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The mechanical properties of olivine-rich rocks are key to determining the mechanical coupling between Earth’s lithosphere and asthenosphere. In crystalline materials, the motion of crystal defects is fundamental to plastic flow 1 – 4 . However, because the main constituent of olivine-rich rocks does not have enough slip systems, additional deformation mechanisms are needed to satisfy strain conditions. Experimental studies have suggested a non-Newtonian, grain-size-sensitive mechanism in olivine involving grain-boundary sliding 5 , 6 . However, very few microstructural investigations have been conducted on grain-boundary sliding, and there is no consensus on whether a single or multiple physical mechanisms are at play. Most importantly, there are no theoretical frameworks for incorporating the mechanics of grain boundaries in polycrystalline plasticity models. Here we identify a mechanism for deformation at grain boundaries in olivine-rich rocks. We show that, in forsterite, amorphization takes place at grain boundaries under stress and that the onset of ductility of olivine-rich rocks is due to the activation of grain-boundary mobility in these amorphous layers. This mechanism could trigger plastic processes in the deep Earth, where high-stress conditions are encountered (for example, at the brittle–plastic transition). Our proposed mechanism is especially relevant at the lithosphere–asthenosphere boundary, where olivine reaches the glass transition temperature, triggering a decrease in its viscosity and thus promoting grain-boundary sliding. Amorphization at grain boundaries in olivine-rich rocks under stress and consequent grain-boundary sliding could explain the decrease in viscosity between the lithosphere and the asthenosphere.

Segregation of rare earth alloying elements are known to segregate to grain boundaries in Mg and suppress grain boundary sliding via strong chemical bonds. Segregation of Mn, however, has recently been found to enhance grain boundary sliding in Mg, thereby boosting its ductility. Taking the Mg (2¯114) twin boundary as an example, we performed a first-principles comparative study on the segregation and chemical bonding of Y, Zn, and Mn at this boundary. We found that both Y-4d and Mn-3d states hybridized with the Mg-3sp states, while Zn–Mg bonding was characterized by charge transfer only. Strong spin-polarization of Mn pushed the up-spin 3d states down, leading to less anisotropic Mn–Mg bonds with more delocalized charge distribution at the twin boundary, and thus promotes grain boundary plasticity, e.g., grain boundary sliding.

Although microstructural evolution is critical to strain‐dependent processes in Earth's mantle, flow laws for dunite have only been calibrated with low‐strain experiments. Therefore, we conducted a series of high‐strain torsion experiments on thin‐walled cylinders of iron‐rich olivine aggregates. Experiments were performed in a gas‐medium apparatus at 1200°C and constant strain rate. In our experiments, each at a different strain rate, a peak stress was observed followed by significant strain weakening. We first deformed samples to high enough strain that a steady state microstructure was achieved and then conducted strain rate stepping tests to characterize the creep behavior of each sample with constant microstructure. A global fit to the data yields a stress exponent of 4.1 and a grain‐size exponent of 0.73, values which agree well with those from previous small‐strain experiments conducted on olivine in the dislocation‐accommodated grain‐boundary sliding (GBS) regime. Strong crystallographic preferred orientations provide support for GBS accommodated by movement of (010)[100] dislocations. The observed strain weakening is not entirely explained by grain‐size reduction; thus, we propose that the remaining 30% reduction in stress is related to CPO development. To incorporate microstructural evolution in a constitutive description of GBS in olivine, we (1) derive a flow law for high‐strain deformation with steady state microstructure, which results in an apparent stress exponent of 5.0, and (2) present a system of evolution equations that recreate the observed strain weakening. Our results corroborate flow‐law parameters and microstructural observations from low‐strain experiments and provide a means for incorporating strain weakening into geodynamic simulations. Key Points We explore the high‐strain behavior of olivine with a novel experiment design We quantify the effect of grain size and crystallographic fabric on deformation We develop a model that describes the strain dependence of olivine deformation

Seismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid‐ocean ridge (MOR) dynamics. Yet, interpreting anisotropy in the context of grain‐scale deformation processes and strain observed in laboratory experiments and natural olivine samples has proven challenging due to incomplete seismological constraints and length scale differences spanning orders of magnitude. To bridge this observational gap, we estimate an in situ elastic tensor for oceanic lithosphere using co‐located compressional‐ and shear‐wavespeed anisotropy observations at the NoMelt experiment located on ∼70 Ma seafloor. The elastic model for the upper 7 km of the mantle, NoMelt_SPani7, is characterized by a fast azimuth parallel to the fossil‐spreading direction, consistent with corner‐flow deformation fabric. We compare this model with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Direct comparison to olivine deformation experiments indicates strain accumulation of 250%–400% in the shallow mantle. We find evidence for D‐type olivine lattice‐preferred orientation (LPO) with fast [100] parallel to the shear direction and girdled [010] and [001] crystallographic axes perpendicular to shear. D‐type LPO implies similar amounts of slip on the (010)[100] and (001)[100] easy slip systems during MOR spreading; we hypothesize that grain‐boundary sliding during dislocation creep relaxes strain compatibility, allowing D‐type LPO to develop in the shallow lithosphere. Deformation dominated by dislocation‐accommodated grain‐boundary sliding (disGBS) has implications for in situ stress and grain size during MOR spreading and implies grain‐size dependent deformation, in contrast to pure dislocation creep. Plain Language Summary Earth's upper mantle is composed primarily of the mineral olivine, which responds to deformation by organizing its seismically fast axis in the flow direction. During seafloor spreading, olivine crystals align with the spreading direction and become frozen into the lithosphere preserving the near‐ridge deformation history. The resulting rock fabric can be observed in place via the directional dependence of seismic wavespeeds (seismic anisotropy) as well as in hand‐sample rocks collected from the field. However, interpreting seismic anisotropy observations in the context of laboratory and field data remains a challenge, due to large differences in length scale and incomplete seismic constraints. Here, we bridge this observational gap by incorporating multiple data types to solve for the complete anisotropic structure of oceanic lithosphere that formed ∼70 Myr ago. By comparing our model to laboratory data, we infer the magnitude of shear strain and style of olivine deformation during seafloor spreading for the first time. Our results indicate large shear strains and an olivine fabric type different to that typically assumed, consistent with deformation that is sensitive to the presence of grain boundaries even though most of the strain is produced by dislocations. This has new implications for the formation and evolution of oceanic plates. Key Points We develop an in situ elastic tensor for oceanic lithosphere that incorporates co‐located compressional and shear wavespeed anisotropy constraints Seismic anisotropy is consistent with corner flow during spreading and shear strains of 250%–400% Girdled D‐type olivine fabric implies activation of multiple olivine easy slip systems during mid‐ocean ridge spreading

—The microcrack initiation at a grain junction during strain induced athermal grain-boundary sliding under the stresses of the virtual dislocation pile-up of a planar shear mesodefect, the stresses of a junction disclination, and an external stress is investigated. The existing criteria are shown not to allow one to take into account the influence of the stress field of a junction disclination on the crack initiation. A new energy criterion, which is based on the concept of spontaneous nucleation of a crack of the minimum possible length for a crystalline body, is proposed. This criterion is used to analyze the conditions necessary for crack initiation. The dependences of the critical external load required for crack initiation on the strength of junction disclinations are obtained at various values of the fragmented structure parameters (fragment boundary size, rotational-shear mesodefect strength) and the threshold athermal sliding stress. The presence of a sufficiently strong negative disclination at a grain junction is shown to significantly facilitate crack initiation.

Microstructural analyses of mylonites next to the Median Tectonic Line (MTL), SW Japan, reveal a transition in the dominant deformation mechanism of quartz from grain‐size‐insensitive dislocation creep to grain‐size‐sensitive grain‐boundary sliding (GBS). The transition occurred under greenschist‐facies conditions (∼300–400°C) during grain‐size reduction by dynamic recrystallization. The stereologically corrected grain size for the transition is approximately 4.3 μm. At the boundary between the fields of dislocation creep and GBS, as calculated from creep constitutive relations, the differential stress and strain rate for this corrected grain size are estimated to be ∼280 MPa and 1.2 × 10−11 s−1 for 300°C, and ∼110 MPa and 1.0 × 10−10 s−1 for 400°C. The strain rates estimated for the mylonites next to the MTL are much higher than those estimated for the surrounding metamorphic rocks (∼10−14 s−1), and the displacement rates calculated based on the thickness of high‐strain mylonites and their strain rates are comparable with the average slip rates of the most active intraplate faults in Japan. These inferences suggest that the high‐strain mylonite zones next to the MTL are the exhumed downward extension of a seismogenic fault in the ductile region. The zones were highly localized (<10 m) and experienced very high strain rates (10−11 to 10−10 s−1). Key Points Identification of deformation mechanism transition in mylonites Quantitative estimation of stress and strain rate in mylonites Evaluation of strain localization within the ductile region of the crust

A theoretical model is established to describe the effect of cooperative grain boundary (GB) sliding and migration on dislocation emission from the tip of branched crack in deformed nanocrystalline solids. The explicit solutions of complex potentials are obtained by means of complex variable method and conformal mapping technique. The critical stress intensity factors (SIFs) for the first lattice dislocation emission from the tip of branched crack are calculated. The effects of the lengths of branched crack and main crack, and the angle between their planes on the critical SIFs for dislocation emission are evaluated in detail. The results indicate that the emission of lattice dislocations from the tip of branched crack is strongly influenced by cooperative GB sliding and migration. When main crack approaches the branched crack, dislocation emission from the tip of branched crack will be suppressed. The main crack tends to propagate while shorter branched crack is prone to be blunted by emitting lattice dislocations from its tip. As a special case, when the planes of main crack and the branched crack are flattened out into one, the present results are in good agreement with previously known results.

We review some of the factors that influence the hardness of polycrystalline materials with grain sizes less than 1 µm. The fundamental physical mechanisms that govern the hardness of nanocrystalline materials are discussed. The recently proposed dislocation curvature model for grain size-dependent strengthening and the 60-year-old Hall–Petch relationship are compared. For grains less than 30 nm in size, there is evidence for a transition from dislocation-based plasticity to grain boundary sliding, rotation, or diffusion as the main mechanism responsible for hardness. The evidence surrounding the inverse Hall–Petch phenomenon is found to be inconclusive due to processing artefacts, grain growth effects, and errors associated with the conversion of hardness to yield strength in nanocrystalline materials.

Superplasticity describes a material’s ability to sustain large plastic deformation in the form of a tensile elongation to over 400% of its original length, but is generally observed only at a low strain rate (~10 −4  s −1 ), which results in long processing times that are economically undesirable for mass production. Superplasticity at high strain rates in excess of 10 −2  s −1 , required for viable industry-scale application, has usually only been achieved in low-strength aluminium and magnesium alloys. Here, we present a superplastic elongation to 2000% of the original length at a high strain rate of 5 × 10 −2  s −1 in an Al 9 (CoCrFeMnNi) 91 (at%) high-entropy alloy nanostructured using high-pressure torsion. The high-pressure torsion induced grain refinement in the multi-phase alloy combined with limited grain growth during hot plastic deformation enables high strain rate superplasticity through grain boundary sliding accommodated by dislocation activity. Superplasticity at high strain rates is challenging to achieve in high strength materials. Here, the authors show superplastic elongation in excess of 2000% in a high entropy alloy nanostructured by high-pressure torsion.

A different type of defect, the coherency disclination, is added to disclination types. Disconnections that include disclination content are considered. A criterion is suggested to distinguish disconnections with dislocation content from those with disclination content. Electron microscopy reveals unit disconnections in a low albite grain boundary, defects important in grain boundary sliding. Disconnections of varying step heights are displayed and shown to define both deformed and recovered structures.