Dislocation Creep Research Articles

Microstructure Evolution and Mechanical Properties of Ti–6Al–4Zr–4Nb Alloys Fabricated by Spark Plasma Sintering (SPS)

The near-α Ti–6Al–4Zr–4Nb (wt pct) alloy is a recently developed alloy with potential for aerospace applications. This study evaluates the microstructure and mechanical properties of Ti–6Al–4Zr–4Nb produced by spark plasma sintering (SPS). The SPS samples sintered in the α + β regions exhibited equiaxed α with a neighboring thin β phase. Above the β-transus temperature, the α/β lamellar structure formed, allowing control of the grain size (100 ~ 200 μm). The compressive strength and the creep property of the SPS samples were compared with the LBPDed and the forged samples. The compressive strength of the SPS sample was lower than that of the LPBFed sample but similar to the forged sample. The SPS samples exhibited longer creep rupture life (2220 hours) than LPBFed samples (1730 hours), but shorter than the forged sample (4109 hours). The creep deformation mechanism of the lamellar structure in the SPS sample was dislocation creep.

Plastic deformation of dry fine-grained olivine aggregates under high pressures

Abstract 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 solgel olivine in the same assembly) in the same sample assembly under high-pressure (P = 2.9–8.8 GPa) and modest temperatures (T = 980–1250 K) at a fixed strain-rate. Evolution of strength was studied using the radial X-ray diffraction from various diffraction planes. We found that San Carlos and sol-gel olivine show similar rheological behaviour (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 micro-structural 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 a correction of 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 in comparison 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.

The influence of viscous slab rheology on numerical models of subduction

Abstract. Numerical models of subduction commonly use diffusion and dislocation creep laws from laboratory deformation experiments to determine the rheology of the lithosphere. The specific implementation of these laws varies from study to study, and the impacts of this variation on model behavior have not been thoroughly explored. We run simplified 2D numerical models of free subduction in SULEC, with viscoplastic slabs following (1) a diffusion creep law, (2) a dislocation creep law, and (3) both simultaneously, as well as several variations of model 3 with reduced resistance to bending. We compare the results of these models to a model with a constant-viscosity slab to determine the impact of the implementation of different lithospheric flow laws on subduction dynamics. In creep-governed models, higher subduction velocity causes a longer effective slab length, increasing slab pull and asthenospheric drag, which, in turn, affect subduction velocity. Numerical and analogue models implementing constant-viscosity slabs lack this feedback but still capture morphological patterns observed in more complex models. Dislocation creep is the primary deformation mechanism throughout the subducting lithosphere in our models. However, both diffusion creep and dislocation creep predict very high viscosities in the cold core of the slab. At the trench, the effective viscosity is lowered by plastic failure, rendering effective slab thickness the primary control on bending resistance and subduction velocity. However, at depth, plastic failure is not active, and the viscosity cap is reached in significant portions of the slab. The resulting high slab stiffness causes the subducting plate to curl under itself at the mantle transition zone, affecting patterns in subduction velocity, slab dip, and trench migration over time. Peierls creep and localized grain size reduction likely limit the stress and viscosity in the cores of real slabs. Numerical models implementing only power-law creep and neglecting Peierls creep are likely to overestimate the stiffness of subducting lithosphere, which may impact model results in a variety of respects.

Understanding the high-temperature deformation behavior of additively manufactured γ'-forming Ni-based alloys by microstructure heterogeneities-integrated creep modelling

Additively manufactured (AM) alloys present unique and heterogeneous microstructures due to the complex, highly dynamic laser-material interactions. These AM-inherent heterogeneities impede the widespread adoption of AM components, necessitating a profound comprehension of their impact on mechanical properties. Despite extensive research on AM of Ni-based alloys, limited attention has been paid to their creep behavior due to the time-intensive nature of creep tests and the long research cycles. Moreover, experiments and conventional alloy-centric approaches to creep modelling are deemed insufficient in quantifying the effects of AM-specific heterogeneities on creep cavity acceleration and in incorporating the microstructural evolution during creep. To address this critical knowledge gap, a novel computational framework was developed within the structure-property paradigm to unravel the intricate mechanisms governing creep properties. A mechanistic creep model was formulated based on fundamental dislocation creep mechanisms, encompassing dislocation climb-glide motion controlled by γ' precipitates, grain-boundary-sliding (GBS) resistance resulting from M23C6 carbides, and the kinetics of cavity formation. The framework integrates the in situ nucleation, precipitation, and coarsening of γ' precipitates during creep by a precipitation model. The results revealed an excellent agreement in terms of γ' precipitate evolution, creep strain, and strain-rate evolution, the predicted creep life, and times to 1 % strain. By elucidating the intricate interplay between microstructural heterogeneities and creep behavior on the cavity nucleation and GBS mechanisms, the developed computational framework provided valuable insights for enhancing the performance of Ni-based alloys manufactured through AM.

Identifying multiple synergistic factors on the susceptibility to stress relaxation cracking in variously heat-treated weldments

The 347H austenitic stainless steel has been widely used for pressure vessels and pipeline (PVP) applications due to its excellent creep and corrosion resistance, which fit ideally to the harsh conditions in petrochemical industries, fossil fuel or nuclear power plants, and modern energy storages. However, a failure mode has been commonly observed with cracks emerging at the heat affected zone (HAZ) of weldments during post-weld heat treatment (PWHT) or under intermediate to high temperature service conditions. This phenomenon is termed as Stress Relaxation Cracking (SRC) since the purpose of PWHT is to relieve the welding-induced residual stress fields, or as Stress Age Cracking (SAC) if failure happens during service. A leading literature explanation of this failure suggests that the residual stress relaxation and the precipitation dissolution and/or re-precipitation occur in the same temperature range, which can lead to locally high strains and thus to crack at the grain boundaries. Since in situ spatial measurements of residual stress fields, microstructural evolution, and failure processes are nearly infeasible, this work recourses to a micromechanical finite element framework that models the high temperature failure as the nucleation and growth of grain boundary cavities, whereas various parameters such as thermomechanical loading history and its evolution, the competition of grain-interior dislocation creep and grain-boundary diffusion in failure lifetime, and microstructural heterogeneities (such as the precipitate free zone near grain boundaries) can be quantitatively incorporated. It can be concluded from these microstructure-explicit simulations that an accurate knowledge of residual stress evolution and a carefully calibrated set of material constitutive parameters are the essential prerequisites for lifetime predictions. The understanding of individual governing factors also leads to a mechanistic interpretation of the observed SRC susceptibility C-curves. These results suggest that the criticality of residual stress evolution, but not the precipitation-induced local strains, be the leading factor for SRC.

Understanding the sigmoidal primary creep behaviour in a multi-component solid-solution Co-base alloy between 700 – 850 °C

This investigation was motivated by the lack of understanding of the role of solid solution strengthening and substructure evolution during the creep of multi-component alloys. Thus, we studied the tensile creep behaviour of a multi-component solid-solution Co-base alloy (Co41-Ni23-Cr26-W4.7-Fe2-Mn1-C0.5) at 650 – 900 °C and stress (σApplied) range: 80 – 400 MPa. Uncommon sigmoidal primary creep (SPC) was witnessed at 700–850 °C and in the intermediate stress range. The stress exponent (∼5 – 7) and activation energy (∼270 kJ/mol ≈ Qlattice) suggest that dislocation creep controls the creep rate. Transmission electron microscopy (TEM) studies revealed substructural evolution with creep strain. At 750 °C and 160 MPa, at creep strain (ε) = 0.25 %, multiple pinning points along the dislocation length were seen, and a thorough analysis confirmed them as jogs. Friction stress estimation from dislocation curvature from multiple TEM micrographs revealed that effective stress on dislocations is high (>0.6σApplied) while solid solution strengthening is nominal during creep. Overall analysis suggests that the creep rate in the multi-component alloy is recovery controlled while jogs' motion determines the SPC creep rate.

Deformation microstructures of blueschists in Alpine Corsica, France, and implications for seismic anisotropy and the low-velocity layer in subducting oceanic crust

Understanding the deformation microstructures and seismic properties of blueschists is important for interpreting the seismic anisotropy at the top of subducting oceanic crust. The deformation microstructures and seismic properties of epidote blueschist, epidote-lawsonite blueschist, and lawsonite blueschist from Alpine Corsica, France, were studied using electron backscatter diffraction mapping and transmission electron microscopy. The crystal-preferred orientations (CPOs) of glaucophane show a strong maximum of the [001] axes aligned sub-parallel to the lineation. The maximum of the (010) poles of epidote is aligned sub-parallel to the lineation. The [001] axes of lawsonite are strongly aligned sub-normal to the foliation. Intracrystalline misorientation, dislocation, and occasional fragmentation of glaucophane indicate that glaucophane CPO was developed mainly by dislocation creep. The intracrystalline deformation features of epidote, such as subgrain boundaries and dislocations, indicate that the CPO of the epidote was formed by dislocation creep. In contrast, our data indicate that lawsonite CPO was developed by a combination of rigid-body rotation and dislocation creep with a minor effect of twinning. The P-wave anisotropy (AVp) and maximum S-wave anisotropy (max.AVs) of epidote blueschists (AVp = 12.3–16.9% and max.AVs = 6.53–11.75%) are higher than those of lawsonite blueschists. The seismic velocities of the blueschists are lower than those of mantle peridotites. The coexistence of epidote and lawsonite in the blueschist results in variations in seismic anisotropy and P-wave velocity in the whole rock, depending on the modal content of epidote and lawsonite. A high content ratio of epidote to lawsonite in blueschist produces strong P- and S-wave anisotropies of blueschist when the glaucophane content is more than ∼50%. The P-wave velocity of blueschist decreases proportionally with increasing epidote content. Our results indicate that the seismic properties of deformed blueschists contribute to the seismic anisotropy and the low-velocity layer in subducting oceanic crusts in subduction zones.

High temperature tensile and creep properties of the new cladding steel of 15-15Ti-Y

The 15-15Ti austenitic stainless steel is commonly used as a fast reactor cladding material due to its favorable comprehensive properties. To further enhance its high temperature strength, yttrium (Y) modified 15-15Ti (15-15Ti-Y) steel was developed. In this study, the tensile properties of 15-15Ti-Y were evaluated across the temperature range of 200–700 °C, and the constant load creep properties of 20 % cold worked (CW) 15-15Ti-Y were measured at temperatures of 550–700 °C under various applied stresses. The results indicated that Y addition led to enhanced intragranular precipitates and delayed intergranular precipitates, which resulted in a YS (YS) that was ∼ 100 MPa higher than that of Y-free steel at temperatures ranging from 200 to 600 °C, without compromising the plasticity and ductility. The creep fracture life of 15-15Ti-Y steel ranged from 21 to 5761 h, and the deformation and damage processes were found to adhere to the Norton power-law and Modified Monkman Grant relationship. The stress exponent revealed that the deformation was dislocation creep mechanism, while the creep damage tolerance parameter suggested that the creep fracture was caused by necking. These findings provide data and theoretical support for the application of Y-modified 15-15Ti steel as a fast reactor cladding material in high temperature environments.

Microstructural study of the Praid Salt Diapir (Transylvanian basin, Romania) and its implication on deformation history and hydrogen storage potential

The Transylvanian basin is one of the major Tertiary sedimentary basins in the Carpathian-Pannonian region. Its thick sedimentary fill contains prominent Middle Miocene age salt that forms major diapir structures at the basin margins. The microstructural characteristics of the rock salt represent one of the main factors that determines the potential of a salt body for storage of hydrogen. The main aim of this study is to extend our understanding of the deformation mechanism of Praid rock salt located at the eastern margin of the Transylvanian basin. Based on petrography, we identified two types of rock salt: (1) layered salt with rather uniform grain size distribution showing alternation of greyish (clay mineral bearing) and white (clear halite) layers, and (2) massive grey salt with large, elongated halite crystals, accompanied by sub-micrometer size grains of halite. To shed light on the microstructure of the rock salt, we performed electron backscatter diffraction (EBSD) mapping, and studied gamma-irradiated samples both in the massive and layered salt samples. Dislocation creep and pressure solution creep were identified which acted concurrently in the Praid rock salt. The total strain rate falls between 1.2 and 1.3×10−10 s−1. The results of this study reveal a complex deformation history of the salt body where coexisting and migrating fluids have played an important role. The outcome of this project contributes to the hydrogen storage potential assessment for the Transylvanian salt and to a better understanding of the structural evolution of the Transylvanian basin.

Analysing the microscopic creep characteristics of carbonate rocks using nanoindentation experiments

ABSTRACT In recent years, the depth of oil drilling has exceeded 9000 m, and the testing and evaluation of mechanical properties of ultra-deep rocks face great challenges. Obtaining complete core samples has lots of serious problems through ultra-deep drilling, so it is difficult to process standard rock samples for general tests to obtain the mechanical properties. In order to study the creep behaviour of deep rocks, we conducted continuous step holding nanoindentation creep experiments on carbonate rocks from the Tarim Oilfield in Xinjiang, China. We first investigated the effects of load and holding time on the creep behaviour; Furthermore, by calculating the stress exponent, the deformation mechanism is revealed; At last, four traditional creep models and fractional order Maxwell model were used for regression to explore the applicability of models in describing the creep behaviour. The results show the creep displacement is positively correlated with the load and holding time. Creep curves with different shapes were obtained in nanoindentation (named A1 and A2 respectively). The Burgers and the two-dashpot Kelvin model are in good agreement with experimental data under long holding time. The stress exponents of A2 are greater than 3, indicating that the creep mechanism of A2 is dislocation creep.

Dehydration‐driven deformation of eclogite: Interplay between fluid discharge and rheology

AbstractAqueous fluids released during dehydration of a subducting slab have a large effect on the rheology of the subduction interface. While high‐pressure experiments and natural‐case studies link deformation with critical dehydration reactions during eclogitization, the exact interplay between these processes remains ambiguous. To investigate fluid–rock interaction and associated deformation at high‐pressure, we studied a suite of eclogites from the Tsäkkok Lens of the Scandinavian Caledonides that record prograde metamorphism within an Early Palaeozoic cold subduction zone. Our results show that in‐situ dehydration during the blueschist to eclogite facies transition produces fluid fluxes leading to rheological weakening and densification, consequently promoting ductile‐brittle deformation. Petrographic evidence, supported by thermodynamic modelling and thermobarometry, attest to a prograde passage from lawsonite‐blueschist to peak eclogite facies of ~2.5 GPa and ~620°C. Phengite‐bearing eclogites imply interaction with an externally‐derived fluid, whereas rare phengite‐free, kyanite‐eclogites only record internally‐derived fluid production. Models predict that prograde breakdown of chlorite, lawsonite and amphibole between 500 and 610°C lead to progressive dehydration and release of up to 4.6 wt.% of aqueous fluid. Microstructural data reveal elongated shapes of highly strained omphacite porphyroblasts, displaying minor yet gradual changes in misorientation towards the grain boundaries. Occasionally, these intragranular structures form subgrain cells that have similar sizes to those of neoblasts in the rock matrix. These observations point to the potential onset of dynamic recrystallization processes via dislocation creep. Moreover, the omphacite neoblasts and rutile show non‐random crystallographic preferred orientations (CPOs), which are characterized by the subparallel alignment of point‐like maxima in rutile [001] and [100] axes to those of [001] and (010) of omphacite neoblasts, respectively. Additionally, the [001] axes of these minerals are also subparallel to the weak stretching mineral lineation, and the (100) of rutile and the (010) of omphacite neoblasts are distributed in the plane of the foliation. This suggests that the development of their CPOs was coeval and structurally controlled. Garnet microfractures normal to the foliation are dilated and sealed predominantly by omphacite. The lack of obliquity between CPO and foliation plane, as well as the systematic orientation of garnet microfracture orientations, are consistent with coaxial deformation at peak‐pressure conditions. Unlike other studies, we show that neither an external fluid source nor channelized fluid flow is needed to facilitate a ductile‐brittle deformation of eclogite in a subduction setting.

High‐Temperature Deformation of Enstatite‐Olivine Aggregates

AbstractSynthesized 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.

Crystallographic texture and multiscale boundaries mediated creep anisotropy in additively manufactured Ni-based Hastelloy C276 superalloy

The microstructure and high-temperature creep mechanisms of Ni-based Hastelloy C276 superalloy fabricated using wire and arc-based directed energy deposition were investigated systematically and innovatively. The microstructural investigation revealed that the as-fabricated samples comprise γ-Ni matrix and topologically close-packed (TCP) P phase precipitates. The γ matrix subgrains and grains are spread over multiple highly textured columnar dendrites, with a majority of γ <001> crystallographic orientations closely aligned along the deposition direction. Moreover, the interdendritic regions exhibit severe Mo segregation, P phase particles, and dislocation bands. Creep tests were conducted on miniature samples under various temperature and stress conditions, loaded either in the deposition direction (DD) or travel direction (TD). DD samples exhibit lower minimum strain rates, greater strains-to-failure, and longer creep rupture lifetimes than TD samples, indicating significant creep anisotropy. Dislocation creep was identified as the primary creep mechanism for both DD and TD conditions. During creep, dynamic precipitation of TCP phases occurred in the interdendritic regions, resulting in varying creep resistance between interdendritic and dendritic core regions. Isostress and isostrain models, considering both crystallographic texture and precipitation strengthening, reasonably predicted the observed creep anisotropy during the secondary creep stage. Additionally, variations in the Schmid factor led to significant deformation incompatibility among dendrites in TD samples. Dislocation accumulation in TD sample interdendritic regions promoted new grain nucleation, triggering dynamic recrystallisation, facilitating grain boundary sliding, and accelerating tertiary creep. Furthermore, TCP phase particles in the interdendritic regions contributed to microcrack development, further accelerating creep fracture, especially in the TD condition.

Enhanced impression creep performance of gravity and suction cast Al-12Ce alloy with Si and Mg additions

The current investigation examines the microstructure and impression creep behavior of both gravity and suction-cast Al-12Ce alloy containing 4Si and 4Si+0.4Mg (all in wt%) additions across stress and temperature ranges of 335–480 MPa and 473–548 K, respectively. The Al11Ce3 phase volume fraction, interlamellar spacing (ILS), and aspect ratio decrease when 4Si and/or 0.4Mg are incorporated into the Al-12Ce alloy. However, the aspect ratio and volume fraction of CeAlSi2 phase increased, which was more pronounced in suction casting. The gravity-cast Al-12Ce-X exhibits superior impression creep resistance contrary to Al-12Ce alloy, which further enhanced after suction casting. Remarkably, the suction-cast Al-12Ce-4Si-0.4Mg alloy displays the maximum hardness (38% higher) and highest creep resistance (63% higher) than Al-12Ce alloy. For all the alloys, the prevailing creep mechanism was pipe diffusion-dominated dislocation creep. High creep resistance of suction cast Al-12Ce-4Si-0.4Mg alloy was due to the smallest grain size, less Al11Ce3 phase content, lowest interlamellar spacing, solid solution strengthening by Mg, maximum CeAlSi2 phase content, and highest dislocation density.

Effect of Low Viscosity Contrast between Quartz and Plagioclase on Creep Behavior of the Mid-Crustal Shear Zone

Ultramylonites are among the most extreme fault rocks that commonly occur in the mid-crustal brittle–plastic transition and are mainly characterized by intensely sheared fine-grained microstructures and well-mixed mineral phases. Although the deformation mechanism of ultramylonites is key to understanding the rheological behavior of the mid-crustal shear zone, their microstructural development is still controversial owing to their intensely fine-grained textures. To investigate the possible crustal deformation mechanisms, we studied 13 mylonites obtained from the Kashio shear zone along the Median Tectonic Line that is the largest strike-slip fault in Japan. In particular, we investigated various mixed quartz–plagioclase layers developed within tonalitic mylonite, which are representative of the common mean grain size and crystal fabric of quartz among the studied samples. A high-quality phase-orientation map obtained by electron backscattered diffraction showed not only a wide range of quartz–plagioclase mixing (10%–80% in quartz modal composition) but also revealed a correlation between grain size reduction and crystal fabric weakening in quartz, indicating a change in the deformation mechanism from dislocation creep to grain-size-sensitive creep in the mixed quartz-plagioclase layers. In contrast, plagioclase showed an almost consistent fine grain size and weak to random crystal fabrics regardless of modal composition, indicating that grain size-sensitive creep is dominant. Combined with laboratory-determined flow laws, our results show that the Kashio shear zone could have developed under deformation mechanisms in which the viscosities of quartz and plagioclase are nearly comparable, effectively within 1017–1019 Pa·s, thereby possibly enabling extensive shearing along the Median Tectonic Line.

Characterizing the Complexity of Subduction Zone Flow With an Ensemble of Multiscale Global Convection Models

AbstractSubduction 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.

On Dislocation Climb as an Important Deformation Mechanism for Planetary Interiors

An understanding of the rheological behavior of the solid Earth is fundamental to provide a quantitative description of most geological and geophysical phenomena. The continuum mechanics approach to describing large-scale phenomena needs to be informed by a description of the mechanisms operating at the atomic scale. These involve crystal defects, mainly vacancies and dislocations. This often leads to a binary view of creep reduced to diffusion creep or dislocation creep. However, the interaction between these two types of defects leading to dislocation climb plays an important role, and may even be the main one, in the high-temperature, low strain rate creep mechanisms of interest to the Earth sciences. Here we review the fundamentals of dislocation climb, highlighting the specific problems of minerals. We discuss the importance of computer simulations, informed by experiments, for accurately modeling climb. We show how dislocation climb increasingly appears as a deformation mechanism in its own right. We review the contribution of this mechanism to mineral deformation, particularly in Earth's mantle. Finally, we discuss progress and challenges, and we outline future work directions. ▪Dislocations can be sources or sinks of vacancies, resulting in a displacement out of the glide plane: climb.▪Dislocation climb can be a recovery mechanism during dislocation creep but also a strain-producing mechanism.▪The slow natural strain rates promote the contribution of climb, which is controlled by diffusion.▪In planetary interiors where dislocation glide can be inhibited by pressure, dislocation climb may be the only active mechanism.

A TEM study of nanostructures and interfaces in the hot-press sintered ZrB2–SiC–Si3N4 composites

A fully dense ZrB2–30 vol% SiC composite containing 5 wt% Si3N4 and 4 wt% phenolic resin (1.6 wt% carbon) was sintered using the hot-pressing route under the external pressure of 10 MPa at 1900 ºC for 2 h. The microstructural evolution and interfacial phenomena were scrutinized using advanced microscopy facilities such as high-resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM). The FESEM images showed the ZrB2 and SiC grains without any evidence of Si3N4. The formation of the hexagonal BN (hBN) phase was proven in the sintered composite. The hBN nanosheets had a graphite-like morphology with an average thickness of 20 nm. This phase has a perpendicular orientation to the pressure direction and prevents abnormal ZrB2 grain growth. Two types of ZrB2/SiC interfaces were detected, which exhibited an amorphous phase along with the grain boundary and a clean/smooth interface, resulting from the Si3N4 addition. HRTEM and inverse fast Fourier transform (IFFT) observations disclosed that the d-spacing value in the ZrB2 grain (0.335 nm) is higher than those reported in the literature. Furthermore, it was found that the exerted pressure during the sintering distorted atomic planes. The presence of numerous dislocations within the ZrB2 grains confirmed dislocation creep as the main densification mechanism.

Deformation microstructures of low- and high-strain epidote-blueschist (Ryukyu arc, Japan): Implications for subduction interface rheology

We present field and microstructural data from an exhumed subduction complex in the Ryukyu arc, Japan, where epidote-blueschist, Triassic, Tomuru metamorphic rocks with block-in-matrix structure crop out. With the aim to constrain epidote-blueschist rheology, we investigate fabric development and infer deformation mechanisms of blocks and matrix through microstructural analyses on the main fabric-forming minerals: glaucophane, epidote and albite. The blocks have a poorly developed, discontinuous foliation. In contrast, the matrix has a well-developed, continuous foliation. Glaucophane is the principal foliation-forming phase, arranged in interconnected layers that surround epidote and albite, which suggests that glaucophane controls epidote-blueschist rheology. Deformation mechanisms inferred from petrographic and electron backscatter diffraction (EBSD) analysis are similar in blocks and matrix, with glaucophane deformed by diffusion creep accompanied by reaction-driven creep, albite by diffusion creep, and epidote by brittle fracture and incipient dislocation creep, based on degrees of internal misorientations, lattice preferred orientation (LPO) development and grain size. During deformation, shear stress must have been relatively low, which is implied by the predominance of diffusion and reaction creep as the dominant deformation mechanisms, and by the small viscosity contrast between blocks and matrix, reflected by their similarity in dominant deformation mechanism.

A key role for diffusion creep in eclogites: Omphacite deformation in the Zermatt-Saas unit, Italian Alps

Eclogites commonly form from subducted oceanic crust and as such carry key information on subduction zone rheology. Using a combination of microanalytical techniques on deformed eclogites from the Zermatt-Saas unit of the Italian Alps, we explore the mechanisms which resulted in both an omphacite shape and lattice preferred orientation. Omphacite defines both foliation and lineation in these rocks and displays a strong S-type lattice preferred orientation. Scarcity of microstructures associated with dislocation creep combined with observed sharp asymmetrical chemical zonation in foliated grains suggest diffusion creep plays an important role in the development of omphacite lattice preferred orientation in these eclogites. Modelling of the pressure-temperature conditions possible for observed mineralogy and mineral geochemistry, and textural relationships between omphacite and retrogressive minerals, place the action of diffusion creep, at the latest, by the onset of retrogression of these Zermatt-Saas eclogites. We propose a model of eclogite deformation that occurred initially via small amounts of dislocation creep which moved quickly into a dominant diffusion creep field, particularly as exhumation/retrogression of these eclogite rocks began. This result suggests that diffusion creep can dominate eclogite deformation at high P-T conditions in subduction zones.