′-strengthened Co–Al–W–Ta–Ti single crystal superalloy an attractive candidate for further study as a high temperature structural material.

Structural Evolution and Transitions of Mechanisms in Creep Deformation of Nanocrystalline FeCrAl Alloys.

FeCrAl alloys have been suggested as one of the most promising fuel cladding materials for the development of accident tolerance fuel. Creep is one of the important mechanical properties of the FeCrAl alloys used as fuel claddings under high temperature conditions. This work aims to elucidate the deformation feature and underlying mechanism during the creep process of nanocrystalline FeCrAl alloys using atomistic simulations. The creep curves at different conditions are simulated for FeCrAl alloys with grain sizes (GS) of 5.6-40 nm, and the dependence of creep on temperature, stress and GS are analyzed. The transitions of the mechanisms are analyzed by stress and GS exponents firstly, and further checked not only from microstructural evidence, but also from a vital comparison of activation energies for creep and diffusion. Under low stress conditions, grain boundary (GB) diffusion contributes more to the overall creep deformation than lattice diffusion does for the alloy with small GSs. However, for the alloy with larger GSs, lattice diffusion controls creep. Additionally, a high temperature helps the transition of diffusional creep from the GB to the dominant lattice. Under medium- and high-stress conditions, GB slip and dislocation motion begin to control the creep mechanism. The amount of GB slip increases with the temperature, or decreases with GS. GS and temperature also have an impact on the dislocation behavior. The higher the temperature or the smaller the GS is, the smaller the stress at which the dislocation motion begins to affect creep.

Microstructure evolution and creep behavior of nitrogen-bearing austenitic Fe–Cr–Ni heat-resistant alloys with various carbon contents

In the petrochemical industry, centrifugal cast Fe–Cr–Ni heat-resistant alloy tubes are critical components that are exposed to severe conditions for long-time service. The life of the alloy tubes is primarily limited by creep damages, which can evolve into catastrophic failures. In order to improve the creep properties in a low-cost approach, the influence of N + C content on the microstructure evolution and creep behavior of the 25Cr35NiNb alloys has been studied in this work. Creep tests were carried out at 1173 K (900 °C) and 1323 K (1050 °C) with different stress levels. Microstructure investigation on precipitates revealed that the C/N ratio significantly affects the content and morphology of Cr-rich carbides and the phase transition of Nb-rich carbonitrides after thermal exposure. By adopting the threshold stress analysis, the true creep activation energy and stress exponent of all the alloys were found to be ∼270 kJ mol−1 and ∼5, respectively, which indicated that dislocation climb is the rate-controlling mechanism of creep. Furthermore, the tailored novel N-bearing alloy possessed superior comprehensive properties of creep resistance and damage tolerance compared to the 25Cr35NiNb grade alloys. Our current findings not only realize improved creep properties by the combined modification of N addition and C/N ratio, but also provide new insights into understanding the microalloying mechanisms of high-temperature materials in general.

Unravelling the microstructure – indentation creep resistance relationships for friction stir welded modified 9Cr-1Mo steel and LN-type 316 stainless-steel dissimilar joints

In this article, the indentation creep characteristics and the microstructures are assessed for the friction stir welded modified 9Cr-1Mo steel and LN-type 316 SS dissimilar joint. Microstructures are characterized for the post-weld heat treated and indentation crept samples at different conditions with optical microscopy, scanning electron microscope and energy dispersive X-ray spectroscopy. Distinct heat-affected zones were revealed with the refined and reformed martensite (α′) and austenite (γ) structures that formed in view of the distinct thermal conductivity property and the differences in lower critical(Ac1) and upper critical(Ac3) transformation temperatures. The size of prior austenite grain sizes was found to be varied significantly at the different regions. Indentation creep data were acquired in distinct weld zones at 873, 898 and 923 K. Zone-wise indentation depth, steady-state creep behaviour (SSCR) and activation energy ( Q) were evaluated and correlated with the microstructures. The formation of small prior austenitic grain (PAG) structures and the grain boundary vacancies has led to maximum creep deformation (86.83%) at the heat affected zone of the modified 9Cr-1Mo steel. Furthermore, the reduction in lath structures and the formation of tiny α′ with soft substructures have lowered the hardness values to around 210 ± 21 HV, which is 20 HV less than the post weld heat treated sample. In the stir zone, the presence of dynamically refined and rich γ along with fine α′ layers has improved the steady-state creep rate of 9.112 E-7, which is less than the modified 9Cr-1Mo steel and the heat affected zone at modified 9Cr-1Mo side. The coarse austenite structures that existed in the LN-type 316 base metal have resisted more creep damage and exhibited high activation energy (i.e. 490 kJ mol−1) than the stir zone and heat affected zone of the LN-type 316 SS.

The Activation Energy of Strain Bursts during Nanoindentation Creep on Polyethylene.

In the present investigation, statistical characterization of strain bursts observed during the load-controlled deformation of high-density polyethylene (HDPE), which arise within the crystalline phase during plastic deformation, was carried out via high-resolution nanoindentation creep experiments. Discrete deformation processes occurred during the nanoindentation creep tests, which indicated that they arose from the break-off of dislocation avalanches, i.e., dislocation climb is a possible mechanism for indentation creep deformation. Characterization of the strain bursts, in terms of the associated height and number, demonstrated that these quantities followed a Gaussian distribution depending on the load and loading rate. This analysis enabled the accurate measurement of creep activation energy. Our method used nanoindentation tests to measure the creep activation energy of HDPE within both the crystalline and amorphous phases. The activation energy of the creep process within the crystalline phase was evaluated using two methods. The frequency of jumps within the crystalline phase, as a function of the strain rate, showed two peaks related to the 5 nm and 10 nm jump sizes that corresponded to the block size within the crystalline lamellae. The results indicated that the intervals coincided with the mean free path of dislocations and the block grain boundaries acted as dislocation barriers. From the dependence of burst frequency on the strain rate and temperature, the activation energy and thermally activated length of the dislocation segment for the plastic slip activation were determined to be 0.66 eV and 20 nm, respectively. Both numbers fit well to the Peterson's model for the nucleation and motion of thermally activated dislocation segments. A similar activation energy resulted from the differential mechanical analysis of the literature for the αI-transition, which occurred near room temperature in polyethylene. The transition was described as the generation of screw dislocation and its motion along a block grain boundary; therefore, this process is suggested to be the basic mechanism underlying the strain bursts observed in this study.

Effects of lattice distortion and chemical short-range order on creep behavior of medium-entropy alloy CoCrNi

Creep behavior of multi-principal element alloys (MPEAs) is an intriguing topic to explore for their potential high-temperature applications. The challenge on this topic is to elucidate what effect the local chemical fluctuation has on the creep behavior of MPEAs. By using large-scale molecular dynamics (MD) simulations, we investigate the creep performances of CoCrNi medium-entropy alloys (MEAs) with and without chemical short-range order (CSRO) as well as the Average-atom (A-atom) counterpart (no lattice distortion (LD)) under different uniaxial tensile stresses at various elevated temperatures. A power-law model is adopted to analyze the implicit stress exponent and activation energy, which are associated with the creep mechanism and creep resistance, respectively. The results reveal CSRO rather than LD plays an important role in creep performance. Specifically, with the introduction of CSRO, the activation energy for creep in CoCrNi MEA is significantly increased and finally close to the activation energy for the diffusion of the Cr element that is the highest among the three elements (Co, Cr, and Ni). The CoCr clusters in the MEA with CSRO make it difficult to increase the shear strain in the creep process, resulting in a much lower creep rate than that of the CoCrNi MEA without CSRO. For the CoCrNi MEA without CSRO, grain boundary diffusion is the main creep mechanism. For the MEAs with CSRO, the predominant creep mechanism depends on the applied stress. At low stress, grain boundary diffusion still prevails and there is no obvious dislocation glide or grain boundary sliding. When the applied stress is high enough, grain boundary diffusion and dislocation slip dominate the creep behavior, accompanied by the occurrence of grain boundary sliding.

The creep behavior of (TiB + TiC + Y2O3)/α-Ti composites at high temperature and short-term

In present study, the creep behavior of (TiB + TiC+Y 2 O 3 )/α-Ti composite with C-1 and C-2 microstructure were investigated with applied stress of 200 MPa and at 650–700 °C. The steady state creep rate increased by an order of magnitude from 650 °C to 700 °C, and the C-2 microstructure shown worse creep performance compared to C-1 microstructure. The creep activation energy of C-1 and C-2 microstructure were 359.5 kJ/mol and 380.2 kJ/mol, respectively, which were higher than the lattice diffusion in α-Ti. And the less Q G due to coarse microstructure was responsible for its higher creep activation energy in C-2. In addition to cracks were observed at β grain boundary, α colony boundary, α/β phase boundary, they were also observed near the fracture TiB whiskers and near decohesion reinforcements. The cracks in C-2 were mainly concentrated near the reinforcement, while those in C-1 were mainly concentrated in the matrix part. Further analysis confirmed that the creep deformation mechanism of C-2 microstructure was controlled by reinforcements, but the creep deformation mechanism of C-1 microstructure was controlled by matrix microstructure. Summarily, the effect of reinforcement on creep deformation of (TiB + TiC+Y 2 O 3 )/α-Ti composite could be divided in two aspect: microstructure influence and the reinforcement itself. For reinforcement itself, it indeed affected the dislocation movement and creep fracture behavior, and then improved the creep performance. However, for microstructure influence, the fine grain (more grain boundary), coarsen microstructure (more equiaxed characteristics of α lamellar) caused more grain boundary slip to participate in creep deformation, which in turn deteriorated creep performance. In this study, the latter effect was somewhat bigger. • The steady state creep rate increases by an order of magnitude from 650 °C to 700 °C. • The creep fracture mechanism is dominated by reinforcements in C-2 microstructure, but by matrix in C-1 microstructure due to the distribution of cracks. • More percentage of grain boundary and fine microstructure in composite promote more creep deformation by grain boundary slip, which resulting in poor creep resistance.

Enhancing the high-temperature creep properties of Mo alloys via nanosized La2O3 particle addition

Molybdenum (Mo) alloys with different La 2 O 3 particle additions (0.6, 0.9, 1.5 wt.%) were prepared by powder metallurgy to investigate the effect of La 2 O 3 particles on microstructural evolution and creep behavior of the alloy. Pure Mo, annealed at 1500 °C for 1 h, presented a fully recrystallized microstructure characterized by equiaxed grains. The alloys doped with La 2 O 3 particles (Mo-La 2 O 3 alloys), on the other hand, exhibited fibrous grains elongated in the rolling direction of the plate. In contrast to the shape of the grains, the average grain size of the alloys was found to be insensitive to the addition of La 2 O 3 particles. Nanosized La 2 O 3 particles with diameters ranging from 65 nm to 75 nm were distributed within the grain interior. Tensile creep tests showed that dislocation creep was the predominant deformation mode at intermediate creep rate (10 −7 s −1 -10 −4 s −1 ) in the present alloys. The creep stress exponent and activation energy were found to decrease with increasing temperature, particularly within the low creep rate regime (<10 −7 s −1 ). The Mo-La 2 O 3 alloys exhibited remarkably greater apparent stress exponent and activation energy than pure Mo. A creep constitutive model based on the interaction between particles and dislocations was utilized to rationalize the nanoparticle-improved creep behavior. It was demonstrated that low relaxed efficiency of dislocation line energy, which is responsible for an enhanced climb resistance of dislocations, is the major creep strengthening mechanism in the Mo-La 2 O 3 alloys. In addition, the area reduction and creep fracture mode of the Mo-La 2 O 3 alloys were found to be a function of the creep rate and temperature, which can be explained by the effect of the two parameters on the creep and fracture mechanisms.

Anisotropy and variability in thermal creep behaviour of Zr-2.5Nb pressure tube

The Zr-2.5Nb alloy pressure tubes of Indian Pressurized Heavy Water Reactors were manufactured from double melted ingots initially and subsequently from quadruple melted ingots to reduce impurities such as chlorine, phosphorus and carbon. In the current investigation, creep tests were carried out in the stress range of 0.7–0.9 times the yield strength and the temperature range of 350 °C–450 °C to characterize the thermal creep behaviour of Zr-2.5Nb alloy pressure tubes fabricated from double and quadruple melted ingots. The rupture time, minimum creep rate, stress exponent, activation energy and threshold stresses were obtained by carrying out the creep tests of the samples fabricated with their axis parallel to the axial and transverse directions of the pressure tubes. Experimental analysis revealed that the rupture time for double melted tube was 2–3 times lower than that of quadruple melted tubes, whereas no significant change in minimum creep rate was observed. Stress exponent values were obtained in the range of 3–5 signifying dislocation creep as the dominant creep mechanism. To predict the long-term creep properties, the master curves employing the Larson-Miller and Monkman-Grant methods were also evaluated. The effect of microstructural anisotropy, alloying elements and impurities on the thermal creep behaviour of Zr-2.5Nb pressure tube has been investigated.

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 behavior of an additively manufactured 9Cr steel in the as-built condition

Limited studies have evaluated the creep behavior of additively manufactured (AM) ferritic-martensitic (FM) steels. This work investigated the creep behavior of a 9Cr FM steel fabricated by powder blown directed energy deposition (DED) technique. The creep testing at 550–650 °C and 150 MPa for the specimens along the deposition direction in the as-built condition, together with corresponding microstructural characterization, revealed a threshold temperature between 600 and 625 °C, below which the steel has creep resistance comparable with Grade 91 cross-welds and noticeably greater than 9Cr-1Mo steel. The threshold temperature distinguishes the creep behavior in two regimes differentiated in creep activation energy, creep deformation, and failure mechanism. Unlike the creep rupture surface ∼45° from the loading direction when tested above the threshold temperature, the creep rupture for testing below the threshold temperature resembles type IV failure in the cross-welds of ferritic steels. The DED-induced layer structure in the as-built steel played a significant role on the change of creep behavior.

Impression creep behavior of different zones in friction stir welded ZE41 magnesium-rare earth alloy

The ability of the impression tests to obtain localized creep data is exploited to study the creep behavior of the friction stir welded (FSWelded) joints made on 15 mm thick plates of ZE41 magnesium alloy. The three zones of the FSWelded joint, Nugget zone (NZ), Thermo-Mechanically Affected Zone (TMAZ), and Heat Affected Zone (HAZ), were divided into ten different parts and their local steady-state creep behavior was studied. Impression tests were carried out at stresses of 350, 400, and 450 MPa and temperatures of 513 K, 533 K, and 553 K. Optical and SEM micrographs were used to study the microstructure of various zones of the FSWelded joint both before and after the impression test. Power law was used to obtain the creep exponent (n) and activation energy (Q) for different parts of the weld and the weld zones as a whole. The creep exponent was found to vary from 3.6 to 10.45 and the activation energy from ∼81 kJ/mol to ∼226 kJ/mol. The creep rate was found to vary between the upper and lower surfaces of the weld zones and also among the advancing and retreating sides. The weak zone was found to be temperature-dependent and varied when the temperature or stress changes. Within the parameters studied, the lower part of the TMAZ on the AS showed poor creep resistance at a lower temperature, while the lower part of TMAZ on the RS showed poor creep resistance at higher temperatures. The upper part of the TMAZ on the RS showed better creep resistance throughout the range of temperatures studied. To rationalize the high n and Q values, the threshold stress approach and the Eyring relationship were adopted. Using the threshold approach, creep exponents of 4.87 and 5.17 were obtained for TMAZ and HAZ, respectively. Using Eyring hyperbolic sine relationship, single activation energies of 143.5 kJ/mol and 261.5 kJ/mol were obtained for TMAZ and HAZ, respectively. Based on the obtained n and Q values, it is proposed that two mechanisms, namely climb and cross-slip of dislocations, are operative in the creep of the ZE41 FS weld joint. However, the dominant/rate controlling mechanism could not be arrived at conclusively.

Tensile creep behavior of HfNbTaTiZr refractory high entropy alloy at elevated temperatures

Tensile creep, which is one of the most important deformation modes for high temperature applications, is rarely reported for refractory high entropy alloys (RHEAs). In the present study, the optical floating zone (OFZ) technique was used to fabricate HfNbTaTiZr with grain size larger than 1 mm on average; tensile creep tests under vacuum at 1100-1250°C and stepwise loading of 5–30 MPa were conducted. The stress exponents and creep activation energies were determined to be 2.5–2.8 and 273 ± 15 kJ mol–1, respectively. The stress exponents determined have suggested solute drag creep behavior, and deformation was governed by a/2<111> type dislocations. To elucidate the effect of alloying constituents on solute drag creep, intrinsic diffusion coefficients of all elements were determined by simulation, and theoretical minimum creep strain rates were compared with those of experimental values. Analysis suggests that creep rate of HfNbTaTiZr appears to be controlled by Ta, which possesses the lowest intrinsic diffusivity and contributes the most to drag dislocations. To our knowledge, this work is the first to report tensile creep deformation mechanism of HfNbTaTiZr, especially up to 1250°C.

High temperature creep deformation behavior of heat-treated (α + β)-Mg-9Li-4Al-1Zn alloy

In this study, the high temperature uniaxial tensile creep deformation behavior of (α + β)-Mg-9 mass%Li-4 mass%Al-1 mass%Zn (LAZ941) alloy was investigated. The creep strength of the LAZ941 alloy was significantly improved by heat treatment. The creep deformation behavior of the heat-treated LAZ941 alloy was unique. As the results of creep tests, it is found that the stress exponent of LAZ941 was about 3. The activation energies of creep at the early and middle stages of deformation were 114.7 and 84.6 kJ･mol −1 , respectively, indicating that the diffusing elements controlling the viscous glide of the dislocation changed during creep deformation. • The creep strength of the LAZ941 alloy is improved by heat treatment. • The time to reach 0.1 creep strain is about 9 times longer due to heat treatment. • The creep deformation behavior of heat-treated LAZ941 alloy is unique. • The activation energy of creep for LAZ941 alloy changes during deformation.

How fluid infiltrates dry crustal rocks during progressive eclogitization and shear zone formation: insights from H2O contents in nominally anhydrous minerals

Granulites from Holsnøy (Bergen Arcs, Norway) maintained a metastable state until fluid infiltration triggered the kinetically delayed eclogitization. Interconnected hydrous eclogite-facies shear zones are surrounded by unreacted granulites. Macroscopically, the granulite–eclogite interface is sharp and there are no significant compositional changes in the bulk chemistry, indicating the fluid composition was quickly rock buffered. To better understand the link between deformation, fluid influx, and fluid–rock interaction one cm-wide shear zone at incipient eclogitization is studied here. Granulite and eclogite consist of garnet, pyroxene, and plagioclase. These nominally anhydrous minerals (NAMs) can incorporate H2O in the form of OH groups. H2O contents increase from granulite to eclogite, as documented in garnet from ~ 10 to ~ 50 µg/g H2O, pyroxene from ~ 50 to ~ 310 µg/g H2O, and granulitic plagioclase from ~ 10 to ~ 140 µg/g H2O. Bowl-shape profiles are characteristic for garnet and pyroxene with lower H2O contents in grain cores and higher at the rims, which suggest a prograde water influx into the NAMs. Omphacite displays a H2O content range from ~ 150 to 425 µg/g depending on the amount of hydrous phases surrounding the grain. The granulitic plagioclase first separates into a hydrous, more albite-rich plagioclase and isolated clinozoisite before being replaced by new fine-grained phases like clinozoisite, kyanite and quartz during ongoing fluid infiltration. Results indicate a twofold fluid influx with different mechanisms to act simultaneously at different scales and rates. Fast and more pervasive proton diffusion is recorded by NAMs that retain the major element composition of the granulite-facies equilibration where hydrogen decorates pre-existing defects in the crystal lattice and leads to OH increase. Contemporaneously, slower grain boundary-assisted aqueous fluid influx enables element transfer and results in progressive formation of new minerals, e.g., hydrous phases. Both mechanisms lead to bulk H2O increase from ~ 450 to ~ 2500 µg/g H2O towards the shear zone and convert the system from rigid to weak. The incorporation of OH groups reduces the activation energy for creep, promotes formation of smaller grain sizes (phase separation of plagioclase), and synkinematic metamorphic mineral reactions. These processes are part of the transient weakening, which enhance the sensitivity of the rock to deform.

Compressive creep properties and mechanisms of (Ti-Zr-Nb-Ta-Mo)C high entropy ceramics at high temperatures

The compressive creep properties of (Ti-Zr-Nb-Ta-Mo)C high-entropy ceramics (HEC) and TaC were studied at temperature range of 1400–1600 °C with stresses of 200–300 MPa. Microstructure changes were investigated by transmission electron microscopy (TEM). It is found that the HEC has a higher creep resistance than TaC at 1400–1600 °C. The steady-state creep rates of the HEC are 2.13 × 10−8–2.74 × 10−6 s−1. The stress exponent n of the HEC is 2.76 at 1600 °C and the creep activation energy Q is 486.3 kJ/mol at 200 MPa, higher than that of TaC. The creep mechanisms of the HEC are dislocation motion, grain boundary sliding and atom diffusion. Strain whorls, grain boundary gap and complex dislocation configuration were generated within the crept HEC. The Burgers vectors of dislocations in (Ti-Zr-Nb-Ta-Mo)C is identified to be a2[11̅0]. The increment of TaC grain size is about 2 times of that of (Ti-Zr-Nb-Ta-Mo)C at 1600 °C/200 MPa. The atom diffusion is less active in (Ti-Zr-Nb-Ta-Mo)C than in TaC because of sluggish lattice diffusion in the HEC.

Effect of Structure and Hydrogen on the Short-Term Creep of Titanium Ti-2.9Al-4.5V-4.8Mo Alloy.

In this paper, the effect of hydrogenation, in the amount of 0.15 wt.%, on the short-term creep of a titanium Ti-2.9Al-4.5V-4.8Mo alloy in fine-grained (FG) and ultrafine-grained (UFG) states is studied at 723 K. The UFG structure was formed by the method of pressing with the change of the deformation axis and gradual temperature decrease. Creep tests are performed under conditions of uniaxial tension at a constant load for the creep rates at an interval of (10−7 ÷ 10−6) s−1. The UFG alloy’s resistance to creep under the investigated conditions is revealed to be substantially lower than in the FG state. When hydrogen presents in the alloy in a solid solution, a 1.3–2.5-fold rise in the value of the steady-state creep rate for the hydrogenated FG and UFG alloys is observed. The creep of the non-hydrogenated FG and UFG alloys is described by the creep power law. The presence of dissolved hydrogen leads to a violation of the creep power law. The values of stress sensitivity indices, steady-state creep rate, and effective creep activation energy are determined. The relationships between the hydrogenation, structure, and creep mechanisms of the alloy at the steady-state are discussed.