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This study investigates the effect of pressure on diffusion creep of dry San Carlos and synthetic (prepared by sol-gel method) olivine. We prepared dry (water content <9 ppm wt) fine-grained (<1 μm grain size) olivine and deformed the samples (both San Carlos and sol-gel olivine) in the same sample assembly under high pressure (P = 2.9–8.8 GPa) and moderate temperatures (T = 980–1250 K) at a fixed strain rate. The evolution of the sample's strength was studied using radial X-ray diffraction from various diffraction planes. We found that San Carlos and sol-gel olivine show similar rheological behavior (when normalized to the same grain size). Stress estimated by the radial X-ray diffraction increases with time and initially shows similar values for all diffraction planes. In many cases, stress values start to depend on the diffraction planes in the later stage, and time dependence becomes minor. The microstructural observations show that grain size increases during an experiment. The results are interpreted using a theory of radial X-ray diffraction and the theoretical models of diffusion and dislocation creep. We conclude that the initial stage of deformation is by diffusion creep, but deformation in the later stage is by dislocation creep. For dislocation creep, our results are in reasonable agreement with previous low-temperature dislocation creep results after correcting the temperature effect. For diffusion creep, we obtain an activation volume of 7.0 ± 2.4 cm3/mol that is substantially smaller than the values reported on dislocation creep but agrees well with the results on grain growth. By comparing the present results on dry olivine with the previous results on wet (water-saturated) olivine, we found that water enhances diffusion creep but only modestly compared to dislocation creep. The difference in the pressure and water content dependence between diffusion and dislocation creep has an important influence on the dominant deformation mechanisms of olivine in the upper mantle. [ABSTRACT FROM AUTHOR]

To understand the dislocation creep behavior of clinopyroxene in the upper mantle, hot‐pressed diopside aggregates without predrying treatment were triaxially deformed under water‐unsaturated conditions at a confining pressure of 300 MPa, temperatures of 1323–1523 K, and strain rates of 10−6–10−4 s−1, using a Paterson gas‐medium apparatus. Fourier transform infrared measurements of the water contents of the samples before and after deformation revealed that water diffusion loss occurred during the deformation process. A simple diffusion model based on Fick's law was established to predict the variation in the water content with respect to time during deformation. Fitting the mechanical data with a power flow law yielded a stress exponent n of 4.3 ± 0.3, an activation energy Q of 427 ± 31 kJ/mol and a water content exponent r of 1.2 ± 0.2 for the dislocation creep of the diopside aggregates under water‐unsaturated conditions. When the flow law was extrapolated to anhydrous and water‐saturated conditions, the calculated flow strengths of the diopside aggregates were generally in agreement with the strengths determined directly by deformation experiments, but there also existed contribution from grain boundary water under water‐saturated conditions. The results of our study indicate that the strength of diopside or upper mantle clinopyroxene is comparable to the strength of olivine under anhydrous conditions but weaker than that of olivine under water‐saturated conditions in the dislocation creep regime. Therefore, diopside might dominate the rheological behavior in some clinopyroxene‐enriched and hydrous regions of the upper mantle. Plain Language Summary: As one of the major rock‐forming minerals in the upper mantle, clinopyroxene can accommodate more structural water than other major mantle minerals, and can be enriched in some regions of the upper mantle due to primary heterogeneity, metasomatism and crustal material recycling. Therefore, the effect of water on the flow behavior of clinopyroxene is important for understanding many geodynamical processes related to the upper mantle. In this study, we investigated the dislocation creep of diopside under water‐unsaturated conditions by triaxial compression experiments at P–T conditions of 300 MPa and 1323–1523 K. Fitting the mechanical data including flow stress, strain rate and water content, allowed us to establish a flow law for diopside aggregates, which quantifies the effect of dissolved water with a water content exponent of 1.2 ± 0.2. Our mechanical results reveal that diopside is comparable in strength to olivine under anhydrous conditions but weaker than olivine under water‐saturated conditions in the dislocation creep regime. This implies that diopside/clinopyroxene might dominate flow behavior and hence geodynamical processes in some hydrous and clinopyroxene‐enriched regions in the upper mantle. Key Points: Dislocation creep of diopside aggregates was investigated by triaxial compression experiments under water‐unsaturated conditionsThe flow law of water‐unsaturated diopside aggregates was established with the water‐weakening effect being quantified for the first timeDiopside displays a more remarkable water‐weakening effect than olivine and a lower strength than olivine under water‐saturated conditions [ABSTRACT FROM AUTHOR]

The asthenosphere is commonly defined as an upper mantle zone with low velocities and high attenuation of seismic waves, and high electrical conductivity. These observations are usually explained by the presence of partial melt, or by a sharp contrast in the water content of the upper mantle. Low viscosity asthenosphere is an essential ingredient of functioning plate tectonics. We argue that a substantial component of asthenospheric weakening is dynamic, caused by dislocation creep at the base of tectonic plates. Numerical simulations of subduction show that dynamic weakening scales with the surface velocity both below the subducting and the overriding plate, and that the viscosity decrease reaches up to two orders of magnitude. The resulting scaling law is employed in an apriori estimate of the lateral viscosity variations (LVV) below Earth's oceans. The obtained LVV help in explaining some of the long‐standing as well as recent problems in mantle viscosity inversions. Plain Language Summary: The motion of lithospheric plates at the Earth's surface is enabled by a weak underlying layer—the asthenosphere. The origin of this low viscosity layer is still subject of discussion. Presence of water or partial melt were proposed as possible reasons of its reduced viscosity. Another mechanism that may lead to weakening is non‐linear deformation. Rheological description of asthenospheric material includes dislocation creep, a deformation mechanism that depends on the velocity contrast between the lithospheric plate and underlying mantle—the faster the plates are, the weaker the underlying layer becomes and vice versa. Here we argue that a substantial component of asthenospheric weakening is dynamic, caused by this deformation mechanism. We evaluate numerical models of subduction including dislocation creep and derive a relation between the surface velocity of oceanic plates and the magnitude of the underlying asthenospheric viscosity. This allows us to estimate how the viscosity varies under different oceanic plates on Earth, which is otherwise hard to constrain. Our results indicate that the asthenosphere below the Pacific plate should be particularly weak. Key Points: Substantial component of asthenospheric weakening is dynamic, caused by dislocation creep at the base of tectonic platesDynamic weakening scales with the surface velocity both below the subducting and the overriding plateThe resulting scaling law is employed in an apriori estimate of the lateral viscosity variations below Earth's oceans [ABSTRACT FROM AUTHOR]

Synthesized polycrystalline samples composed of enstatite and olivine with different volumetric ratios were deformed in compression under anhydrous conditions in a Paterson gas‐medium apparatus at 1150–1300°C, an oxygen fugacity buffered at Ni/NiO, and confining pressures of 300 or 450 MPa (protoenstatite or orthoenstatite fields). Mechanical data suggest a transition from diffusion to dislocation creep with increasing differential stress for all compositions. Microstructural analyses by optical and scanning electron microscopy reveal well‐mixed aggregates and homogeneous deformation. Crystallographic preferred orientations measured by electron backscatter diffraction are consistent with activation of the slip systems (010)[100] and (010)[001] for olivine and (100)[001] and (010)[001] for enstatite, as expected at these conditions. Nonlinear least‐squares fitting to the full data set from each experiment allowed the determination of dislocation creep flow laws for the different mixtures. The stress exponent is 3.5 for all compositions, and the apparent activation energies increase slightly as a function of enstatite volume fraction. Within the limits of experimental uncertainties, all two‐phase aggregates have strengths that lie between the uniform strain rate (Taylor) and the uniform stress (Sachs) bounds calculated using the dislocation creep flow laws for olivine and enstatite. Calculation of the Taylor and Sachs bounds at strain rate and temperature conditions expected in nature (but not extrapolating in pressure) indicates that using the dislocation creep flow law for monomineralic olivine aggregates provides a good estimate of the viscosity of olivine‐orthopyroxene rocks deforming by dislocation creep in the deeper lithosphere and asthenosphere. Plain Language Summary: The rheology of Earth's upper mantle is generally modeled using mechanical flow laws determined for aggregates composed only of olivine minerals, in spite of the polyphase nature of mantle rocks. In this study, we investigated the effect of phase volume proportions on the high‐temperature deformation properties of aggregates composed of the two most abundant minerals in the upper mantle, olivine and enstatite. The samples were deformed under dry conditions in triaxial compression at 1150–1300°C, under oxygen fugacity fixed at the Ni/NiO solid buffer, and confining pressures of 300 or 450 MPa, at conditions where enstatite has two different crystallographic structures. At both pressures, in the dislocation creep regime, where deformation occurs mostly by the motion of dislocations along slip planes within mineral grains, the strengths of all the two‐phase mixtures lie between the uniform strain rate and the uniform stress bounds, which assume iso‐strain and iso‐stress conditions, respectively, in all the grains comprising each aggregate. Extrapolating these bounds to temperatures and strain rates expected in nature indicates that the viscosity of mantle rocks can be modeled adequately with the dislocation creep flow law for olivine. Key Points: Polycrystalline samples of enstatite and olivine with different volumetric ratios were deformed in compression at high temperatureAll two‐phase aggregates have strengths in dislocation creep that lie between the uniform stress and uniform strain rate boundsIn dislocation creep the flow law for olivine provides a good estimate of the viscosity of olivine‐pyroxene rocks under natural conditions [ABSTRACT FROM AUTHOR]

Subduction zones are fundamental features of Earth's mantle convection and plate tectonics, but mantle flow and pressure around slabs are poorly understood because of the lack of direct observational constraints on subsurface flow. To characterize the linkages between slabs and mantle flow, we integrate high‐resolution representations of Earth's lithosphere and slabs into a suite of global mantle convection models to produce physically plausible present‐day flow fields for Earth's mantle. We find that subduction zones containing wide, thick, and long slabs dominate regional mantle flow in the neighboring regions and this flow conforms to patterns predicted by simpler regional subduction models. These subduction zones, such as Kuril‐Japan‐Izu‐Bonin‐Mariana, feature prismatic poloidal flow coupled to the downgoing slab that rotates toward toroidal slab‐parallel flow near the slab edge. However, other subduction zones, such as Sumatra, deviate from this pattern because of the competing influence of other slabs or longer‐wavelength mantle flow, showing that upper mantle flow can link separate subduction zones and how flow at subduction zones is influenced by broader scale mantle flow. We find that the non‐linear dislocation creep reduces the coupling between slab motion and asthenospheric flow and increases the occurrence of non‐ideal flow, in line with inferences derived from seismological constraints on mantle anisotropy. Key Points: We embed detailed representations of Earth's subduction zones within global convection models to predict mantle flow at subduction zonesSome subduction zones conform to ideal models, with upper mantle flow coupled to downgoing slabs, while others deviate from this patternDislocation creep partially decouples upper mantle flow from slab motion, increasing the fraction of non‐ideal flow [ABSTRACT FROM AUTHOR]

High-temperature resistant material is becoming increasingly significant in numerous industrial applications, including aerospace and automotive use, to increase fuel efficiency and decrease CO2 emissions. Metals with nanoparticles to form metal matrix nanocomposite (MMNCs) are promising to enhance metals for structural/industrial applications at high temperatures. This review provides a comprehensive insight into the nanoparticle effect on the creep behavior of metals and alloys. Firstly, it discusses the different creep mechanisms that are activated in response to the creep strain according to the deformation parameters (temperature, load, and microstructure). The most significant creep mechanisms vary according to the grain shape accommodation mechanism required to facilitate the grain boundary sliding into accommodation by diffusion and accommodation by intragranular dislocation activity. Secondly, it reviews the threshold stress contribution in metal matrix composites as well as the load transfer phenomenon, which enhances creep resistance of metals with nanoparticles. Thirdly, the nanoparticles' influences on the creep behavior of metals, especially Al, Mg, Ti, Zn, and solder alloys, are reviewed in detail. Finally, a summary of this review and the future aspects associated with the behavior of metals with nanoparticles at high temperatures are provided.

Abstract Subduction zones are fundamental features of Earth's mantle convection and plate tectonics, but mantle flow and pressure around slabs are poorly understood because of the lack of direct observational constraints on subsurface flow. To characterize the linkages between slabs and mantle flow, we integrate high‐resolution representations of Earth's lithosphere and slabs into a suite of global mantle convection models to produce physically plausible present‐day flow fields for Earth's mantle. We find that subduction zones containing wide, thick, and long slabs dominate regional mantle flow in the neighboring regions and this flow conforms to patterns predicted by simpler regional subduction models. These subduction zones, such as Kuril‐Japan‐Izu‐Bonin‐Mariana, feature prismatic poloidal flow coupled to the downgoing slab that rotates toward toroidal slab‐parallel flow near the slab edge. However, other subduction zones, such as Sumatra, deviate from this pattern because of the competing influence of other slabs or longer‐wavelength mantle flow, showing that upper mantle flow can link separate subduction zones and how flow at subduction zones is influenced by broader scale mantle flow. We find that the non‐linear dislocation creep reduces the coupling between slab motion and asthenospheric flow and increases the occurrence of non‐ideal flow, in line with inferences derived from seismological constraints on mantle anisotropy.

This work deals with a physically based creep model of the martensitic 9% Cr-steel P91. In particular, we quantitatively study the effect of precipitates located in the subgrain interior on the creep curve and the lifetime of the material. Our creep model is capable of simulating the microstructural evolution parallel to creep curves and has been further developed towards predicting time-to-rupture (TTR) diagrams. We demonstrate how the number density, size and shape of the precipitates in the subgrain interior affect the creep curve and thus the lifetime of the material. Three according parameter studies on the material P91 compare microstructures with/without precipitates and/or assuming specific shapes by introducing aspect ratios. These simulations can be used as a basis for further development of materials with regard to the precipitation phases. [ABSTRACT FROM AUTHOR]

Amphibole‐rich rocks constitute significant components of the mid‐ to lower continental crust, particularly in active orogens characterized with thick and hot crusts. Nevertheless, experimental data on their viscosity remain scarce. We conducted axial compression deformation experiments on synthetic amphibolites under temperature and pressure conditions resembling deep sections of overthickened crust. A novel flow law for a calcic‐amphibole‐rich rock (80% amphibole +20% garnet) in the dislocation creep regime is derived from these experiments. Contrary to common assumptions, our results reveal that calcic‐amphibolite is 1‐2 orders of magnitude weaker than plagioclase‐rich amphibolite, granulite, or gabbro. A calcic‐amphibole‐rich, low viscosity deep crust may not only support the "channel flow" model proposed for the Tibetan Plateau but also explain the observed high crustal seismic anisotropy in the region. Plain Language Summary: The rarity of earthquakes despite strong deformation in the deep crust of mountain belts produced by continental collisions, such as the Himalayas, implies that the deep continental crust is rather weak, easily dissipating imposed stresses by ductile deformation. The major components of the orogenic lower crust are plagioclase, amphibole, pyroxene, and garnet. Experimental data on the ductile deformation of coarse‐grained amphibole‐rich rocks was until now missing. We present new deformation experiments, from which we derive a flow law for an amphibolitic aggregate (80% amphibole +20% garnet). These data show high volumes of amphibole should result in high ductility and, hence, low strength in the deep crust, which should be considered when modeling the mechanical behavior of the lower crust during continental collisional. An amphibole‐rich deep crust also accounts for the high crustal seismic anisotropy in southern Tibet. Key Points: Experimentally based flow law for dislocation creep of calcic‐amphibole‐rich (80 wt.%) rocksA calcic‐amphibole‐rich deep crust supports the "channel flow" model in southern TibetExperimental amphibole crystal preferred orientations (CPO) may explain deep crustal seismic anisotropy [ABSTRACT FROM AUTHOR]

In applications critical to the geological, materials, and engineering sciences, deformation occurs at strain rates too small to be accessible experimentally. Instead, extrapolations of empirical relationships are used, leading to epistemic uncertainties in predictions. To address these problems, we construct a theory of the fundamental processes affecting dislocations: storage and recovery. We then validate our theory for olivine deformation. This model explains the empirical relationships among strain rate, applied stress, and dislocation density in disparate laboratory regimes. It predicts the previously unexplained dependence of dislocation density on applied stress in olivine. The predictions of our model for Earth conditions differ from extrapolated empirical relationships. For example, it predicts rapid, transient deformation in the upper mantle, consistent with recent measurements of postseismic creep. [ABSTRACT FROM AUTHOR]

Transient creep may play an important role in short‐term (small strain) time‐dependent deformation such as the post‐glacial rebound and the post‐seismic deformation. Among various mechanisms of transient creep, I focus on inter‐granular transient dislocation creep caused by plastic anisotropy in each grain. The essence of this process is the evolution of strain accommodation across grain‐boundaries from elastic to plastic (viscous) accommodation. Extending the previous studies, I show that evolution of strain accommodation leads to two asymptotic behaviors: at small strain, strain accommodation across grain‐boundaries is by elastic deformation and the strength is controlled largely by that of the soft slip system. In contrast, when strain exceeds a threshold value, strain accommodation occurs mostly by plastic deformation and the strength is largely controlled by that of the hard slip system. The model predicts that the threshold strain is on the order of elastic strain (depending also on the degree of plastic anisotropy) and the strength contrast between transient and steady‐state creep depends on the degree of plastic anisotropy of the crystal. Consequently, transient creep plays an important role in these short‐term time‐dependent processes. Since the soft and the hard slip systems have different properties, applications of rheological properties inferred from short‐term time‐dependent deformation to long‐term geodynamic processes needs to be made with a great care. Plain Language Summary: Rheological properties of Earth's interior can be inferred from the short‐term time‐dependent deformation such as the post‐glacial crustal motion or the post‐seismic deformation. These phenomena involve small strain on the order of elastic strain. In contrast, deformation in the geological time scale such as mantle convection involves much larger strain. In contrast to elastic deformation, plastic deformation of solids is not instantaneous and hence strain dependent and consequently the link between these two types of deformation is not straightforward. This paper examines the link between these two types of deformation with an emphasis on deformation of a polycrystalline aggregate made of plastically anisotropic crystals such as olivine. It is shown that those two types of deformation reflect different types of defect motion and therefore the rheological properties inferred from short‐term time‐dependent deformation are different from those relevant to long‐term geological deformation. Key Points: Plastic deformation at small and large strain is controlled by different rheological propertiesThe threshold strain between transient and steady‐state deformation in on the order of elastic strainThe strength contrast between two regimes is determined by the plastic anisotropy of minerals [ABSTRACT FROM AUTHOR]

Creep properties of ZK60 alloy and ZK60/SiCw composite have been investigated after extrusion and precipitation hardening by accelerated creep test. Creep tests were conducted at 150 °C in the stress range of 10–120 MPa. At low stresses, the stress exponents of 1.93 and 1.75 were obtained for the unreinforced alloy and the composite, respectively. Stress exponents of the unreinforced alloy and composite sample were 5.82 and 7.07, respectively, at high stresses. The creep mechanism changed by increasing the stress from grain boundary sliding (GBS) to dislocation creep due to the fact that the average true creep activation energy changed from 55 to 95.06 kJ/mol. Based on the microstructural observations, at low stresses, the grain refinement induced by twinning caused the GBS mechanism. However, at high stresses, slip changed from basal planes to the pyramidal secondary slip system which was associated with increase in twin density. Examination of the fracture surfaces revealed that cavity nucleation in the grain corners and around the precipitates was the main reason for creep failure. [ABSTRACT FROM AUTHOR]

The creep behavior of ice is believed to be dominated by dislocation creep at high stresses (>1 MPa) and high homologous temperatures (>0.9). Dislocation creep of ice is often described by the Glen law, ε̇=Bσn, with a canonical value of n= 3, and is independent of grain size. Laboratory studies of ice deformation at elevated pressures, however, suggest that dislocation creep of ice is characterized by a value of n≈ 4. Here, we deformed ice samples with initial grain sizes of 0.23 and 0.63 mm, at 263 K and confining pressures of 10 or 20 MPa. The experiments yield a value of n= 3.6 using the peak stresses obtained at a strain ε≈ 0.02, and reveal a marked dependence of the stress on initial grain size. The nominally constant flow stresses at larger strains, up to 0.2, yield a value of n= 3.9, and no influence of the initial grain size on the stress is observed. The lower value of n at peak stresses and the decrease in peak stress with decreasing initial grain size suggest a contribution to the strain rate from a grain‐size‐sensitive process, such as grain boundary sliding. Our data yield two flow laws, one for isotropic ice and one for highly anisotropic ice deformed to large strains, the latter of which may be used to model flow in high‐strain natural environments. Plain Language Summary: The flow of ice in glaciers and ice sheets is believed to be dominated by the dislocation creep mechanism, in which the strain rate depends on stress raised to a power n and is insensitive to grain size. A canonical value of n=3 is widely used; however, laboratory experiments often reveal a value of n=4. To reconcile this discrepancy, we carried out laboratory deformation experiments on ice and analyzed mechanical data at different strains. Data obtained at small strains, representing a homogeneous, isotropic microstructure, reveal n=3.6. Data obtained at larger strains, representing an anisotropic microstructure, reveal n=3.9. More importantly, a grain‐size dependence was found in the putative "dislocation creep" regime, suggesting that grain boundary sliding contributes to the deformation. This discovery implies that the rheological behaviors of ice need to be reinvestigated. Key Points: Peak‐stress data from constant‐strain‐rate tests reveal a stress exponent of 3.6, for ice with a homogeneous, isotropic microstructureFlow‐stress data from constant‐strain‐rate tests reveal a stress exponent of 3.9, for ice with an anisotropic microstructureA grain‐size dependence is observed in the peak‐stress data, suggesting a contribution to the strain rate from grain boundary sliding [ABSTRACT FROM AUTHOR]

The plastic deformation of olivine has been studied for decades. However, the precise role of grain boundaries during deformation in, for example, the dislocation‐accommodated grain‐boundary sliding regime, remains poorly understood. Specifically, we lack knowledge regarding the manner in which grain boundaries interact with other defects, such as dislocations, during deformation. To investigate the interaction of dislocations and grain boundaries, we analyzed the structure and distribution of grain boundaries in a polycrystalline aggregate of Fo50 deformed in torsion (Hansen, Zimmerman, Dillman, & Kohlstedt, 2012, https://doi.org/10.1016/j.epsl.2012.04.016). We characterized the microstructure of the aggregate using electron‐backscatter diffraction and transmission electron microscopy in three perpendicular directions. An increase in plastic strain is associated with the development of a strong crystallographic preferred orientation and a grain‐boundary plane distribution that evolves from a uniform distribution to one dominated by (010)‐type planes. We use the m' factor, to evaluate the potential for transmission of dislocations across grain boundaries based on the relative orientations of slip systems between neighboring grains. With progressive deformation, our analysis indicates an increase in abundance of apparently slip‐transparent boundaries until moderate strains (γ = 4) are reached. Based on these observations, we propose that specific types of grain boundaries are created by dislocation activity and that the input of dislocations into grain boundaries facilitates grain‐boundary sliding. Our results provide insight into the microphysics of olivine deformation and highlight the importance of the coupled study of inter‐ and intragranular mechanisms during rock deformation. Key Points: We investigated the interaction of grain boundaries and dislocations in olivine deformed experimentallyOur results suggest that high‐angle boundaries evolve from low‐angle grain boundaries by absorbing dislocations. This process geometrically requires adjacent grains to slide past each otherOur results provide evidence that the rate of deformation in the dislocation‐accommodated grain‐boundary sliding regime is controlled by the rate of assimilation of dislocations into the grain boundaries [ABSTRACT FROM AUTHOR]

Transient creep occurs during geodynamic processes that impose stress changes on rocks at high temperatures. The transient is manifested as evolution in the viscosity of the rocks until steady‐state flow is achieved. Although several phenomenological models of transient creep in rocks have been proposed, the dominant microphysical processes that control such behavior remain poorly constrained. To identify the intragranular processes that contribute to transient creep of olivine, we performed stress‐reduction tests on single crystals of olivine at temperatures of 1,250°C–1,300°C. In these experiments, samples undergo time‐dependent reverse strain after the stress reduction. The magnitude of reverse strain is ∼10−3 and increases with increasing magnitude of the stress reduction. High‐angular resolution electron backscatter diffraction analyses of deformed material reveal lattice curvature and heterogeneous stresses associated with the dominant slip system. The mechanical and microstructural data are consistent with transient creep of the single crystals arising from accumulation and release of backstresses among dislocations. These results allow the dislocation‐glide component of creep at high temperatures to be isolated, and we use these data to calibrate a flow law for olivine to describe the glide component of creep over a wide temperature range. We argue that this flow law can be used to estimate both transient creep and steady‐state viscosities of olivine, with the transient evolution controlled by the evolution of the backstress. This model is able to predict variability in the style of transient (normal vs. inverse) and the load‐relaxation response observed in previous work. Plain Language Summary: At high temperatures and over long timescales, rocks can flow in a similar manner to viscous fluids. If the stresses in Earth that drive the flow of rocks change suddenly in magnitude (e.g., after an earthquake), the viscosity of the rock changes and evolves. This evolution in viscosity influences how quickly the stress is relaxed or starts to build up again. We do not currently understand the underlying physics controlling this evolution in viscosity, which reduces our ability to predict and evaluate many aspects of flow in Earth's interior. Key Points: Stress‐reduction experiments reveal that transient creep of olivine is controlled by dislocation interactionsEquations describing deformation due to glide of dislocations and the buildup of backstress are applicable at low and high temperaturesOur proposed framework for dislocation creep allows prediction of yield stresses, strain hardening, transient creep, and load relaxation [ABSTRACT FROM AUTHOR]

We present a flow law for dislocation‐dominated creep in wet quartz derived from compiled experimental and field‐based rheological data. By integrating the field‐based data, including independently calculated strain rates, deformation temperatures, pressures, and differential stresses, we add constraints for dislocation‐dominated creep at conditions unattainable in quartz deformation experiments. A Markov Chain Monte Carlo (MCMC) statistical analysis computes internally consistent parameters for the generalized flow law: ε˙ = AσnfH2Ore−(Q+VP)/RT. From this initial analysis, we identify different effective stress exponents for quartz deformed at confining pressures above and below ∼700 MPa. To minimize the possible effect of confining pressure, compiled data are separated into "low‐pressure" (<560 MPa) and "high‐pressure" (700–1,600 MPa) groups and reanalyzed using the MCMC approach. The "low‐pressure" data set, which is most applicable at midcrustal to lower‐crustal confining pressures, yields the following parameters: log(A) = −9.30 ± 0.66 MPa−n−r s−1; n = 3.5 ± 0.2; r = 0.49 ± 0.13; Q = 118 ± 5 kJ mol−1; and V = 2.59 ± 2.45 cm3 mol−1. The "high‐pressure" data set produces a different set of parameters: log(A) = −7.90 ± 0.34 MPa−n−r s−1; n = 2.0 ± 0.1; r = 0.49 ± 0.13; Q = 77 ± 8 kJ mol−1; and V = 2.59 ± 2.45 cm3 mol−1. Predicted quartz rheology is compared to other flow laws for dislocation creep; the calibrations presented in this study predict faster strain rates under geological conditions by more than 1 order of magnitude. The change in n at high confining pressure may result from an increase in the activity of grain size sensitive creep. Plain Language Summary: At conditions present in the middle and lower crust, rocks generally deform by ductile processes, instead of brittle fracture which is dominant closer to the Earth's surface. As a volumetrically dominant mineral in many crustal rocks, quartz is commonly thought to control the overall strength of the crust. Much of our understanding of how quartz behaves comes from deformation experiments; but to produce the same type of deformation we observe in middle‐ or lower‐crustal rocks, experiments must be performed at conditions different from what natural rocks experience. The difference poses a potential issue in extrapolating experimental results to natural conditions. Here, we integrate mechanical data from deformation experiments and naturally deformed rocks to arrive at a quantitative relationship that relates material properties and deformation conditions to the rate at which quartz deforms. Because we add constraints at deformation conditions not attainable in experiments, we likely produce a more accurate representation of ductile quartz behavior. Our results indicate that quartz‐rich rocks in the middle and lower crust deform at least an order of magnitude faster than previous predictions, which has important implications for crustal strength, geodynamic modeling, and potentially for the loading of seismically active faults. Key Points: We use Bayesian statistical analysis on compiled rock deformation data to produce a flow law for dislocation creep in wet quartzThis analysis indicates a low sensitivity to water fugacity, while other parameters are broadly consistent with existing valuesUnder geological conditions, wet quartz aggregates deform faster than is predicted by existing flow laws [ABSTRACT FROM AUTHOR]

To explore the rheology of dolomite and investigate recent findings regarding the so‐called inversion of activation energy between dislocation and diffusion creep, we compressed medium‐grained Fangshan dolomite (113 ± 42 µm) at effective confining pressures of 50–300 MPa, temperatures of 27°C–900°C, and strain rates of 10−6 to 2 × 10−4 s−1 using a Paterson gas‐medium apparatus. Two end‐member deformation regimes with corresponding diagnostic flow laws and microstructures were identified. At temperatures ≤500°C, low‐temperature plasticity (LTP), which is characterized by microstructures of predominant abrupt undulatory extinctions and f‐twins, was determined to dominate the deformation of Fangshan dolomite. The corresponding flow behavior can be described by anε˙=ε˙0×exp(α×σ) with α=0.0806±0.0078 and lnε˙0=−76.66±6.24 (Regime 1). At temperatures ≥800°C, dislocation creep, which shows characteristic microstructures of smooth undulating extinction and new recrystallized grains, dominated the deformation of Fangshan dolomite. The corresponding flow behavior can be expressed by a power law equation, ε˙=Aσnexp(−QRT) with n=4.75±0.58, Q=436±54kJ/mol, and logA=3.48±1.41(Regime 2). At temperatures between ∼500 and 800°C, a transition regime between LTP and dislocation creep was identified (Regime 3) with the dependence of flow stress on strain rate increasing gradually with increasing temperature. When extrapolated to natural conditions, our flow law of dislocation creep for dolomite in combination with that of diffusion creep reported by Davis et al. (2008) suggests that the dislocation creep regime of dolomite is limited to a relatively narrow region of high temperature and relatively high stress, whereas the diffusion creep regime dominates the deformation of dolomite in tectonic settings with low stress levels. Key Points: Low‐temperature plasticity and dislocation creep of dolomite were recognized at T ≤ 500°C and T ≥ 800°C, respectivelyStress exponent (n = 4.75) and activation energy (Q = 436 kJ/mol) were determined for the dislocation creep of Fangshan dolomiteQ for dislocation creep is not only higher than that for diffusion creep, but also is comparable to the values of calcite and magnesite [ABSTRACT FROM AUTHOR]

Abstract Subduction channels are commonly occupied by deformed and metamorphosed basaltic rocks, together with clastic and pelagic sediments, which form a zone up to several kilometers thick to depths of at least 40 km. At temperatures above ~ 350 °C (corresponding to depths of > 25–35 km), the subduction zone undergoes a transition to aseismic behavior, and much of the relative motion is accommodated by ductile deformation in the subduction channel. Microstructures in metagreywacke suggest deformation occurs mainly by solution-redeposition creep in quartz. Interlayered metachert shows evidence for dislocation creep at relatively low stresses (8–13 MPa shear stress). Metachert is likely to be somewhat stronger than metagreywacke, so this value may be an upper limit for the shear stress in the channel as a whole. Metabasaltic rocks deform mainly by transformation-assisted diffusional creep during low-temperature metamorphism and, when dry, are somewhat stronger than metachert. Quartz flow laws for dislocation and solution-redeposition creep suggest strain rates of ~ 10−12 s−1 at 500 °C and 10 MPa shear stress: this is sufficient to accommodate a 100 mm/yr. convergence rate within a 1 km wide ductile shear zone. The up-dip transition into the seismic zone occurs through a region where deformation is still distributed over a thickness of several kilometers, but occurs by a combination of microfolding, dilational microcracking, and solution-redeposition creep. This process requires a high fluid flux, released by dehydration reactions down-dip, and produces a highly differentiated deformational fabric with alternating millimeter-scale quartz and phyllosilicate-rich bands, and very abundant quartz veins. Bursts of dilational microcracking in zones 100–200 m thick may cause cyclic fluctuations in fluid pressure and may be associated with episodic tremor and slow slip events. Shear stress estimates from dislocation creep microstructures in dynamically recrystallized metachert are ~ 10 MPa.

Stress and temperature dependence of strain rates in dislocation-mediated plasticity is usually associated with the depinning process. This is an escape-from-the-well process, thus such dependence is of Arrhenius type, exp − Δ G / T. Function exp − Δ G / T arises as an asymptotic approximation in the escape-from-the-well problem as T → 0 or Δ G / T → ∞. The activation energy Δ G is usually accepted to be a linear function of stress σ. To describe stress–strain curves at high strain rates, Langer, Bouchbinder and Lookman (Acta Mater., 2010, 58, 3718–3732) successfully employed a double-exponential function exp − T p / T exp − σ / σ T or Δ G = T p exp − σ / σ T , with two parameters, T p and σ T. Parameter σ T was associated with the flow stress. The major difference from the usual exponential dependence is the existence of an upper bound for strain rates no matter how high stresses are. For large stresses, the power Δ G / T vanishes and the question arises as to whether the double-exponential stress dependence is consistent with the escape-from-the-well problem. In this paper, we give a positive answer to this question. The key point is that the boundedness of strain rates is caused by the boundedness of dislocation velocities: dislocation velocities cannot exceed the shear wave speed. Accordingly, the velocity–force relation is nonlinear. Incorporation of such nonlinear relation into the escape-from-the-well problem results in a strain rate–stress dependence which has a form of a double-exponential curve. [ABSTRACT FROM AUTHOR]

The present investigation deals with the microstructural modification following the Bi + Sr additions to the squeeze-cast AZ91 alloy and its effect on impression creep response. The Bi + Sr additions form the Al4Sr and Sr2Bi phases besides the α-Mg and β-Mg17Al12 phases, and improves creep resistance of the AZ91 alloy. The AZ91 + 1.0Bi + 0.5Sr alloy reveals the best creep resistance among the alloys. The stress exponent and the activation energy values of all the alloys are in the range of 4–7 and 100.2–112.7 kJ mol−1, respectively, depicting the pipe diffusion-controlled dislocation creep is the governing creep mechanism. The post-creep microstructural study confirms several dislocations pile-ups around the Al4Sr and Sr2Bi phases resulting in improved creep resistance of the modified AZ91 alloys. [ABSTRACT FROM AUTHOR]

To investigate the effect of water on the rheological properties of enstatite, we have conducted triaxial compressive creep experiments on enstatite aggregates using a gas medium apparatus at a confining pressure of 300 MPa and temperatures of 1373–1473 K under both water‐saturated and anhydrous conditions. Samples were mainly deformed in the dislocation creep regime; analyses of mechanical data yield activation energies of 603 and 567 kJ/mol for hydrous and anhydrous conditions, respectively. Under similar differential stress and temperature, the creep rate of enstatite under water‐saturated conditions is a factor of ~50 greater than that under anhydrous conditions. Based on a comparison of creep strength between olivine and enstatite in the dislocation creep regime, our results suggest that, at least under limited pressure and temperature conditions, enstatite is weaker than olivine. The results from this study provide a solid database for modeling geological processes occurring within Earth's interior. Key Points: The effect of water on the dislocation creep of enstatite was quantified for the first timeFlow laws for enstatite under both water‐saturated and anhydrous conditions were establishedUnder hydrous conditions, the enstatite creep rate is a factor of ~50 greater than that under anhydrous conditions [ABSTRACT FROM AUTHOR]

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The objective of this paper is to assess the creep law of natural salt in a small deviatoric stress range. In this range, creep is suspected to be much faster than what is predicted by most constitutive laws used in the cavern and mining industries. Five 2-year, multistage creep tests were performed with creep-testing devices set in a gallery of the Altaussee mine in Austria to take advantage of the very stable temperature and humidity conditions in this salt mine. Each stage was 8-month long. Dead loads were applied, and vertical displacements were measured through gages that had a resolution of 12.5 nm. Loading steps were 0.2, 0.4, and 0.6 MPa, which are much smaller than the loads that are usually applied during creep tests (5–20 MPa). Five salt samples were used: two samples were cored from the Avery Island salt mine in Louisiana, United States; two samples were cored from the Gorleben salt mine in Germany; and one sample was cored from a deep borehole at Hauterives in Drôme, France. During these tests, transient creep is relatively long (6–10 months). Measured steady-state strain rates ( ε ˙ = 10−13–10−12 s−1) are much faster (by 7–8 orders of magnitude) than those extrapolated from relatively high-stress tests (σ = 5–20 MPa). When compared to n = 5 within the high-stress domain for Gorleben and Avery Island salts, a power-law stress exponent within the low-stress domain appears to be close to n = 1. These results suggest that the pressure solution may be the dominant deformation mechanism in the steady-state regime reached by the tested samples and will have important consequences for the computation of caverns or mines behavior. This project was funded by the Solution-Mining Research Institute. [ABSTRACT FROM AUTHOR]

We report that high strain deformation in ductile shear zones greatly enhanced high to ultrahigh pressure (HP-UHP) metamorphic transformation of a two-pyroxene granite in deep continental subduction zone. Integrated structural and petrologic study of rocks from the two-pyroxene granite block and shear zones in the block in the UHP shear complex at Yangkou, Sulu UHP metamorphic belt, reveals that the undeformed granite has preserved granitic texture and primary mineral assemblage of hypersthene, augite, plagioclase, K-feldspar, quartz and biotite. Metamorphic transformation of the granite is limited to grain boundaries due to low fluid activity and low reaction kinetics. In contrast rocks from the shear zones have a mylonitic texture and most of the primary minerals are transformed to HP-UHP phase minerals. Dislocation creep accommodated by dynamic recrystallization results in significant grain size reduction of the primary minerals in the mylonite, which increases reaction surface area and enhances metamorphic transformation. Fluid released by biotite transformation to HP-UHP minerals leads to diffusion mass transfer that greatly accelerates the metamorphic process in the shear zone. Deformation conditions are estimated ca 546 °C and pressure 2.5 GPa. • Undeformed pyroxene granite survived ultrahigh pressure metamorphism. • Deformation in shear zones in the granite enhanced metamorphic transformation. • Dislocation creep and fluid-enhanced diffusion creep are deformation mechanisms. • Deformation takes place at a minimum temperature 546 °C and pressure 2.5 GPa. [ABSTRACT FROM AUTHOR]

It is shown that the dislocation creep strength of a solid-solution-strengthened Ni-base superalloy is critically dependent upon the cooling rate from the annealing temperature. This behavior is correlated with the effect of cooling rate on the morphology of M23C6-type carbide which precipitates at grain boundaries during cooling. Experiment shows that the creep strength is enhanced by rapid cooling which results in discrete carbide particles with irregular shape following a zigzag path along the grain boundaries. However, the creep strength is degraded as the morphology is changed into lamellar type by a discontinuous grain boundary reaction which occurs during slower cooling rates. Due to the transition from the discrete particle into the lamellar morphology, plastic deformation becomes more localized alongside the grain boundaries which accelerates the creep rate leading to premature intergranular failure. On the other hand, it is also shown that the stacking fault energy of the alloy increases with temperature which can allow dislocations to rearrange themselves into subgrain structure with relative ease during steady-state creep. [ABSTRACT FROM AUTHOR]

Grain size reduction via dynamic recrystallization (DRX) is intimately related to the softening of crystalline materials under an applied stress, and plays a role in numerous geodynamic processes that involve strain‐weakening feedbacks. Despite its importance, few studies have provided empirical constraints on the rates of DRX and grain size reduction. Here we examine DRX rates using the Johnson‐Mehl‐Avrami‐Kolmogorov theory for phase transformation kinetics. Our analysis uses a compilation of published and newly‐derived DRX data from experimental studies on a wide range of geologic and engineering materials. We find that DRX rates are strongly dependent on the homologous temperature at which deformation occurs, with DRX occurring over a broad range of shear strains (0.01 < γ < 200), and more rapidly with increasing homologous temperature. Moreover, the experimental data can be described by a single fit to a modified form of the Avrami equation that incorporates homologous temperature dependence, implying that, to first order, DRX rates are largely material independent. We also examine the influence of stress, deformation work rate, crystal orientation, and initial grain size on DRX rates and compare DRX data from experimental and natural conditions. Overall, we show that microstructures can evolve over long transient intervals, and provide a framework for determining DRX kinetics from empirical data, which can ultimately be incorporated into geodynamic models of grain size evolution in the Earth. Key Points: Dynamic recrystallization (DRX) rates are examined for a compilation of experimental data using the Avrami theory for transformation kineticsDRX rates are strongly dependent on homologous temperature, with DRX occurring over a broad range of shear strains (0.01 < γ < 200)Experimental DRX data can be fit using a single, material‐independent relationship that incorporates homologous temperature [ABSTRACT FROM AUTHOR]

As Nickel (Ni) is the base of important Ni-based superalloys for high-temperature applications, it is important to determine the creep behavior of its nano-polycrystals. The nano-tensile properties and creep behavior of nickel polycrystalline nanopillars are investigated employing molecular dynamics simulations under different temperatures, stresses, and grain sizes. The mechanisms behind the creep behavior are analyzed in detail by calculating the stress exponents, grain boundary exponents, and activation energies. The novel results in this work are summarized in a deformation mechanism map and are in good agreement with Ashby’s experimental results for pure Ni. Through the deformation diagram, dislocation creep dominates the creep process when applying a high stress, while grain boundary sliding prevails at lower stress levels. These two mechanisms could also be coupled together for a low-stress but a high-temperature creep simulation. In this work, the dislocation creep is clearly observed and discussed in detail. Through analyzing the activation energies, vacancy diffusion begins to play an important role in enhancing the grain boundary creep in the creep process when the temperature is above 1000 K.

We present a comprehensive record of deformation, metamorphism, and the state of stress during interaction between garnet-rich skarn and infiltrating fluids in an Eo-Hellenic subduction zone (Preveli nappe, Crete). The structural data and deformation microfabrics of quartz, coupled with petrological constraints, suggest that subduction of the Preveli rocks occurred under epidote-blueschist-facies conditions (T = 360 ± 40 °C, P > 1.0 GPa) and differential stresses >70 MPa. Localized deformation of fine-grained monomineralic grandite of the skarn led to mode I fractures and discrete shear zones along which synkinematic ferri-winchite developed. Brittle-viscous deformation of grandite in high-strain domains was accommodated by cataclasis, incongruent dissolution precipitation creep (IDPC), and incipient dislocation creep (DC). Cataclasis led to strong grain-size reduction and increased permeability resulting in fluid infiltration and enhanced IDPC as is documented by serrated reaction seams along which actinolite and ferri-winchite grew in sequence while replacing grandite. Incipient DC of grandite is indicated by lattice distortion and possible subgrains. Brittle-viscous deformation of grandite occurred at high differential stress (>250 MPa) and was significantly controlled by externally derived fluids, which contributed to hydraulic fracturing and the redistribution and addition of mobile elements. • Brittle-viscous deformation of wet garnet at T < 400 °C. • High differential stress during brittle-viscous deformation of garnet. • Fluid-assisted grain-boundary migration of garnet. • Incongruent dissolution-precipitation creep of garnet at T < 400 °C. [ABSTRACT FROM AUTHOR]

A piezometric relationship is derived between subgrain size and stress in plastically deformed crystalline materials, based on the assumption that subgrain boundaries become stabilized once their strain energy per unit area exceeds that in the adjacent lattice. The subgrain diameter at this point depends on the density of geometrically necessary dislocations (GND), many of which are housed in the subgrain walls, and on their spacing in the walls. Subgrains grow by merging, which results in a linear relationship between size and misorientation. The piezometric relationship for any given misorientation has a stress exponent close to unity. The subgrain piezometer can be calibrated using literature-based estimates of the size of dynamically recrystallized grains produced by subgrain rotation (SGR) for different minerals. Grain-size piezometers reflect processes of nucleation and post-nucleation modification during dynamic recrystallization. New grains formed by these different processes occupy different regions in grain-size/stress space. These regions are bounded by the subgrain piezometer and the Dmin line, which marks the smallest size that newly formed grains can have without being eliminated by surface-energy driven grain-boundary migration. The piezometers are statistically defined by the scatter of grain-sizes produced by these different processes. Piezometers defined in this way for quartz, olivine, and calcite correspond well within uncertainties to experimentally determined piezometers. No experimental piezometer exists for feldspars; a piezometer based on the concepts advanced in this paper is suggested that can be tested by experiment. [Display omitted] • Theoretically derived subgrain piezometers are consistent with experimental data 82. • Different subgrain piezometers are derived for different misorientations 74. • Subgrain piezometers are calibrated using the grain-size produced by SGR nucleation 85. • Grain-size piezometers are a function of recrystallization mechanism 75. • A new grain-size piezometer is predicted from theory for feldspars 68. [ABSTRACT FROM AUTHOR]

The high-temperature deformation process of the recrystallized 16CrODS ferritic steel was investigated at 1000°C for the stress loading perpendicular to the elongated grain structure. The strain rate was varied in the range from 1.0×10−2 to 1.0×10−5s−1. At the strain rate over 1.0×10−4s−1, deformation is dominated by the conventional dislocation creep. Decreasing strain rate from 1.0×10−4s−1, grain boundary sliding becomes prominent. Accommodation process for the localized stress induced by grain boundary sliding could be dislocation creep at 1.0×10−4s−1, and by diffusional creep at 1.0×10−5s−1 or less. These were verified through the observation of void formation and localized strain accumulation by KAM map.

Abstract Numerical modeling of salt tectonics is a rapidly evolving field; however, the constitutive equations to model long-term rock salt rheology in nature still remain controversial. Firstly, we built a database about the strain rate versus the differential stress through collecting the data from salt creep experiments at a range of temperatures (20–200 °C) in laboratories. The aim is to collect data about salt deformation in nature, and the flow properties can be extracted from the data in laboratory experiments. Moreover, as an important preparation for salt tectonics modeling, a numerical model based on creep experiments of rock salt was developed in order to verify the specific model using the Abaqus package. Finally, under the condition of low differential stresses, the deformation mechanism would be extrapolated and discussed according to microstructure research. Since the studies of salt deformation in nature are the reliable extrapolation of laboratory data, we simplified the rock salt rheology to dislocation creep corresponding to power law creep (n = 5) with the appropriate material parameters in the salt tectonic modeling.

The simplified model of numerical analyses of discrete dislocation motion and emission from a stressed source was applied to predict the yield stress, dislocation creep, and fatigue crack growth rate of metals dominated by dislocation motion. The results obtained by these numerical analyses enabled us to link various dynamical effects on the yield stress, dislocation creep, and fatigue crack growth rate with the experimental results of macroscopic phenomena, as well as to link them with theoretical results obtained by the concept of static, continuously distributed infinitesimal dislocations for the equilibrium state under low strain or stress rate conditions. This will be useful to holistic research approaches with concern for time and space scales, that is, in a time scale ranging from results under high strain rate condition to those under static or low strain rate condition, and in a space scale ranging from meso-scale to macro-scale mechanics. The originality of results obtained by these analyses were found by deriving the analytical formulations of number of dislocation emitted from a stressed source and a local dynamic stress intensity factor at the pile-up site of dislocations as a function of applied stress or stress rate and temperature material constants. This enabled us to develop the predictive law of yield stress, creep deformation rate, and fatigue crack growth rate of metals dominated by dislocation motion. Especially, yielding phenomena such as the stress rate and grain size dependence of yield stress and the delayed time of yielding were clarified as a holistic phenomenon composed of sequential processes of dislocation release from a solute atom, dislocation group moving, and stress concentration by pile-up at the grain boundary.

Localization processes in the viscous lower crust generate ductile shear zones over a broad range of scales affecting long‐term lithosphere deformation and the mechanical response of faults during the seismic cycle. Here we use centimeter‐scale numerical models in order to gain detailed insight into the processes involved in strain localization and rheological weakening in viscously deforming rocks. Our 2‐D Cartesian models are benchmarked to high‐temperature and high‐pressure torsion experiments on Carrara marble samples containing a single weak Solnhofen limestone inclusion. The models successfully reproduce bulk stress‐strain transients and final strain distributions observed in the experiments by applying a simple, first‐order softening law that mimics rheological weakening. We find that local stress concentrations forming at the inclusion tips initiate strain localization inside the host matrix. At the tip of the propagating shear zone, weakening occurs within a process zone, which expands with time from the inclusion tips toward the matrix. Rheological weakening is a precondition for shear zone localization, and the width of this shear zone is found to be controlled by the degree of softening. Introducing a second softening step at elevated strain, a high strain layer develops inside the localized shear zone, analogous to the formation of ultramylonite bands in mylonites. These results elucidate the transient evolution of stress and strain rate during inception and maturation of ductile shear zones. Key Points: First‐order numerical softening laws successfully reproduce strain localization and stress transients observed in laboratory torsion testsOur models provide a virtual way of analyzing viscous process zone evolution as it nucleates near an inclusion and propagates into a matrixThe degree of weakening controls the process zone geometry, the finite width of shear bands, and the potential formation of ultramylonites [ABSTRACT FROM AUTHOR]

Abstract Shearing mechanisms of the primary strengthening phase in cobalt-base and cobalt-nickel-base superalloys, γ′ (L1 2), are investigated at the elementary defect level by using a combination of generalized-stacking-fault energy (GSF) calculations and phase field simulations. The GSF energy surfaces of the γ and γ′ phases, as determined from available experimental data and ab initio calculations, are used in the phase field simulations. Sophisticated deformation pathways leading to various planar defects including antiphase boundaries (APB), remnant superlattice intrinsic stacking faults (SISF), APB-SISF-APB ribbons and SISF islands are predicted as a function of alloy composition and particle shapes. The predicted stacking fault configurations for both alloys are consistent with recent transmission electron microscopy observations. Effects of dislocation line tension difference in γ and γ′ phases and planar defect energy variation due to segregation at the planar defects are discussed. The detailed dislocation core structures, effects of dislocation line tension differences on deformation mechanisms, and unique deformation mechanisms uncovered, which include stacking fault ribbon shearing, antiphase boundary shearing, and mixed modes, could be used to improve constitutive microstructure-property relationships in advanced crystal plasticity modeling and to assist in alloy design. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract A primary creep model of nickel-base superalloys is developed for the low-temperature-high-stress regime, which provides insight into the active micro-mechanisms, extracted from features of the experimental creep curves. A two-parameter model is established to account for the accumulated primary creep strain, which contains a “threshold stress” representing the possible shear resistance during primary creep. Consideration of this threshold stress leads to a stress exponent n in the order of 3 ∼ 4 , in sharp contrast to the conventional analysis that gives an ultra high value of n > 10. A general stress-dependence of dislocation velocity is then proposed based on the framework of thermally-activated deformation, which leads to another two-parameter model for the primary creep rate. Applications to two superalloys demonstrate a good agreement between the model and experimental data. In addition, two micro-mechanistic characteristics of creep, i.e., dislocation drag coefficient and activation volume, can be unambiguously determined from the model parameters. The values of these physical quantities obtained for the two superalloys considered suggest a coupled dislocation motion of glide and climb at the γ/γ' interface as the rate-controlling mechanism during primary creep in superalloys. Highlights • A predictive model is developed for primary creep in Ni-base superalloys. • The conventional high stress exponent (>10) is now reduced to 3–4. • Micro-mechanism "fingerprints" are extracted directly from experimental creep curves. • A coupled dislocation motion of glide and climb at the γ/γ′ interface is suggested. [ABSTRACT FROM AUTHOR]

Abstract The reduction of grain size is believed to play a critical role in strain localization to form shear zones. Although many mechanisms have been proposed, the source of grain size reduction remains debated. The South Armorican Shear Zone (SASZ) is a crustal-scale strike-slip fault that deforms granitoids at mid-crustal conditions. The SASZ records the transition from protolith to ultramylonite, representative of increasing ductile shear strain. To investigate the evolution of strain localization, the different states of deformation were studied using a combination of detailed microstructural, chemical and electron backscatter diffraction analyzes. Increasing strain from protolith to ultramylonite resulted in (1) grain size reduction, (2) the development of interconnected monophase layers of mica and incipient mixed-phase zones composed of phengite-quartz ± K-feldspar in protomylonite and low-strain mylonite, and (3) the formation of fine-grained mixed-phase zones composed of K-feldspar-quartz ± phengite in high-strain mylonite and ultramylonite. We propose that the causes for interconnection of mica are the formation of cracks in the protolith combined with fluid-assisted nucleation. The latter process also plays a major role in phase mixing, as attested by the precipitation of K-feldspar at triple junctions in quartz-rich layers and in fine-grained tails of inherited K-feldspar porphyroclasts in ultramylonite. The transition from quartz-rich layers and mixed-phase zones is accompanied by a strong dispersion of the quartz lattice preferred orientation. These microstructural and textural evidences suggest that phase nucleation is the major process behind phase mixing, possibly accompanied by the action of grain-boundary sliding to open cavities during deformation. Instead of a single process, we therefore highlight a succession of weakening processes in the evolution of the SASZ, starting with the crystallization of mica as a first weakening material, and then evolving with the formation of very fine-grained mixed-phase zones made principally of feldspar, quartz and phengite. Highlights • Grain size reduction and phase mixing increase with increasing strain. • Phase nucleation is the dominant process behind phase mixing. • Phase nucleation is driven by chemical reactions and grain-boundary sliding. • Succession in weakening processes related to the accumulation of strain itself. [ABSTRACT FROM AUTHOR]

Shear experiments on quartz gouge were performed at elevated confining pressures (predominantly 1.5 GPa) and temperatures (500 °C - 1000 °C) at shear strain rates of 3.5·10 −6 s −1 to 2·10 −3 s −1 to study the brittle-to-viscous transition. An unsystematic temperature dependence of strength at low temperatures changes towards a clear temperature weakening dependence above 650 °C. The transition from a pressure strengthening to a pressure weakening relationship takes place continuously between 650 °C and 800 °C. Strain rate stepping experiments reveal power-law breakdown at low temperatures (∼650 °C). Between 800 °C and 1000 °C, a stress exponent of n  = 1.9 ± 0.6 and an activation energy of Q = 170 ± 72 kJ/mol indicate a combination of diffusion and dislocation creep. The Goetze criterion is confirmed as the upper stress limit for viscous deformation mechanisms. Localised deformation in the form of semibrittle shear bands with Riedel geometry at low temperatures changes to homogeneous deformation with a pervasive foliation accompanied by a continuous texture evolution between 700 °C and 1000 °C. Fracturing dominates at low temperatures accompanied by increasing amounts of dissolution and precipitation in fine-grained zones with increasing temperature. Above 650 °C, dislocation and diffusion creep are the dominating deformation processes, with dislocation creep being favoured in larger grains while dissolution-precipitation is active in the fine-grained fraction. [ABSTRACT FROM AUTHOR]

We quantify the long term creep closure of salt cavities under in-situ conditions. We use an incompressible Carreau viscosity model to treat combined dislocation and pressure solution creep of rock salt. We build three material models based on the published compilations of experimental data. Analytical expressions are given for the maximum closure velocity under combined pressure and shear loads in the far field. A non-linear finite element model is used to study the apparent viscosity around a circular hole. In the low stress regime, depending on the grain size, creep rate can increase several orders of magnitude by the action of pressure solution. The influence of grain size dominates over all other parameters such as temperature or dislocation creep parameters. In general, pressure solution is dominant in cold fine-grained salts under low loads, while dislocation creep is dominant in warm coarse-grained salts under high loads. In the dislocation creep regime, the hole closure rate is n (stress exponent) times more sensitive to the far field shear stress than to the pressure differences. Significant hole closure can be reached in less than one day in very fine-grained salts under in-situ loads in the order of 1 MPa. A thorough understanding of salt grain size and water availability is needed to properly assess the relative contribution of dislocation and pressure solution creep in hole closure. A direct relationship relating grain size with deformation mechanism cannot be established in general. The closure of a cavity in a typical salt having a 10 mm grain size can either be dominated by pressure solution or dislocation creep depending on the temperature and salt type. Best practices to prevent hole closure include increasing hole pressure, and avoiding, when possible, the salt layers which are known to be fined-grained and under high deviatoric stress. [ABSTRACT FROM AUTHOR]