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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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

Deformation experiments on olivine aggregates were performed under hydrous conditions using a deformation-DIA apparatus combined with synchrotron in situ X-ray observations at pressures of 1.5-9.8 GPa, temperatures of 1223-1800 K, and strain rates ranging from 0.8 × 10 to 7.5 × 10 s. The pressure and strain rate dependencies of the plasticity of hydrous olivine may be described by an activation volume of 17 ± 6 cm mol and a stress exponent of 3.2 ± 0.6 at temperatures of 1323-1423 K. A comparison between previous data sets and our results at a normalized temperature and a strain rate showed that the creep strength of hydrous olivine deformed at 1323-1423 K is much weaker than that for the dislocation creep of water-saturated olivine and is similar to that for diffusional creep and dislocation-accommodated grain boundary sliding, while dislocation microstructures showing the [001] slip or the [001](100) slip system were developed. At temperatures of 1633-1800 K, a much stronger pressure effect on creep strength was observed for olivine with an activation volume of 27 ± 7 cm mol assuming a stress exponent of 3.5, water fugacity exponent of 1.2, and activation energy of 520 kJ mol (i.e., power-law dislocation creep of hydrous olivine). Because of the weak pressure dependence of the rheology of hydrous olivine at lower temperatures, water weakening of olivine could be effective in the deeper and colder part of Earth's upper mantle. [ABSTRACT FROM AUTHOR]

We theoretically derived flow laws for quartz dislocation creep using climb-controlled dislocation creep models and compared them with available laboratory data for quartz plastic deformation. We assumed volume diffusion of oxygen-bearing species along different crystallographic axes (// c, ⊥ R, and ⊥ c) of α-quartz and β-quartz, and pipe diffusion of H2O, to be the elementary processes of dislocation climb. The relationships between differential stress ( σ) and strain rate ( [ABSTRACT FROM AUTHOR]

A kinetic model of creep-rupture strength is constructed using the physicomathematical theory of irreversible strains of metals. An algorithm for the mathematical modeling of the processes occurring during tension of samples is proposed. Results of experimental verification of a uniaxial model of creep-rupture strength are given. It is shown that the proposed model differs from available models in that it contains physical structural parameters (scalar densities of dislocations and microcracks) and kinetic equations for them. [ABSTRACT FROM AUTHOR]

There is a heated debate about the long-term rheology of salt at rest. As many researches show, pressure solution creep and dislocation creep mechanisms dominate mechanical behavior of salt. We summarize the database of researches of salt rheology for the first time. A new method is used to evaluate salt rheology through inverting the gravitational sinking of anhydrite inclusions (also called 'stringers') embedded in Zechstein salt at rest during the Tertiary. Seismic data can be used to compare with numerical model of sinking stringer. Results show that under the control of dislocation creep, the fragments has almost no movement but under the control of pressure solution creep, the fragments will sink obviously through the salt section during a few Ma (some fragments can sink hundreds of meters and touch the presalt). In conclusion, through inverting the sinking of stringer fragments the long-term Zechstein salt rheology can be investigated and dislocation creep dominates its deformation during Tertiary. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Microstructural analysis of pyrite from a single sample of Witwatersrand conglomerate indicates a complex deformation history involving components of both plastic and brittle deformation. Internal deformation associated with dislocation creep is heterogeneously developed within grains, shows no systematic relationship to bulk rock strain or the location of grain boundaries and is interpreted to represent an episode of pyrite deformation that predates the incorporation of detrital pyrite grains into the Central Rand conglomerates. In contrast, brittle deformation, manifest by grain fragmentation that transects dislocation-related microstructures, is spatially related to grain contacts and is interpreted to represent post-depositional deformation of the Central Rand conglomerates. Analysis of the low-angle boundaries associated with the early dislocation creep phase of deformation indicates the operation of <010>{100} slip systems. However, some orientation boundaries have geometrical characteristics that are not consistent with simple <010>{100} deformation. These boundaries may represent the combination of multiple slip systems or the operation of the previously unrecognized <001>{120} slip system. These boundaries are associated with order of magnitude enrichments in As, Ni and Co that indicate a deformation control on the remobilization of trace elements within pyrite and a potential slip system control on the effectiveness of fast-diffusion pathways. The results confirm the importance of grain-scale elemental remobilization within pyrite prior to their incorporation into the Witwatersrand gold-bearing conglomerates. Since the relationship between gold and pyrite is intimately related to the trace element geochemistry of pyrite, the results have implications for the application of minor element geochemistry to ore deposit formation, suggest a reason for heterogeneous conductivity and localized gold precipitation in natural pyrite and provide a framework for improving mineral processing. [ABSTRACT FROM AUTHOR]

Current creep models are derived using data from constant stress (or load) creep tests and are capable of accurately predicting creep behaviour when applied conditions are constant or near constant. However, analyses of creep curve shapes for the nickel based superalloy Waspaloy, when applied stress and/or temperature vary greatly during testing, have shown that predictive methods based purely on strain, time or life fraction are insufficient and cannot predict the observed creep rates. This is important when considering stress concentration features where stress relaxation due to creep can significantly alter the distribution of stress and thus affect fatigue life. When both stress and temperature are changed during a creep test, dislocation movement must proceed through a dislocation network formed under different conditions, resulting in greater than expected creep rates. It is proposed that this is due to a reduction in effective internal stress due to changes in dislocation structure. [ABSTRACT FROM AUTHOR]

In the present study, the grain size ( d) and shape of 225 magnetite grains, that crystallized at T>600°C in a syntectonic granite (Godhra Granite, India) are evaluated and implications of data to decipher deformation mechanism of magnetite are discussed. Fractal (ruler) dimension ( D) analysis of magnetite grains is performed and it is demonstrated that they show fractal behaviour. Smaller magnetite grains tend to be more serrated than the larger ones, which is manifested in the higher fractal (ruler) dimension ( D) of the former. Assuming a natural strain rate ranging between 10 s and 10 s, the grain size data fall dominantly in the dislocation creep field of the existing deformation mechanism map of magnetite for 630°C. However, SEM-EBSD studies reveal that subgrains are absent in the magnetite grains and they did not undergo dislocation creep. Thus it is inferred that the shape of magnetite grains was not controlled by dislocation creep. It is concluded that the higher serration and increased fractal dimension of finer magnetite grains implies the importance of diffusion creep as an important deformation mechanism at high-T for magnetite in polymineralic rocks. [ABSTRACT FROM AUTHOR]

An undeformed glomeroporphyritic andesite from the Sunda Arc of Java, Indonesia, contains zoned plagioclase and amphibole glomerocrysts in a fine-grained groundmass and records a complex history of adcumulate formation and subsequent magmatic disaggregation. A suite of xenocrystic zircon records Proterozoic and Archaean dates whilst a discrete population of zoned, euhedral, igneous zircon yields a SHRIMP U-Pb crystallisation age of 9.3 ± 0.2 Ma. Quantitative microstructural analysis of zircon by electron backscatter diffraction (EBSD) shows no deformation in the inherited xenocrysts, but intragrain orientation variations of up to 30° in 80% of the young zircon population. These variations are typically accommodated by both progressive crystallographic bending and discrete low angle boundaries that overprint compositional growth zoning. Dispersion of crystallographic orientations are dominantly by rotation about an axis parallel to the zircon c-axis [001], which is coincident with the dominant orientation of misorientation axes of adjacent analysis points in EBSD maps. Less common <100> misorientation axes account for minor components of crystallographic dispersion. These observations are consistent with zircon deformation by dislocation creep and the formation of tilt and twist boundaries associated with the operation of <001>{100} and <100>{010} slip systems. The restriction of deformation microstructures to large glomerocrysts and the young magmatic zircon population, and the absence of deformation within the host igneous rock and inherited zircon grains, indicate that zircon deformation took place within a low-melt fraction (<5% melt), mid-lower crustal cumulate prior to fragmentation during magmatic disaggregation and entrainment of xenocrystic zircons during magmatic decompression. Tectonic stresses within the compressional Sunda Arc at the time of magmatism are considered to be the probable driver for low-strain deformation of the cumulate in the late stages of initial crystallisation. These results provide the first evidence of crystal plastic dislocation creep in zircon associated with magmatic crystallisation and indicate that the development of crystal-plastic microstructures in zircon is not restricted to high-strain rocks. Such microstructures have previously been shown to enhance bulk diffusion of trace elements (U, Th and REE) in zircon. The development of deformation microstructures, and therefore multiple diffusion pathways in zircon in the magmatic environment, has significant implications for the interpretation of geochemical data from igneous zircon and the trace element budgets of melts due to the potential enhancement of bulk diffusion and dissolution rates. [ABSTRACT FROM AUTHOR]

Dislocation creep, which is the dominant deformation mechanism in the upper mantle, results in a non-Newtonian anisotropic rheology. The implication of non-Newtonian rheology has been quite extensively studied in geodynamic models but the anisotropic aspect remains poorly investigated. In this paper, we propose to fill this gap by (1) introducing a simple mathematical description of anisotropic viscosity and (2) illustrating the link between plastic crystal deformation and bulk material rheology. The study relies on the highest symmetry of the anisotropic tensor, a cubic symmetry, for which anisotropy is characterized by one parameter only, δ. First-order implications of anisotropy are quantitatively explored as a function of δ. The effective rheology of the material is described as a function of the orientation of the crystals and of the imposed stress and the validity of the isotropic approximation is discussed. The model, applied to ringwoodite, a cubic crystal with spinel-type structure, predicts that the dynamics of the transition zone in the Earth's mantle is going to be strongly affected by mechanical anisotropy. [ABSTRACT FROM AUTHOR]

A finite element method was recently designed to model the mechanisms that cause superplastic deformation (A.F. Bower and E. Wininger, A Two-Dimensional Finite Element Method for Simulating the Constitutive Response and Microstructure of Polycrystals during High-Temperature Plastic Deformation, J. Mech. Phys. Solids, 2004, 52, p 1289-1317). The computations idealize the solid as a collection of twodimensional grains, separated by sharp grain boundaries. The grains may deform plastically by thermally activated dislocation motion, which is modeled using a conventional crystal plasticity law. The solid may also deform by sliding on the grain boundaries, or by stress-driven diffusion of atoms along grain boundaries. The governing equations are solved using a finite element method, which includes a fronttracking procedure to monitor the evolution of the grain boundaries and surfaces in the solid. The goal of this article is to validate these computations by systematically comparing numerical predictions to experimental measurements of the elevated-temperature response of aluminum alloy AA5083 (M.-A. Kulas, W.P. Green, E.M. Taleff, P.E. Krajewski, and T.R. McNelley, Deformation Mechanisms in Superplastic AA5083 materials. Metall. Mater. Trans. A, 2005, 36(5), p 1249-1261). The experimental work revealed that a transition occurs from grain-boundary sliding to dislocation (solute-drag) creep at approximately 0.001/s for temperatures between 425 and 500 °C. In addition, increasing the grain size from 7 to 10 lm decreased the transition to significantly lower strain rates. Predictions from the finite element method accurately predict the effect of grain size on the transition in deformation mechanisms. [ABSTRACT FROM AUTHOR]

We have developed a channel flow model that dynamically couples plate motion and mantle stress with a composite rheology (diffusion creep and dislocation creep) to study the rheological and anisotropic structures of the oceanic upper mantle. A semi-analytic approach is used to solve for mantle stress and viscosity, allowing fast calculations and exploration of a wide range of rheological parameters. Mantle stress in our model is due to shearing by a moving plate. By comparing mantle stress with a transition stress for dislocation creep, we identify regions where either diffusion creep or dislocation creep is active. Deformation by dislocation creep results in a mineral fabric that may be responsible for observed seismic anisotropy. Our study suggests that there is an important relation between plate motion, seismic anisotropy, mantle viscosity and transition stress. Using laboratory results for rheological parameters, we find that dislocation creep exists only in a layer at certain depths in the upper mantle. For a plate velocity of 10 cm yr−1, an asthenospheric viscosity of 1019 Pa s and an asthenospheric transition stress of 0.1 MPa, our model predicts a ∼200 km thick dislocation creep layer, which is broadly consistent with the observations of seismic anisotropy. For a plate velocity of 10 cm yr−1 and an asthenospheric transition stress of 0.1 MPa, the asthenospheric viscosity needs to be greater than to produce any dislocation creep deformation, and the asthenospheric viscosity needs to be larger for slower plate motion or larger transition stress. Slower plate motion leads to a thinner dislocation creep layer, which may partially explain the observed asymmetry in anisotropic structure in the East Pacific Rise. [ABSTRACT FROM AUTHOR]

Lherzolite samples synthesized from fine-grained powders prepared from a natural xenolith were deformed at P = 300 MPa and 1373 ≤ T ≤ 1573 K in a high-resolution gas-medium deformation apparatus. Below and above the solidus of Ts &ap; 1433 K, fine-grained lherzolite exhibits a transition from diffusional creep at low stress to dislocation creep at high stress. The transition occurs at a differential stress σ &ap; 100 MPa in samples with a grain size d &ap; 20 µm and a melt fraction ϕ &ap; 0·03. Extrapolation to upper-mantle conditions suggests that a similar transition will occur in the mantle for d = 2 mm at σ &ap; 0·5 MPa. Therefore, both creep mechanisms are likely to contribute to deformation of partially molten regions of the mantle. We determined an empirical relationship for the dependence of strain rate ε˙ on ϕ using data for melt-free olivine, analysis of the subsolidus and hypersolidus creep behavior of lherzolite and constraints on ϕ(T) from our experimentally determined melting curve. We find that ε˙ϕ increases approximately exponentially with increasing ϕ in both the diffusional and dislocation creep regimes. The observed enhancement in ε˙ϕ as a result of melt is much larger than anticipated based on deformation models that consider only an isotropic distribution of melt in triple junctions and do not incorporate the presence of melt along some grain boundaries and the melt preferred orientation that develops during deformation. The results for lherzolite are similar to results reported previously for olivine plus basaltic melt samples, indicating a minor effect of pyroxene content for deformation of partially molten peridotites. Our constitutive equation provides a basis for modeling geodynamic processes in the partially molten mantle beneath mid-ocean ridges and in the mantle wedge above subducting slabs. [ABSTRACT FROM AUTHOR]

High-temperature creep behavior in Ni2GeO4 spinel was investigated using synthetic polycrystalline aggregates with average grain sizes ranging from submicron to 7.4 microns. Cylindrical samples were deformed at constant load in a gas-medium apparatus at temperatures ranging from 1223 to 1523 K and stresses ranging from 40 to 320 MPa. Two deformation mechanisms were identified, characterized by the following flow laws: where σ is in MPa, d is in μm and T is in Kelvin. These flow laws suggest that deformation was accommodated by dislocation creep and grain-boundary diffusion (Coble) creep, respectively. A comparison with other spinels shows that an isomechanical group can be defined for spinels although some differences between normal and inverse spinels can be identified. When creep data for olivine and spinel are normalized and extrapolated to Earth-like conditions, spinel (ringwoodite) has a strength similar to olivine in the dislocation creep regime and is considerably stronger than olivine in the diffusion creep regime at coarse grain size. However, when grain-size reduction occurs, spinel can become weaker than olivine due to its high grain-size sensitivity (Coble creep behavior). Analysis of normalized diffusion creep data for olivine and spinel indicate that spinel is weaker than olivine at grain sizes less than 2 μm. [ABSTRACT FROM AUTHOR]

Superplastic forming is a manufacturing process during which a sheet is blow formed into a die to produce lightweight and strong components. In this paper, the microstructural mechanism of grain growth during superplastic deformation is studied. A new model, which considers grain growth, is proposed and applied to conventional superplastic materials. The relationships among the strain, strain rate, test temperature, initial grain size, and grain growth in superplastic materials are discussed. According to the proposed model, theoretical predictions for superplastic forming processes are presented, and comparison with experimental data is given. The new constitutive equation of superplasticity is introduced into a finite element method program to study superplastic blow forming. The effects of the geometric shape parameters of the die on the superplastic blow forming process are investigated, and the inhomogeneity in the thickness distribution of the specimen is analyzed. [ABSTRACT FROM AUTHOR]

Plastically accommodated creep in an inclusion bearing material (particle- or discontinuous fiber-reinforced metal matrix composite) without any interfacial relaxation mechanisms has been examined. For a material with an elastic-viscoplastic matrix, the non-uniform strain rate in steady state creep is derived using Eshelby’s solution for elastic strain outside an inclusion. The obtained creep strain increment is impotent and does not generate any additional internal stress. During this creep deformation, a dislocation comes in from one direction and goes out in another direction, so that no dislocation nor internal stress but a heterogeneous plastic strain remains in the material. The concrete trajectory of the dislocations climbing over a cylindrical inclusion is calculated and illustrated. [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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]