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Takehiko Hiraga - One of the best experts on this subject based on the ideXlab platform.
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grain to multiple grain scale deformation processes during Diffusion Creep of forsterite diopside aggregate 1 direct observations
Journal of Geophysical Research, 2017Co-Authors: G Maruyama, Takehiko HiragaAbstract: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 with 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 sub-grain boundaries, which indicate intragranular dislocation processes. Overall, the surface microstructures reflect the deformation state of the materials well.
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grain to multiple grain scale deformation processes during Diffusion Creep of forsterite diopside aggregate 2 grain boundary sliding induced grain rotation and its role in crystallographic preferred orientation in rocks
Journal of Geophysical Research, 2017Co-Authors: G Maruyama, Takehiko HiragaAbstract: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.
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olivine crystals align during Diffusion Creep of earth s upper mantle
Nature, 2013Co-Authors: T Miyazaki, Kenta Sueyoshi, Takehiko HiragaAbstract:The crystallographic preferred orientation (CPO) of olivine produced during dislocation Creep is considered to be the primary cause of elastic anisotropy in Earth’s upper mantle and is often used to determine the direction of mantle flow. A fundamental question remains, however, as to whether the alignment of olivine crystals is uniquely produced by dislocation Creep. Here we report the development of CPO in iron-free olivine (that is, forsterite) during Diffusion Creep; the intensity and pattern of CPO depend on temperature and the presence of melt, which control the appearance of crystallographic planes on grain boundaries. Grain boundary sliding on these crystallography-controlled boundaries accommodated by Diffusion contributes to grain rotation, resulting in a CPO. We show that strong radial anisotropy is anticipated at temperatures corresponding to depths where melting initiates to depths where strongly anisotropic and low seismic velocities are detected. Conversely, weak anisotropy is anticipated at temperatures corresponding to depths where almost isotropic mantle is found. We propose Diffusion Creep to be the primary means of mantle flow. In Earth’s mantle, the shape change of olivine grains depending on temperature and the presence of melt can result in the development of olivine crystallographic preferred orientation during Diffusion Creep, meaning that the process may be the principal mechanism of mantle flow. The crystallographic-preferred orientation of olivine produced during dislocation Creep is considered the primary cause of elastic anisotropy in the Earth's upper mantle, and is often used by seismologists to determine the direction of mantle flow. Tomonori Miyazaki et al. now show that iron-free olivine grains coexisting with either diopside or melt can result in the development of crystallographic-preferred orientation of olivine during Diffusion Creep. They show that strong radial anisotropy is anticipated from such Diffusion Creep at temperatures approaching the solidus temperature.
David L Kohlstedt - One of the best experts on this subject based on the ideXlab platform.
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Diffusion Creep of enstatite at high pressures under hydrous conditions
Journal of Geophysical Research, 2017Co-Authors: S Mei, Guinan Zhang, Maoshuang Song, David L KohlstedtAbstract: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 (similar to 6m) samples of enstatite at high pressures (3.8-6.3GPa) and high temperatures (1323-1573K) 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 similar to 200kJ/mol and an activation volume of similar to 14x10(-6)m(3)/mol. The magnitude of the water-weakening effect is similar to that for olivine with a water fugacity exponent of r approximate to 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.
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observations of grain size sensitive power law Creep of olivine aggregates over a large range of lattice preferred orientation strength
Journal of Geophysical Research, 2016Co-Authors: Jacob A Tielke, Lars N Hansen, Miki Tasaka, Cameron Meyers, Mark E Zimmerman, David L KohlstedtAbstract:Grain size sensitive (GSS) power law Creep of San Carlos olivine aggregates was investigated by comparing strain rates measured in laboratory deformation experiments to strain rates determined from a micromechanical model of intragranular dislocation processes. The plastic flow behavior of olivine aggregates due solely to intragranular slip was determined using flow laws for olivine single crystals in combination with grain orientations measured by electron backscatter diffraction. Measured strain rates were compared to results from the micromechanical model for samples deformed in compression to an axial strain of <0.2 and in torsion to a shear strain of up to 7.4. Olivine aggregates deform up to a factor of 4.6 times faster than the maximum possible rates determined from the micromechanical model of intragranular slip. Comparison of our data to published flow laws indicates that Diffusion Creep cannot account for this difference. The ratio of experimentally determined strain rates to those from the micromechanical model is strongly dependent upon grain size but is independent of stress and strength of lattice-preferred orientation. These observations indicate that GSS power law Creep, consistent with dislocation-accommodated grain boundary sliding, occurs in both weakly and strongly textured olivine aggregates at the studied conditions.
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rheology of the upper mantle and the mantle wedge a view from the experimentalists
Geophysical monograph, 2013Co-Authors: Greg Hirth, David L KohlstedtAbstract:In this manuscript we review experimental constraints for the viscosity of the upper mantle. We first analyze experimental data to provide a critical review of flow law parameters for olivine aggregates and single crystals deformed in the Diffusion Creep and dislocation Creep regimes under both wet and dry conditions. Using reasonable values for the physical state of the upper mantle, the viscosities predicted by extrapolation of the experimental flow laws compare well with independent estimates for the viscosity of the oceanic mantle, which is approximately 10 19 Pa s at a depth of ∼100 km. The viscosity of the mantle wedge of subduction zones could be even lower if the flux of water through it can result in olivine water contents greater than those estimated for the oceanic asthenosphere and promote the onset of melting. Calculations of the partitioning of water between hydrous melt and mantle peridotite suggest that the water content of the residue of arc melting is similar to that estimated for the asthenosphere. Thus, transport of water from the slab into the mantle wedge can continually replenish the water content of the upper mantle and facilitate the existence of a low viscosity asthenosphere.
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anomalous compressibility of ferropericlase throughout the iron spin cross over
Proceedings of the National Academy of Sciences of the United States of America, 2009Co-Authors: R M Wentzcovitch, David A. Yuen, J F Justo, C R S Da Silva, David L KohlstedtAbstract:The thermoelastic properties of ferropericlase Mg1−xFexO (x = 0.1875) throughout the iron high-to-low spin cross-over have been investigated by first principles at Earth's lower mantle conditions. This cross-over has important consequences for elasticity such as an anomalous bulk modulus (KS) reduction. At room temperature the anomaly is somewhat sharp in pressure but broadens with increasing temperature. Along a typical geotherm it occurs across most of the lower mantle with a more significant KS reduction at ≈1,400–1,600 km depth. This anomaly might also cause a reduction in the effective activation energy for Diffusion Creep and lead to a viscosity minimum in the mid-lower mantle, in apparent agreement with results from inversion of data related with mantle convection and postglacial rebound.
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the effect of large melt fraction on the deformation behavior of peridotite
Earth and Planetary Science Letters, 2006Co-Authors: T Scott, David L KohlstedtAbstract:To determine the influence of a large melt fraction, 0.15 ≤ ϕ ≤ 0.30, on the rheological behavior of partially molten rocks, we performed a series of high-temperature, triaxial compressive Creep experiments on dry, synthetic peridotites in both the Diffusion and dislocation Creep regimes. We deformed samples with an olivine grain size of either 10 or 50 μm in a gas-medium apparatus at a confining pressure of 300 MPa and temperatures from 1498 to 1556 K. Stress vs. strain rate data reveal a drop in rock viscosity of several orders of magnitude when the melt fraction is increased from ϕ = 0.25 to ϕ = 0.30, indicative of a rheologically critical melt fraction (RCMF). In all of these experiments, we observed only a small amount of grain growth. Over the range 0 ≤ ϕ ≤ 0.25, the flow behavior in both the Diffusion and grain size sensitive dislocation Creep regimes is well described by the published flow law for partially molten samples of olivine + MORB deformed under anhydrous conditions in which e˙ ∝ exp(αϕ) with α = 21 for Diffusion Creep (Newtonian) and α = 32 for dislocation-accommodated GBS (grain boundary sliding) Creep.
G Maruyama - One of the best experts on this subject based on the ideXlab platform.
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grain to multiple grain scale deformation processes during Diffusion Creep of forsterite diopside aggregate 1 direct observations
Journal of Geophysical Research, 2017Co-Authors: G Maruyama, Takehiko HiragaAbstract: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 with 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 sub-grain boundaries, which indicate intragranular dislocation processes. Overall, the surface microstructures reflect the deformation state of the materials well.
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grain to multiple grain scale deformation processes during Diffusion Creep of forsterite diopside aggregate 2 grain boundary sliding induced grain rotation and its role in crystallographic preferred orientation in rocks
Journal of Geophysical Research, 2017Co-Authors: G Maruyama, Takehiko HiragaAbstract: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.
Herbert Gleiter - One of the best experts on this subject based on the ideXlab platform.
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stress enhanced grain growth in a nanocrystalline material by molecular dynamics simulation
Acta Materialia, 2003Co-Authors: Andrew J Haslam, Simon R Phillpot, D Wolf, Dorel Moldovan, V Yamakov, Herbert GleiterAbstract:Molecular-dynamics simulations are used to elucidate the coupling between grain growth and grain-boundary Diffusion Creep in a polycrystal consisting of 25 grains with an average grain size of about 15 nm and a columnar grain shape. Consistent with our earlier simulations of grain-boundary Diffusion Creep, albeit in the absence of grain growth, we find that initially, i.e. prior to the onset of significant grain growth, the deformation proceeds via the mechanism of Coble Creep. Also, consistent with our earlier grain-growth simulations in the absence of stress, two growth mechanisms are observed during the deformation: growth due to curvature-driven GB migration and growth resulting from grain rotation-induced grain coalescence. The comparison of the grain growth observed in the presence of the applied stress with that solely in response to temperature as the driving force enables us to identify the mechanisms by which external stress affects grain growth. In particular, we find that both GB migration and grain rotation are accelerated by the deformation.
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grain boundary Diffusion Creep in nanocrystalline palladium by molecular dynamics simulation
Acta Materialia, 2002Co-Authors: V Yamakov, D Wolf, Simon R Phillpot, Herbert GleiterAbstract:Molecular-dynamics (MD) simulations of fully three-dimensional (3D), model nanocrystalline face-centered cubic metal microstructures are used to study grain-boundary (GB) Diffusion Creep, one mechanism considered to contribute to the deformation of nanocrystalline materials. To overcome the well-known limitations associated with the relatively short time interval used in our MD simulation (typically <10−8 s), our simulations are performed at elevated temperatures where the distinct effects of GB Diffusion are clearly identifiable. In order to prevent grain growth and thus to enable steady-state Diffusion Creep to be observed, our input microstructures were tailored to (1) have a uniform grain shape and a uniform grain size of nm dimensions and (2) contain only high-energy GBs which are known to exhibit rather fast, liquid-like self-Diffusion. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state Diffusion Creep that is homogeneous, with a strain rate that agrees quantitatively with that given by the Coble-Creep formula. The grain-size scaling of the Coble Creep is found to decrease from d−3 to d−2 when the grain diameter becomes of the order of the GB width. For the first time a direct observation of the grain-boundary sliding as an accommodation mechanism for the Coble Creep, known as Lifshitz sliding, is reported.
Holger Stunitz - One of the best experts on this subject based on the ideXlab platform.
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Evidence of phase nucleation during olivine Diffusion Creep: A new perspective for mantle strain localisation
Earth and Planetary Science Letters, 2016Co-Authors: Jacques Précigout, Holger StunitzAbstract:For the past decades, grain size reduction leading to Diffusion Creep in olivine is believed to be a very important process for strain localisation in the lithospheric mantle. However, the mechanisms of grain size reduction in this regime are still poorly understood (e.g., Platt, 2015). Here we show new experimental results that document grain size reduction and material weakening during wet olivine Diffusion Creep. While occurring for both, mono-phase and two-phase aggregates, grain size reduction is coeval with strain localisation and local phase mixing in olivine–pyroxene aggregates. Based on evidence of fluid inclusions and cracks filled with a fine-grained phase mixture, we conclude that grain size reduces as a result of fluid-assisted nucleation that takes place in the presence of an aqueous fluid during Diffusion Creep. Cavitation induced by grain boundary sliding (Creep cavitation) can be inferred, and may play a critical role for olivine grain size reduction. Amongst their implications for rock rheology in general, our findings highlight a key process for strain localisation in the ductile uppermost mantle.
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Creep cavitation bands control porosity and fluid flow in lower crustal shear zones
Geology, 2015Co-Authors: Luca Menegon, Florian Fusseis, Holger Stunitz, Xianghui XiaoAbstract:Shear zones channelize fluid flow in Earth’s crust. However, little is known about deep crustal fluid migration and how fluids are channelized and distributed in a deforming lower crustal shear zone. This study investigates the deformation mechanisms, fluid-rock interaction, and development of porosity in a monzonite ultramylonite from Lofoten, northern Norway. The rock was deformed and transformed into an ultramylonite under lower crustal conditions (temperature = 700–730 °C, pressure = 0.65–0.8 GPa). The ultramylonite consists of feldspathic layers and domains of amphibole + quartz + calcite, which result from hydration reactions of magmatic clinopyroxene. The average grain size in both domains is <25 mm. Microstructural observations and electron backscatter diffraction analysis are consistent with Diffusion Creep as the dominant deformation mechanism in both domains. Festoons of isolated quartz grains define C'-type bands in feldspathic layers. These quartz grains do not show a crystallographic preferred orientation. The alignment of quartz grains is parallel to the preferred elongation of pores in the ultramylonites, as evidenced from synchrotron X-ray microtomography. Such C'-type bands are interpreted as Creep cavitation bands resulting from Diffusion Creep deformation associated with grain boundary sliding. Mass-balance calculation indicates a 2% volume increase during the protolith-ultramylonite transformation, which is consistentmore » with synkinematic formation of Creep cavities producing dilatancy. Thus, this study presents evidence that Creep cavitation bands may control deep crustal porosity and fluid flow. Nucleation of new phases in Creep cavitation bands inhibits grain growth and enhances the activity of grain size–sensitive Creep, thereby stabilizing strain localization in the polymineralic ultramylonites.« less
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transition from fracturing to viscous flow in granulite facies perthitic feldspar lofoten norway
Journal of Structural Geology, 2013Co-Authors: Luca Menegon, Holger Stunitz, Renee Heilbronner, P Nasipuri, Henrik SvahnbergAbstract:Abstract Recrystallization of perthites in granulite facies ( T = 700–730 °C, P = 0.65–0.8 GPa) shear zones in mangerite-charnockite rocks from Lofoten (Norway) is localized along intracrystalline bands parallel to fractures. Fracturing preferentially occurred along the cleavage planes (010) and (001). EBSD analysis of perthite porphyroclasts indicates a very low degree of internal misorientation (within 5°) and the lack of recovery features. Recrystallized grains show coarsening with increasing width of the bands, and chemical changes with respect to the host grains. Crystallographic orientation of the new grains does not show a host-control relation to the parent perthite grains. In summary, the microstructure and CPO data consistently indicate intragranular recrystallization by nucleation and growth from fractured grains. Perthite porphyroclasts are surrounded by a matrix of recrystallized plagioclase + K-feldspar ± amphibole ± biotite. There is extensive evidence of syndeformational nucleation of new phases and of phase boundary migration in the matrix, with plagioclase grains forming bulges and protrusions towards K-feldspar. The spatial distribution of K-feldspar and plagioclase in the recrystallized matrix is characterized by the predominance of phase boundaries over grain boundaries. All these observations are consistent with Diffusion Creep as the dominant deformation mechanism in the matrix, associated with grain boundary sliding. Accordingly, recrystallized plagioclase and K-feldspar show a very weak crystallographic preferred orientation, which is interpreted in terms of oriented growth during Diffusion Creep. Fracturing of perthites promoted extensive grain size reduction, recrystallization, fluid infiltration, and operation of grain-size sensitive Creep, resulting in strain localization.
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quartz grain size reduction in a granitoid rock and the transition from dislocation to Diffusion Creep
Journal of Structural Geology, 2011Co-Authors: Rudiger Kilian, Renee Heilbronner, Holger StunitzAbstract:Abstract In the Gran Paradiso metagranodiorite (Western Alps) small scale lower amphibolite facies shear zones record the transition from a mylonite composed of polycrystalline mineral aggregates to a homogeneous ultramylonite with a grain scale phase mixture. Polycrystalline quartz aggregates in the mylonite deform by dislocation Creep developing a crystallographic preferred orientation (CPO) and a monoclinic surface orientation distribution function (ODF). The polymineralic matrix of the mylonite and the ultramylonite deform by Diffusion Creep. In the ultramylonite the quartz CPO is randomized and the surface ODF becomes orthorhombic. The transition from mylonite to ultramylonite is accompanied by a grain size decrease and a disintegration of quartz aggregates, concomitant with the precipitation of K-feldspar (±biotite) between quartz grains. In quartz, reduction from the dynamically recrystallized grain size in the aggregates (110 μm) to the size of the dispersed grains in the ultramylonite (25 μm) occurs through the following processes: K-feldspar precipitates at opening sites along grain boundaries (strain incompatibility) pinning the grain size in quartz aggregates. Coalescence of K-feldspar leads to enhanced grain boundary sliding and disintegration of the quartz aggregates. Solution precipitation reduces the size of the dispersed grains to less than subgrain size (∼45 μm).