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High-pressure sintering (HPS) is a promising technique for producing nanocrystalline ceramics with unique properties. However, the densification mechanisms of HPS for nanocrystalline ceramics at different sintering stages remain controversial. This study focuses on Gd 2 Zr 2 O 7 (GZO) nanocrystalline ceramics, investigating their microstructure evolution and densification behavior under varying pressures, temperatures, and dwelling times. The HPS process involves distinct stages: cold compaction, hot compaction, and isothermal progression. During cold compaction, densification is driven by the breakdown of aggregates, particle rearrangement, and local plastic deformation, resulting in a relative density of 84.8 %. The intermediate stage achieves an increase to ∼93.7 % in relative density by grain boundary-mediated plastic deformation mechanism. The final isothermal stage, held at 500 °C/5 GPa, achieves nearly full density (∼99.2 %) through diffusion creep, eliminating microstructural defects. This comprehensive understanding of HPS densification mechanisms, exemplified by GZO ceramics, contributes to advancing nanocrystalline ceramic applications. [ABSTRACT FROM AUTHOR]

Currently, studies of structural superplasticity (SP) are of great interest, since the use of this mode in metalworking technologies allows for the production of parts of various shapes in one operation (with high repetition accuracy of even very complex shapes), while requiring less energy and material resources (relatively low pressures and tool wear) compared to deformation in the ‘normal plasticity’ mode. Other advantages of using a structural SP are improved physical and mechanical characteristics of the finished product: better surface quality after deformation, high ductility at elevated temperatures, increased strength at temperatures close to room one without reducing ductility (most often there is an increase in ductility), increased cyclic strength, hardness, impact resistance, elevated corrosion-resistance durability, and absence of anisotropy of properties after superplastic deformation.

Currently, studies of structural superplasticity (SP) are of great interest, since the use of this mode in metalworking technologies allows for the production of parts of various shapes in one operation (with high repetition accuracy of even very complex shapes), while requiring less energy and material resources (relatively low pressures and tool wear) compared to deformation in the 'normal plasticity' mode. Other advantages of using a structural SP are improved physical and mechanical characteristics of the finished product: better surface quality after deformation, high ductility at elevated temperatures, increased strength at temperatures close to room one without reducing ductility (most often there is an increase in ductility), increased cyclic strength, hardness, impact resistance, elevated corrosion-resistance durability, and absence of anisotropy of properties after superplastic deformation. [ABSTRACT FROM AUTHOR]

In a previous study (Okamoto & Hiraga, 2022, https://doi.org/10.1029/2022jb024638), we concluded that diffusion creep and grain growth in polymineralic rocks proceed by a common diffusional mechanism. Here, we built on that finding and estimated lower mantle grain size and viscosity during a single mantle convection cycle dominated by diffusion creep. We approximated the lower mantle as a two‐phase material consisting of bridgmanite + ferropericlase and post‐perovskite + ferropericlase, depending on depth. We used previously reported self‐diffusivities for bridgmanite and post‐perovskite. We predict a bridgmanite grain‐size of tens to hundreds of microns shortly after the phase transition at ∼660 km depth. This size remains relatively constant until the mantle material enters the post‐perovskite zone, which is marked by significant grain growth up to ∼9 mm just prior to upwelling. This size is sufficient to prevent further grain growth until the mantle material reaches the top of the lower mantle. These grain sizes combined with the diffusivities yield viscosities that vary laterally and with depth. At a lateral temperature difference of up to 800 K in the lower mantle, fine‐grained cold downwelling mantle is almost as viscous as, or more likely to be softer than coarse‐grained hot upwelling mantle. The lateral viscosity variations cannot be more than 2 orders of magnitude, and we estimated viscosities of 1018–1020 Pa · s in the upper lower mantle, 5 × 1020–5 × 1022 Pa · s in the lower bridgmanite zone, and 1017–1019 Pa · s in the post‐perovskite zone, which compare well with the values estimated in previous geophysical modeling studies. Plain Language Summary: We estimated lower mantle viscosity during a mantle convection cycle by assuming that the mantle flows by diffusion creep. Because diffusion creep is a grain‐size sensitive mechanism, grain size is a key parameter for determining mantle viscosity. We recently concluded that diffusion creep and grain growth in polymineralic rocks occur by the same diffusional mechanism. With previously reported self‐diffusivities for bridgmanite and post‐perovskite, we were able to estimate how the grain size of these minerals evolves after the ∼660‐km phase transition in downwelling mantle. Using these grain sizes and diffusivities, we were able to determine the mantle viscosity. We demonstrate that lower mantle viscosity has temporal and spatial variations induced by grain growth. At a lateral temperature difference of up to 800 K in the lower mantle, fine‐grained cold downwelling mantle is almost as viscous as, or more likely to be softer than coarse‐grained hot upwelling mantle. The estimated overall viscosity structure compares well with a one‐dimensional depth structure of the lower mantle viscosity estimated in previous geophysical modeling studies. Key Points: We estimated lower mantle viscosity during a mantle convection cycle by assuming mantle flow with a diffusion creep mechanismOur estimated viscosity structure agrees well with the viscosity structures predicted in previous geophysical modelsTemperature and grain size dependence of diffusion creep suppresses lateral changes in the viscosity of the lower mantle [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]

Part II of the paper presents comprehensive investigations aimed at demonstrating the validation and effectiveness of the proposed integrated model for simulating Coble creep deformation and void nucleation/growth in polycrystalline solids for material design. Quantitative comparisons between the model’s predictions of void area fraction in a 2D cross-section and experimental results using Alloy 201, a commercially pure nickel, demonstrated a high degree of agreement. Furthermore, the numerical simulation results align with theoretical equations, affirming the model’s validity and its capacity to represent complex phenomena accurately. Although traditional laboratory testing for pure Coble creep is challenging due to constraints such as time and equipment limitations, the proposed model offers a distinct advantage, allowing numerical simulation of Coble creep in materials with arbitrary polycrystalline morphologies under various environmental conditions. Leveraging this capability, the study conducted comprehensive numerical simulations to quantitatively explore the effects of environmental and polycrystalline morphology factors on Coble creep deformation and void nucleation/growth in polycrystalline solids. These aspects, which have not been comprehensively addressed in existing studies, were thoroughly investigated. The findings presented here establish a novel foundation for designing heat-resistant materials applicable across diverse equipment scenarios.

This study proposes an integrated model for simulating Coble creep deformation and void nucleation/growth in a 3D polycrystalline solid. Part I of the paper provides a theoretical framework for the proposed model. A representative volume element approach is employed to predict the effects of 3D polycrystalline morphology. The model comprises two distinct but interconnected stages: deformation and void nucleation/growth. The deformation stage of the proposed model comprises two sub-models: grain boundary (GB) migration and GB diffusion. The void nucleation/growth stage is composed of three consecutive calculations: void nucleation, void growth, and postprocessing. The process simulated in the void nucleation/growth stage is driven by relative GB velocity of the respective GBFs and diffusional fluxes between adjacent GBFs, which are provided from the deformation stage. The void nucleation rate is quantified using the relative GB velocity, and the initial void size is determined based on the stability condition for void existence derived from Helmholtz free energy. The void growth rate is evaluated by the atomic diffusion on the GBF and the surface of each void.

The flow law for diffusion creep of chromite spinel has been determined from uniaxial compressive creep experiments at temperatures of 1400° to 1600 °C and a pressure of 0.1 MPa on polycrystalline samples of Mg(Cr,Al)2O4 with grain sizes of ~ 10 μm and chromium mole fractions, XCr, of 0.25, 0.50, and 0.75. Under our experimental conditions, creep rate is dominated by grain boundary diffusion of Cr. The dependence of strain rate on composition is manifest through the compositional dependence of the activation energy. The activation energy for grain boundary diffusion can be described in terms of melting temperature, Tm, as a function of composition in the form Q Cr gb = g Cr gb R T m (X Cr) , where g Cr gb is a material-dependent scaling parameter. The value obtained for this scaling parameter of g Cr gb = 19.7 ± 0.5 yields activations energies of approximately 405, 416, and 427 kJ/mol for samples with XCr = 0.25, 0.50, and 0.75, respectively. Extrapolation of our results to conditions appropriate for chromitite pods enveloped by peridotites from the Oman ophiolite indicates that lattice diffusion, rather than grain boundary diffusion, largely dominates plastic deformation of chromite spinel, which is significantly stronger than olivine. [ABSTRACT FROM AUTHOR]

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

Recent progress on the study of olivine LPO after (Karato et al., 2008) is reviewed with the emphasis on three issues: (i) LPO formed by the rotation of olivine crystals with anisotropic shape (euhedral crystals) in diffusion creep (Miyazaki et al., 2013), (ii) B-type LPO in the olivine + basaltic melt (Holtzman et al., 2003), and (iii) pressure change in the influence of LPO (Ohuchi and Irifune, 2013). Regarding the role of euhedral crystals, we show that euhedral olivine crystals occur in a mixture of forsterite and diopside (used by (Miyazaki et al., 2013)) but not in a mixture of olivine and enstatite. Consequently, the results by reported by (Miyazaki et al., 2013) are not applicable to the Earth's upper mantle where olivine co-exists mostly with enstatite. Also we show that the LPO reported by (Miyazaki et al., 2013) is not consistent with the shape of olivine, and the observed LPO is likely due to dislocation glide (A-type fabric) under the conditions near the diffusion-dislocation creep regime boundary. Regarding the LPO of olivine with the presence of melt, (Qi et al., 2018)'s experimental study with the torsion geometry did not reproduce the B-type fabric reported by (Holtzman et al., 2003) indicating that the B-type fabric reported by (Holtzman et al., 2003) was indeed an artifact of the direct shear experiments. The weak LPO found by (Qi et al., 2018) (compared to that by (Zimmerman et al., 1999)) can be explained by the smaller grain size in their experiments. I conclude that a majority of the experimental results on olivine LPO at relatively low pressures (<2 GPa) can be understood based on the basics of deformation mechanism map and LPO caused by various slip systems in olivine. Regarding a claim by (Ohuchi and Irifune, 2013) that the A-type LPO (a-slip) dominates at high water content and c-slip dominates at low water content at pressures higher than ∼7 GPa, a compilation of experimental studies by (Masuti et al., 2019) and the observed LPO of the ultra-deep xenolith do not support their claim. However, experimental studies under these high-pressure conditions are limited and there remain large uncertainties regarding the LPO at high pressures (P>3 GPa). [ABSTRACT FROM AUTHOR]

In this study, we investigate the finite deformation of a polycrystalline mixture of bridgmanite (Br) and ferropericlase (Fp) by diffusion creep at the lower mantle-like temperature and pressure by using the self-consistent approach. We explore the influence of volume fraction of Fp, viscosity contrast, and strain dependence (effect of shape change) under both axial (coaxial deformation) and simple shear (non-coaxial deformation). Our present study shows: i) the strength (viscosity) contrast between Fp and Br increases with strain since the viscosity of Fp significantly decreases as Fp grain elongates, and (ii) deformation starts from nearly homogeneous strain to finally nearly homogeneous stress under simple shear whereas deformation behavior remains nearly homogeneous strain under axial deformation. A more substantial creep rate partitioning occurs in simple shear than in axial deformation. These results imply that strain localization via diffusion creep might occur in the lower mantle, particularly in regions where the simple shear is dominated (i.e. , in the boundary layers (e.g. , the D″ layer)). [Display omitted] • Diffusion creep of the bridgmanite and ferropericlase mixture is investigated. • The self-consistent method is used to find the rheology of the two-phase mixture. • The creep rate is largely localized in ferropericlase under simple shear. • Diffusion creep would lead to substantial shear localization in the lower mantle. [ABSTRACT FROM AUTHOR]

[Display omitted] • A model integrating Coble creep deformation and void nucleation/growth is proposed. • Part I provides a comprehensive theoretical framework and numerical modelling. • The model is based on the RVE approach simulating a 3D polycrystalline structure. • The deformation stage has two sub-models: grain boundary migration and diffusion. • The void nucleation/growth stage is driven by the results from the deform. stage. This study proposes an integrated model for simulating Coble creep deformation and void nucleation/growth in a 3D polycrystalline solid. Part I of the paper provides a theoretical framework for the proposed model. A representative volume element approach is employed to predict the effects of 3D polycrystalline morphology. The model comprises two distinct but interconnected stages: deformation and void nucleation/growth. The deformation stage of the proposed model comprises two sub-models: grain boundary (GB) migration and GB diffusion. The void nucleation/growth stage is composed of three consecutive calculations: void nucleation, void growth, and postprocessing. The process simulated in the void nucleation/growth stage is driven by relative GB velocity of the respective GBFs and diffusional fluxes between adjacent GBFs, which are provided from the deformation stage. The void nucleation rate is quantified using the relative GB velocity, and the initial void size is determined based on the stability condition for void existence derived from Helmholtz free energy. The void growth rate is evaluated by the atomic diffusion on the GBF and the surface of each void. [ABSTRACT FROM AUTHOR]

[Display omitted] • Part II demonstrates the validation and effectiveness of the integrated model. • Detailed investigations and discussions are presented based on numerical simulations. • Model prediction for void nucleation/growth agreed with the experimental results. • Loading and temperature effects were clarified by comprehensive simulations. • Polycrystalline morphology factors' effects on creep performance were also clarified. Part II of the paper presents comprehensive investigations aimed at demonstrating the validation and effectiveness of the proposed integrated model for simulating Coble creep deformation and void nucleation/growth in polycrystalline solids for material design. Quantitative comparisons between the model's predictions of void area fraction in a 2D cross-section and experimental results using Alloy 201, a commercially pure nickel, demonstrated a high degree of agreement. Furthermore, the numerical simulation results align with theoretical equations, affirming the model's validity and its capacity to represent complex phenomena accurately. Although traditional laboratory testing for pure Coble creep is challenging due to constraints such as time and equipment limitations, the proposed model offers a distinct advantage, allowing numerical simulation of Coble creep in materials with arbitrary polycrystalline morphologies under various environmental conditions. Leveraging this capability, the study conducted comprehensive numerical simulations to quantitatively explore the effects of environmental and polycrystalline morphology factors on Coble creep deformation and void nucleation/growth in polycrystalline solids. These aspects, which have not been comprehensively addressed in existing studies, were thoroughly investigated. The findings presented here establish a novel foundation for designing heat-resistant materials applicable across diverse equipment scenarios. [ABSTRACT FROM AUTHOR]

We deformed aggregates of Fe‐free olivine +20 vol% diopside in pure shear to study olivine grain morphology and fabric during diffusion creep. Samples were each deformed to a strain of around 0.7 at different temperatures and experimental durations, which resulted in different grain sizes among the samples due to grain growth. The sample microstructures were observed from each of the three principal stress directions, which revealed various 3D grain morphologies among the samples. Samples with tabular grains displayed a crystallographic preferred orientation (CPO) in olivine, while samples with weakly anisotropic grains had uniform CPOs. The CPO patterns vary from AG‐type (i.e., a uniaxial [010] concentration in the direction of sample shortening) to A‐type (i.e [010] and [100] concentrations in the directions of sample shortening and stretching, respectively), with intermediate types being the most common. The transition from AG‐ to A‐type fabrics is accompanied by a change from oblate to prolate grain morphologies. We explain the direct correlation between the olivine morphology and fabrics based on preferential grain‐boundary sliding (GBS) at boundaries with fewer steps (ledges). Oblate morphologies with well‐developed grain boundaries oriented parallel to (010) provide easy GBS along (010) and [h0l] planes and directions, respectively, while relatively prolate morphologies with boundaries parallel to (010) that are elongated along [100] provide easy GBS along (010) and [100] planes and directions, respectively. Boundaries form parallel to a particular crystallographic plane through differential grain growth in the directions of the crystallographic axes. Such grain growth causes fabric transitions during diffusion creep. Plain Language Summary: We deformed aggregates of Fe‐free olivine +20 vol% diopside in pure shear to study olivine grain morphology and fabric during diffusion creep. Various 3D grain morphologies and fabrics were found among the samples deformed at different temperatures and durations. We found a direct correlation between the olivine morphology and fabric, which is explained based on preferential grain‐boundary sliding at grain boundaries parallel to low‐index crystallographic planes. Such boundaries form through differential grain growth along the crystallographic axes. We propose grain growth to be the cause of fabric transitions during diffusion creep. Key Points: Various olivine grain morphologies and fabrics were observed after deformation to similar strains in pure shear during diffusion creepGrain morphology and fabric types are strongly correlatedGrain morphology changes during grain growth, which results in a fabric transition [ABSTRACT FROM AUTHOR]

We investigate the finite deformation of a weak inclusion embedded in a strong matrix. The goal is to understand the possible causes for shear localization that would explain the presence of geochemical heterogeneity in the Earth's lower mantle. The weak phase in the lower mantle is ferropericlase (Fp), and the strong phase is bridgmanite (Br). We herein consider deformation by diffusion creep for which some supporting evidence is available. When an inclusion deforms by diffusion creep, deformation itself modifies its geometry and stress distribution. The modified stress distribution in turn leads to strain‐dependent rheological properties due to the changes in diffusion path length and the stress gradient that drives diffusion flux. We investigate the evolution of deformation of a weak inclusion (i.e., Fp‐like weak grain embedded in Br‐like strong matrix) using the Eshelby theory (for stress and strain rate fields) combined with a theory of diffusional mass transport caused by the gradient in the normal stress within the inclusion. We found that finite strain leads to a significant strain weakening under simple shear deformation but not under axial deformation. Since diffusion creep is thought to be a dominant mechanism of the lower mantle rheology, the results of the present study provide a basis for investigating the nature of shear localization and its important implication for the preservation of geochemical heterogeneity and the distribution of seismic anisotropy in the lower mantle. Plain Language Summary: Localized deformation is a possible explanation for the preservation of geochemical heterogeneity in the lower mantle. The lower mantle is made of two major phases: bridgmanite (Br [∼70%]) and ferropericlase (Fp [∼20%]) (and other minor phases). Lab data show that Fp is much weaker than Br. We investigate the deformation of a weak Fp‐like inclusion embedded in a strong Br‐like matrix as a function of strain. We consider finite strain deformation by diffusion creep. Deformation of a weaker phase evolves with strain caused by the evolution of internal stress (stress acting on the inclusion) and the shape of an inclusion. A substantial weakening of inclusion is observed particularly for shear deformation. These results provide an important implication for the preservation of geochemical heterogeneity and the distribution of seismic anisotropy in the lower mantle of the Earth. Key Points: Diffusion creep rate is highly sensitive to the shape of the Fp grain (embedded in a Br matrix) and boundary normal stress stateIn the setting of a two‐phase mixture, the diffusion creep rate of Fp dramatically increases under remote simple shear deformationThe strain‐dependent diffusion creep of Fp may lead to shear localization that forms the weak boundary layer within the lower mantle [ABSTRACT FROM AUTHOR]

Rare-earth iron garnets are instrumental in the development of integrated nonreciprocal passive devices such as isolators and circulators in silicon photonics. Unfortunately, monolithic integration of garnet on-chip requires annealing temperatures much higher than the thermal budget of a semiconductor foundry. Here, we report the mechanical exfoliation of large area (0.2 mm × 0.2 mm) nanosheets of a high-gyrotropy cerium-doped terbium iron garnet (CeTbIG) enabled by a strain-enhanced vacancy diffusion process that follows the Nabarro–Herring (lattice diffusion) model. Diffusivities calculated from the strain rate–stress data (1.13 × 10–18 m2s–1) identify iron and rare-earth cations as the rate-determining lattice diffusants. Cross-section scanning transmission electron microscopy reveals an exfoliation gap located ∼30 nm into the film, comparable to the cation diffusion length, which appears to verify the model. With a saturation magnetization of 18 emu cc–1 and a Faraday rotation of −2900°cm–1 at 1550 nm, the magnetic and optical properties of the nanosheets are comparable to their thin-film values. Diffusion-driven exfoliation will open foundry-acceptable pathways for heterogeneous integration of garnets on photonic waveguides and protect devices from the high-temperature processes used in crystallizing garnet films. [ABSTRACT FROM AUTHOR]

A physical model for describing the diffusion creep in perovskite-type material given oxygen non-stoichiometry and tensile–compressive asymmetry is developed. Reference tests for determining the creep parameters in the constitutive equations are discussed. The proposed model is used for combined numerical-and-analytical simulation of diffusion creep in a hollow thick-walled perovskite cylinder under generalized plane strain conditions. The analytical solution for oxygen non-stoichiometry is used. Also, the closed-form general expressions for stresses in a hollow cylinder undergoing diffusion creep at a given oxygen chemical potential gradient are derived. The numerical integration is performed using the-Runge–Kutta–Merson method of the fourth order with automatic step control. The stress redistribution over time in the cylinder under the diffusion creep conditions is analyzed. The numerical results related to tensile–compressive asymmetry and oxygen surface exchange in a perovskite cylinder are discussed. [ABSTRACT FROM AUTHOR]

Pseudotachylytes are fine‐grained fault rocks that solidify from melt that is produced in fault zones during earthquakes. Exposed sections of natural fault zones reveal evidence of postseismic plastic deformation (i.e., reactivation) of pseudotachylyte, which suggests these rocks may contribute to aseismic slip behavior in regions of repeated seismicity. To measure the plastic flow behavior of pseudotachylyte, we performed high‐temperature deformation experiments on pseudotachylyte from the Gole Larghe Fault Zone, Italy. Plastic reactivation of pseudotachylyte occurs at temperatures above 700°C for strain rates accessible during laboratory experiments. Extrapolation of experimental results to natural conditions demonstrates that pseudotachylyte deforms via diffusion creep at crustal conditions and is much weaker than host rocks in seismically active regions. Importantly, the presence of plastically deforming pseudotachylyte may influence the thickness of the seismogenic layer in some fault zones that experience repeated seismicity. Plain Language Summary: While the plastic strength of individual minerals constituting the continental crust are well known, the plastic strength of rocks resulting from coseismic slip in seismically active regions has yet to be measured, despite of the observations of their plastic reactivation. Here, we establish the flow law parameters controlling the strength of natural pseudotachylyte, fine‐grained fault rocks that crystalize from melt produced in fault zones during earthquakes. Experiments where conducted at temperature ranging from 700 to 900°C and at a confining pressure of 300 MPa. Our experimental results demonstrate that pseudotachylyte deforms via diffusion creep at crustal conditions. Flow law parameters derived from plastically deforming pseudotachylyte suggest that the presence of pseudotachylyte along faults drastically reduces the strength of the seismically active continental crust. Finally, the presence of seismically generated pseudotachylyte is likely to strongly influence the thickness of the seismogenic layer, since seismicity is rare in mature crustal faults at depths below which pseudotachylyte deforms plastically. Key Points: Natural pseudotachylyte‐filled shear zones can reactivate plastically at high temperaturePlastic deformation processes are dominated by diffusion creep at low temperaturesRheology of pseudotachylytes can reduce the strength of the continental crust [ABSTRACT FROM AUTHOR]

Abstract Slip on the active Mai'iu low‐angle normal fault in Papua New Guinea that dips 15–24° at the surface has exhumed in its footwall a single, continuous fault surface across a >25‐km‐wide dome. Derived from a metabasaltic protolith, the fault zone consists of a 275°C–370°C. In a foliated cataclasite unit at shallower crustal levels (T ≈ 150°C–275°C), metasomatic reactions accompanied fluid‐assisted mass transfer processes that accommodated aseismic, distributed shearing; pseudotachylites and ultracataclasites in the same unit indicate that such creep was punctuated by episodes of seismic slip—after which creep resumed. At the shallowest levels (T

Slip on the active Mai'iu low‐angle normal fault in Papua New Guinea that dips 15–24° at the surface has exhumed in its footwall a single, continuous fault surface across a >25‐km‐wide dome. Derived from a metabasaltic protolith, the fault zone consists of a <3‐m‐thick zone of gouges and cataclasites that overprint a structurally underlying carapace of extensional mylonites. Detailed microstructural and geochemical data, combined with chlorite‐based geothermometry, reveal changing deformation processes and conditions in the Mai'iu fault rocks as they were exhumed. The microstructure of nonplastically deformed actinolite grains inherited from the fine‐grained (6–35 µm) metabasaltic protolith indicates that shearing at depth was controlled by diffusion creep accompanied by grain‐boundary sliding of these grains together with chlorite neo‐crystallization at T > 275°C–370°C. In a foliated cataclasite unit at shallower crustal levels (T ≈ 150°C–275°C), metasomatic reactions accompanied fluid‐assisted mass transfer processes that accommodated aseismic, distributed shearing; pseudotachylites and ultracataclasites in the same unit indicate that such creep was punctuated by episodes of seismic slip—after which creep resumed. At the shallowest levels (T < 150°C), gouges contain abundant saponite, a frictionally weak mineral that promotes creep on the shallowest dipping (≤24°), most poorly oriented part of the Mai'iu fault. Our field, microstructural and geochemical data of freshly exhumed fault rocks support geodetic, seismological, and geomorphic evidence for mixed seismic‐to‐aseismic slip on this active low‐angle normal fault. Key Points: Fault rock microstructures reveal slow‐to‐fast slip on an active detachment fault that dips 15–24° at the Earth's surfacePseudotachylites, foliated cataclasites, and ultracataclasites developed in a zone of mixed mode, seismic‐to‐aseismic slip behaviorFrictionally weak saponite in fault gouge promotes slip on the most poorly oriented, surficial part (<24°) of the Mai'iu fault [ABSTRACT FROM AUTHOR]

The effect of nickel on surface energy of the solid/gas interface of silver was studied. The measurements were taken using foils according to the previously developed in situ method in an atmosphere of Ar + 10% H2 at high temperatures. This method simultaneously allows one to determine the surface energy and diffusion creep rate. The solubility of nickel in silver is very low, so the measurements were taken for the alloys in the two-phase region with 0.45 at.% Ni and 1.5 at.% Ni, as well as for pure nickel. Nickel is shown to significantly increase the surface energy of silver and slow down the diffusion creep rate. [ABSTRACT FROM AUTHOR]

The lowermost portion of Earth's mantle (D") above the core- mantle boundary shows anomalous seismic features, such as strong seismic anisotropy, related to the properties of the main mineral MgSiO3 postperovskite. But, after over a decade of investigations, the seismic observations still cannot be explained simply by flow models which assume dislocation creep in postperovskite. We have investigated the chemical diffusivity of perovskite and postperovskite phases by experiment and ab initio simulation, and derive equations for the observed anisotropic diffusion creep. There is excellent agreement between experiments and simulations for both phases in all of the chemical systems studied. Single-crystal diffusivity in postperovskite displays at least 3 orders of magnitude of anisotropy by experiment and simulation (Da = 1,000 Db; Db ≈ Dc) in zinc fluoride, and an even more extreme anisotropy is predicted (Da = 10,000 Dc; Dc = 10,000 Db) in the natural MgSiO3 system. Anisotropic chemical diffusivity results in anisotropic diffusion creep, texture generation, and a strainweakening rheology. The results for MgSiO3 postperovskite strongly imply that regions within the D" region of Earth dominated by postperovskite will 1) be substantially weaker than regions dominated by perovskite and 2) develop a strain-induced crystallographic-preferred orientation with strain-weakening rheology. This leads to strain localization and the possibility to bring regions with significantly varying textures into close proximity by strain on narrow shear zones. Anisotropic diffusion creep therefore provides an attractive alternative explanation for the complexity in observed seismic anisotropy and the rapid lateral changes in seismic velocities in D". [ABSTRACT FROM AUTHOR]

Al–Li alloy possesses remarkable advantages such as low density, high specific strength and specific elastic modulus, making it an exceptional aerospace structural material with considerable application potential. However, Al–Li alloy suffers from poor formability, which restricts its broader engineering applications. Alternatively, superplastic forming at high temperatures is widely applied for producing such components. Temperature is a key factor affecting the thermal deformation properties of Al–Li alloy. In this study, tensile tests of 2198 Al–Cu–Li alloy were conducted at different temperatures, and dedicated characterizations were performed to reveal the underlying mechanisms responsible for the hot deformation behavior. Results reveal that three distinct stages are observed in the superplastic deformation process of this alloy. Specifically, in the initial low-temperature range (400 °C–450 °C), incomplete dynamic recrystallization (DRX), accompanied by grain rotation and elongation plays a dominant role in the hot deformation mechanism. In the middle temperature stage (450 °C–500 °C), both DRX and grain boundary sliding (GBS) occur. At the high-temperature stage (500 °C–550 °C), diffusion creep emerges as the prominent mechanism. Moreover, optimum superplasticity is achieved at 500 °C, which results from a synergistic combination of several factors, including grain refinement by DRX, an increased fraction of high-angle grain boundaries (HAGBs), reduced dislocation density and weak texture. [ABSTRACT FROM AUTHOR]

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The chemical widths of grain and phase boundaries in deformed wehrlite (olivine–clinopyroxene; Ol–Cpx) aggregates are characterized by atom probe tomography (APT), a (laser-assisted) field evaporation technique employing time-of-flight mass spectrometry. The wehrlite was deformed to high finite strain in diffusion creep: The effective viscosity measured for the wehrlite is approximately an order of magnitude lower than that of either end-member aggregate; further, phase ordering, in which the spatial density of Ol–Cpx phase boundaries increases with accumulated strain, characterizes the deformation (Zhao et al. in Earth Planet Sci Lett 517:83–94, 2019). The mechanical results imply that the transport properties of the phase boundaries—dictated by their structure and composition—differ from those of grain boundaries. Our APT data show that, indeed, the chemical widths of crystalline Ol–Cpx phase boundaries—3.1–6.6 nm, depending on the element used for their characterization—are up to a factor of two greater than the chemical widths of crystalline Ol–Ol and Cpx–Cpx grain boundaries. Careful statistical analyses of the APT data reveal that the near-boundary compositional profiles of the presented Ol–Cpx phase boundary are consistent with—indeed, evidence for—the rheological model in which diffusion creep is rate-limited by the (mechanism-required) interfacial reactions at the Ol–Cpx phase boundaries. Such an analysis is unavailable by current electron beam/X-ray spectrometry approaches, which have not the requisite spatial precision. APT application to nanometer-scale problems in silicate petrology is challenging, particularly because signal overlap is caused by the evaporation of polyion species. We carefully outline the procedures used to acquire and discriminate the data in order to address the challenges of signal overlap. [ABSTRACT FROM AUTHOR]

Parameters of superplasticity of the Al–7.6% Mg–0.6% Mn–0.25% Cr alloy have been studied in the range of 490–520°C, and the optimum temperature of deformation has been determined. The evolution of the grain structure in the bulk and on the surface of samples during the superplastic deformation at a temperature of 510°C has been analyzed. The contribution of grain-boundary sliding has been estimated in the strain-rate regime of superplastic deformation according to the results of the analysis of changes in the structure of the surface with preliminarily applied marker scratches. The contribution of grain-boundary sliding to the total deformation is 20–30%; the contribution of diffusion creep decreases from 40 to 20% with an increase in the deformation rate by an order of magnitude, from 1 × 10–3 to 1 × 10–2 s–1. The intragranular deformation is localized in the peripheral regions of grains and in the region of striated zones. [ABSTRACT FROM AUTHOR]

We conducted uniaxial compression and grain growth experiments on fine‐grained (~1 μm) forsterite +20 vol% enstatite aggregates. Based on analyses of the sensitivity of the strain rate as a function of stress, we find power law creep at low stress, Newtonian creep at intermediate stress, and again power law creep at high stress, which correspond to interface‐controlled diffusion creep, grain boundary diffusion (Coble) creep, and a dislocation‐controlled process, respectively. The creep rate of these samples is well expressed by a combination of strain rates of these three mechanisms where interface‐controlled and Coble creep rates are combined as series‐sequential processes, while the rate of the dislocation process is added with them as a parallel‐concurrent process. Mechanical data collected continuously during the application of a constant load but while slowly changing temperature were decomposed into data for every 1 °C, which allowed consideration of >600 mechanical data points from 1054 to 1370 °C. The data were analyzed using Bayesian statistics implementing a Markov chain Monte Carlo method imposed on the above constitutive equation, resulting in the best fit flow law parameters for interface‐controlled and Coble creep. Samples were annealed for 500 hr at various temperatures. A comparison of the final grain sizes as a function of temperature on an Arrhenius plot resulted in an activation energy for grain growth similar to that observed for grain boundary diffusion during Coble creep of these materials. Key Points: Power law, Newtonian, and again power law creep regimes are identified at low to high stress in forsterite + enstatite aggregateGreater than 600 creep data were analyzed by Bayesian statistics obtaining flow law parameters for interface‐controlled creep and Coble creepGrain growth rate as a function of temperature is well determined [ABSTRACT FROM AUTHOR]

We determined the rate‐controlling diffusivities for creep and grain growth of forsterite +20 vol% enstatite aggregates from the experimental results presented in Part 1 of this study. Both diffusivities exhibit similar values with similar temperature dependency, indicating that a common grain boundary diffusion process controls both creep and grain growth. This correspondence is due to the presence of pyroxene grains as a secondary phase in the olivine aggregate, which is the case for the majority of mantle‐derived rocks. As a consequence, the viscosity (η) of the material during deformation by grain boundary diffusion creep accompanied by grain growth changes with time (t) and temperature (T) as η∝t34expQGB4RT, where QGB is the activation energy for grain boundary diffusion and R is gas constant. This viscosity relationship is used to interpret the very low activation energy estimated for the viscosity of the oceanic mantle lithosphere. Key Points: The same diffusional process controls diffusion creep and grain growth in aggregates of forsterite +20 vol% enstatiteThe activation energy of diffusion creep accompanied by grain growth is one fourth the activation energy of grain boundary diffusionThe diffusion creep process presented here can account for the mantle viscosity in the oceanic lithosphere [ABSTRACT FROM AUTHOR]

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

Evolution of the deformation behaviour, surface and bulk structures at superplastic flow of the АА7475 aluminium-based alloy were studied by scanning and transmission electron microscopes and a focused ion beam technique. Differences between deformation behaviour at a primary stage with strains below 0.69 and a steady stage with strains above 0.69 were discussed. The research showed the grain growth and grain elongation to the stress direction at a primary stage of deformation. Stabilisation of both mean grain size and grain aspect ratio was found at a steady stage of deformation. Grain neighbour switching, grain rotation, dispersoid free zones and some insignificant intragranular strain were observed at both stages. Appearing and disappearing of the grains on the sample surface, with increased dislocation activity occurred at the steady stage of deformation. The current results highlighted the importance of diffusion creep as a dominant mechanism at the beginning of superplastic deformation and as an accommodation mechanism of the grain boundary sliding at the steady stage of deformation. The dislocation creep as an additional accommodation mechanism of the grain boundary sliding at the steady stage of superplastic deformation is suggested. [ABSTRACT FROM AUTHOR]

The microstructure evolution and superplastic deformation mechanisms of a 2A97 Al–Cu–Li alloy (Al-3.4Cu-1.5Li-0.5Zn-0.4Mg-0.3Mn-0.15Zr) with initial banded grains were studied by using electron backscatter diffraction and focused ion beam techniques. The uniaxial superplastic tensile tests were carried out at a temperature of 430 °C and an initial strain rate of 2 × 10−3 s−1. The surface studies under a large true strain range (0–1.61) were achieved to investigate the superplastic deformation mechanisms. The results showed that the banded grains transformed into equiaxed grains due to dynamic recrystallization during deformation, accompanied by texture spreading, misorientation distribution randomization, and dislocation density reduction. The superplastic deformation process can be divided into two stages according to the microstructure characteristics and deformation mechanisms. In the primary deformation stage, the alloy was mainly composed of banded-grained structures, the intragranular dislocation slip (IDS) dominated the deformation and accounted for 54.1% contribution to the total deformation. When the true strain raised to 1.1, the deformation entered the second stage where the equiaxed grains became dominant, the grain boundary sliding became the main deformation mechanism and accounted for 54.4% contribution to the total deformation. A modified Ashby−Verrall model accompanied by IDS is suggested, which plays a crucial role in the superplastic deformation of the studied alloy. [ABSTRACT FROM AUTHOR]

Mantle convection and large-scale plate motion depend critically on the nature of the lithosphere-asthenosphere boundary and thus on the viscosity structure of Earth's upper mantle, which is determined by the rheological properties of its constituent minerals. To constrain the flow behavior of orthopyroxene, the second most abundant constituent of the upper mantle, deformation experiments were carried out in triaxial compressive creep on fine-grained (~6 μm) samples of enstatite at high pressures (3.8-6.3 GPa) and high temperatures (1323-1573 K) using a deformation-DIA apparatus. Based on results from this study, the deformation behavior of enstatite is quantitatively presented in the form of a flow law that describes the dependence of deformation rate on differential stress, water fugacity, temperature, and pressure. Specifically, the creep rate depends approximately linearly on stress, indicating deformation in the diffusion creep regime. A least squares regression fit to our data yielded a flow law for diffusion creep with an activation energy of ~200 kJ/mol and an activation volume of ~14 × 10−6 m3/mol. The magnitude of the water-weakening effect is similar to that for olivine with a water fugacity exponent of r ≈ 0.7. This strong dependence of viscosity on water fugacity (concentration) indicates that the viscosity of an orthopyroxene-bearing mantle varies from one geological setting to another, depending on the large-scale water distribution. Based on the rheology contrast between olivine and enstatite, we conclude that olivine is weaker than enstatite throughout most of the upper mantle except in some shallow regions in the diffusion creep regime. [ABSTRACT FROM AUTHOR]

We uniaxially deformed fine-grained (~ 1 μm) forsterite + diopside (5 and 20 vol %) aggregates in the diffusion creep regime. Prior to deformation, line markers were milled on a lateral surface of a cylindrical sample to detect single- to multiple-grain-scale deformation. We performed deformation experiments and observations of the marker-etched surface after sample cooling multiple times on the same specimens. The strain measured at the scale of several tens of grains from macroscopic shortening of the markers parallel to the compression axis is consistent with the total strain of the sample. However, microscopically, the markers are intensely segmented and rotated at the grain scale increasing with the sample strain. Meanwhile, essentially, no deformation is observed within the grains in most of the samples. The surface microstructures, including the deformation of the markers, reveal the serial operations of grain boundary migration, grain boundary sliding, rigid body grain rotation, and grain neighbor switching, which correspond well to processes expected in diffusion-controlled superplasticity. This sequence is commonly observed in both samples consisting of forsterite grains of tabular and equiaxed grain shapes, which have been shown to develop notable crystallographic preferred orientation (CPO) and random (or weak) CPO, respectively, during diffusion creep. Intragranular regions of relatively larger forsterite grains in the specimens deformed at stresses near the transition between deformation mechanisms from diffusion creep to dislocation creep reveal marker deformation and formation of surface creases and subgrain boundaries, which indicate intragranular dislocation processes. Overall, the surface microstructures reflect the deformation state of the materials well. [ABSTRACT FROM AUTHOR]

Polycrystalline samples composed of either tabular or equiaxed forsterite grains +diopside (5 and 20 vol %) were deformed with a grid etched onto the lateral surface. In Part 1 of this study, we identified grain boundary sliding (GBS) and rigid body-like grain rotation during deformation by diffusion creep where samples with tabular forsterite grains were shown to develop low-index plane grain boundaries that result in crystallographic preferred orientation (CPO). Here we examine how grain rotation depends on the sample strain, grain size, phases, grain shapes, and orientations relative to the compression axis and long axes of tabular forsterite grains. Based on these results, we model grain rotation due to GBS that occurs preferentially along low-index plane boundaries. The model reproduces all of the characteristics of grain rotation and together with the observed grain rotation rates in tabular and equiaxed grain samples, we estimate that low-index plane boundaries have a lower viscosity by a factor of ~3 relative to general grain boundaries, which results in the development of CPO during diffusion creep. The observed constant rotation rate of ~0.4 (radian/strain) in equiaxed-grain samples and in tabular-grain samples deformed to a strain of >0.5 is considered to be a minimum and further, a material-independent rotation rate during diffusion creep, indicating grain rotation as a primary microprocess during diffusion creep. We discuss the possible consequences of GBS-induced grain rotation and CPO development in rock microstructure and the seismic properties of the Earth's mantle. [ABSTRACT FROM AUTHOR]

Creep tests at very low stresses are often not run long enough to reach the stationary stage. This means that the test results must be analysed with the help of primary creep models. A previously published such model that does not involve adjustable parameters is reformulated to simplify its integration. It is applied to three sets of test data for copper at 95-150 °C, 600 °C and 820 °C. Primary creep data including that at a very early stage follow the φ model, i.e. the creep rate decreases exponentially as a function of time. The primary creep model can accurately reproduce this behaviour. Previously unpublished creep data at 600 MPa and 1–2 MPa are presented. These data are controlled by dislocation creep. This is demonstrated by a stress exponent of 3, a minimum creep rate that is fully consistent with a stationary creep model and primary creep that can be modelled. In spite of this, the predicted Coble diffusion creep rate lies about an order of magnitude above the observations. The third data set is believed to be controlled by diffusion creep. It can also be represented with primary dislocation creep that gives a stress exponent of 1 at low stresses and a transition to stationary creep at higher stress. This shows that dislocation and diffusion creep are competing processes at high temperatures and low stresses and that careful testing is needed to distinguish between the mechanisms. [ABSTRACT FROM AUTHOR]

γ-TiAl-based alloys have attracted intense attentions in the past two decades and are regarded as promising candidate materials for high-temperature applications due to their low density, high specific strength, superior creep and oxidation resistance, etc. It is well known that there are four typical microstructures in TiAl-based alloys, namely, full-lamellar (FL), near-lamellar (NL), duplex (DP) and near-gamma (NG). The FL/NL-TiAl alloys possess superior high temperature mechanical performance such as creep resistance, but their deformability is relatively lower. In contrast, the DP/NG-TiAl alloys have much better formability while the creep resistance is reduced. Hence the DP/NG microstructure is preferable for hot-working and thought to the prerequisite for secondary processing, and the present paper is primarily focused on the high temperature deformation behavior of DP/NG-TiAl alloys. Till date, considerable research efforts have been directed towards the deformation kinetics or mechanical properties of various DP/NG-TiAl alloys, and a mass of experimental data has been accumulated. In order to explore their general features of high temperature deformation kinetics, in this paper the tensile/compression mechanical data of various DP/NG-TiAl alloys from the literature published in the past twenty years has been summarized and reviewed. Three deformation mechanisms (dislocation creep, grain boundary sliding and diffusion creep) have been identified, and for each the effects of major factors (i.e., alloy composition, microstructure and grain size) on deformation kinetics have been analyzed and discussed in detail. The results revealed that the high temperature deformation kinetics of DP/NG-TiAl alloys is less sensitive to composition except some dispersion-hardened alloys such as carbon-containing TiAl alloys. Instead, the γ grain size became the major limiting factor. Based on this observation, a series of unified rate-equations has been successfully developed. [ABSTRACT FROM AUTHOR]