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Effects of dynamic recrystallization on lattice preferred orientation (LPO) in olivine were investigated through the combination of two SEM-based techniques, electron backscattered diffraction (EBSD) technique for crystallographic orientation measurement and backscattered electron imaging (BEI) for dislocation observation. Samples are experimentally deformed olivine aggregates in simple shear geometry. In the sample deformed at T=1473 K and high stresses (480 MPa), only incipient dynamic recrystallization is observed along grain-boundaries. Orientations of these small recrystallized grains are more random than that of relict grains, suggesting an important role of grain-boundary sliding at this stage of recrystallization. In the sample deformed at T=1573 K and low stress (160 MPa), dynamic recrystallization is nearly complete and the LPO is characterized by two [100] peaks. One peak is located at the orientation subparallel to the shear direction and is dominated by grains with high Schmid factor. The other occurs at high angles to the shear direction and is due to the contribution from grains with low Schmid factor. Grains with high Schmid factor tend to have higher dislocation densities than those with low Schmid factor. Based on these observations, we identify two mechanisms by which dynamic recrystallization affects LPO: (1) enhancement of grain-boundary sliding due to grain-size reduction, leading to the modification of LPO caused by the relaxation of constraint for deformation; (2) grain-boundary migration by which grains with lower dislocation densities grow at the expense of grains with higher dislocation densities. Based on the deformation mechanism maps and stress versus recrystallized grain-size relation, we suggest that the first mechanism always plays an important role whereas the second mechanism has an important effect only under limited conditions. 相似文献
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The dislocation annihilation rate in experimentally deformed olivine single crystals was measured as a function of oxygen partial pressure (PO2). It was shown that the dislocation annihilation rate decreased with increasing PO2. This result is inconsistent with the reported PO2 dependence of creep rate () in single olivine crystals, thus indicating that the creep in single olivine crystals is not rate-controlled by recovery, under the experimentally investigated conditions. 相似文献
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利用人工合成的多晶材料研究了镍锗尖晶石在位错域的高温蠕变性质.多晶材料的颗粒尺寸约8μm.单轴压缩试件为圆柱状,使用气体介质围压筒.常压蠕变试验过程中,围压为300MPa,温度为1373-1523K,应力在55-330MPa范围内.从实验结果得出了镍锗尖晶石在位错域的流动律,应力指数n=29±01,表明流动的微观机制为位错蠕变.与其他尖晶石进行对比可以发现一个力学同构群,虽然在正尖晶石和反尖晶石之间存在一定的差异.在位错蠕变域,尖晶石与橄榄石归一化的强度类似.由于尖晶石的剪切模量比橄榄石高50%,其实际强度也比橄榄石高.将橄榄石和尖晶石的蠕变数据外推到地球内部条件时,由于其高应力指数,橄榄石则有可能比尖晶石的强度高. 相似文献
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High-temperature creep behavior in Ni2GeO4 spinel was investigated using synthetic polycrystalline aggregates with average grain sizes ranging from submicron to 7.4
microns. Cylindrical samples were deformed at constant load in a gas-medium apparatus at temperatures ranging from 1223 to
1523 K and stresses ranging from 40 to 320 MPa. Two deformation mechanisms were identified, characterized by the following
flow laws:
where σ is in MPa, d is in μm and T is in Kelvin. These flow laws suggest that deformation was accommodated by dislocation creep and grain-boundary diffusion
(Coble) creep, respectively. A comparison with other spinels shows that an isomechanical group can be defined for spinels
although some differences between normal and inverse spinels can be identified. When creep data for olivine and spinel are
normalized and extrapolated to Earth-like conditions, spinel (ringwoodite) has a strength similar to olivine in the dislocation
creep regime and is considerably stronger than olivine in the diffusion creep regime at coarse grain size. However, when grain-size
reduction occurs, spinel can become weaker than olivine due to its high grain-size sensitivity (Coble creep behavior). Analysis
of normalized diffusion creep data for olivine and spinel indicate that spinel is weaker than olivine at grain sizes less
than 2 μm.
Received: 18 June 2000 / Accepted: 3 April 2001 相似文献
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Shun-ichiro Karato 《Icarus》2011,212(1):14-229
The rheological properties of the mantle of super-Earths have important influences on their orbital and thermal evolution. Mineral physics observations are reviewed to obtain some insights into the rheological properties of deep mantles of these planets where pressure can be as high as ∼1 TPa. It is shown that, in contrast to a conventional view that the viscosity of a solid increases with pressure (at a fixed temperature), viscosity will decrease with pressure (and depth) when pressure exceeds ∼0.1 TPa. The causes for pressure-weakening include: (i) the transition in diffusion mechanisms from vacancy to interstitial mechanism (at ∼0.1 TPa), (ii) the phase transition in MgO from B1 to B2 structure (at ∼0.5 TPa), (iii) the dissociation of MgSiO3 into MgO and SiO2 (at ∼1 TPa), and (iv) the transition to the metallic state (at ∼1 TPa). Some (or all) of them individually or in combination reduce the effective viscosity of constituent materials in the deep interior of super-Earths. Taken together, super-Earths are likely to have low viscosity deep mantle by at least 2-3 orders of magnitude less than the maximum viscosity in the lower mantle of Earth. Because viscosity likely decreases with pressure above ∼0.1 TPa (in addition to higher temperatures for larger planets), deep mantle viscosity of super-Earths will decrease with increasing planetary mass. The inferred low viscosity of the deep mantle results in high tidal dissipation and resultant rapid orbital evolution, and affects thermal history and hence generation of the magnetic field and the style of mantle convection. 相似文献
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A comparison of compressional properties of silicate solids, glasses, and liquids reveals the following fundamental differences: (1) Liquids have much smaller bulk moduli than solids and glasses and the bulk moduli of various silicate melts have a narrow range of values; (2) Liquids do not follow the Birch’s law of corresponding state as opposed to solids and glasses; (3) The Grüneisen parameter increases with increasing pressure for liquids but decreases for solids; (4) The radial distribution functions of liquids show that the interatomic distances in liquids do not change upon compression as much as solids do. The last observation indicates that the compression of silicate melts occurs mostly through the geometrical arrangement of various units whose sizes do not change much with compression, i.e., the entropic mechanism of compression plays a dominant role over the internal energy contribution. All of the other three observations listed above can be explained by this point of view. In order to account for the role of the entropic contribution, we propose a new equation of state for multi-component silicate melts based on the hard sphere mixture model of a liquid. We assign a hard sphere for each cation species that moves in the liquid freely except for the volume occupied by other spheres. The geometrical arrangement of these spheres gives the entropic contribution to compression, while the Columbic attraction between all ions provides the internal energy contribution to compression. We calibrate the equation of state using the experimental data on room-pressure density and room-pressure bulk modulus of liquids. The effective size of a hard sphere for each component in silicate melts is determined. The temperature and volume dependencies of sphere diameters are also included in the model in order to explain the experimental data especially the melt density data at high pressures. All compressional properties of a silicate melt can be calculated using the calibrated sphere diameters. This equation of state provides a unified explanation for most of compressional behaviors of silicate melts and the experimental observations cited above including the uniformly small bulk moduli of silicate melts as well as the pressure dependence of Grüneisen parameters. With additional data to better constrain the key parameters, this equation of state will serve as a first step toward the unified equation of state for silicate melts. 相似文献
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S. Karato 《Pure and Applied Geophysics》1998,153(2-4):239-256
—A microphysical model of seismic wave attenuation is developed to provide a physical basis to interpret temperature and frequency dependence of seismic wave attenuation. The model is based on the dynamics of dislocation motion in minerals with a high Peierls stress. It is proposed that most of seismic wave attenuation occurs through the migration of geometrical kinks (micro-glide) and/or nucleation/migration of an isolated pair of kinks (Bordoni peak), whereas the long-term plastic deformation involves the continuing nucleation and migration of kinks (macro-glide). Kink migration is much easier than kink nucleation, and this provides a natural explanation for the vast difference in dislocation mobility between seismic and geological time scales. The frequency and temperature dependences of attenuation depend on the geometry and dynamics of dislocation motion both of which affect the distribution of relaxation times. The distribution of relaxation times is largely controlled by the distribution in distance between pinning points of dislocations, L, and the observed frequency dependence of Q, Q, Q∝ωα is shown to require a distribution function of P(L)∝L -m with m=4-2α The activation energy of Q ?1 in minerals with a high Peierls stress corresponds to that for kink nucleation and is similar to that of long-term creep. The observed large lateral variation in Q ?1 strongly suggests that the Q ?1 in the mantle is frequency dependent. Micro-deformation with high dislocation mobility will (temporarily) cease when all the geometrical kinks are exhausted. For a typical dislocation density of ～ 108 m?2, transient creep with small viscosity related to seismic wave attenuation will persist up to the strain of ～ 10?6, thus even a small strain (～ 10?6?10?4) process such as post-glacial rebound is only marginally affected by this type of anelastic relaxation. At longer time scales continuing nucleation of kinks becomes important and enables indefinitely large strain, steady-state creep, causing viscous behavior. 相似文献