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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >The down-stress transition from cluster to cone fabrics in experimentally deformed ice
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The down-stress transition from cluster to cone fabrics in experimentally deformed ice

机译：在实验变形的冰上从簇到锥形织物的下压力过渡

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During plastic deformation of polycrystalline ice 1h, ice crystals become crystallographically aligned due to dislocation glide, primarily on the basal slip system. Such crystallographic preferred orientation (CPO) introduces a viscous anisotropy in ice, and thus strongly influences the kinematics of the flow of glaciers and ice sheets. Two key mechanisms exert different controls on CPO. In axial compression, recrystallization dominated by lattice rotation yields a cluster of c-axes parallel to compression, and recrystallization dominated by grain boundary migration (GBM) yields a cone-shaped distribution of c-axes with the cone axis parallel to compression. The transition between these dominant mechanisms of CPO formation has not been well quantified. In this study, we explore how this transition varies with stress. Ice deformation experiments were conducted using a high-pressure, gas-medium apparatus to prevent fracturing of samples at relatively high stresses. Samples were deformed in uniaxial compression at a temperature of similar to-10 degrees C and a confining pressure of 10 MPa. Fabricated ice samples with starting average grain sizes of either similar to 0.23 mm or similar to 0.63 mm were each deformed to an axial strain of similar to 0.2 at a nominally constant strain rate in the range 1.2 x 10(-6) to 2.4 x 10(-4) s(-1), yielding flow stresses of 1.17 to 4.31 MPa. High-quality electron backscatter diffraction reveal the grain size, shape, subgrain structure, and CPOs formed at different stresses. All deformed samples have strong, non-random CPOs with c-axes concentrated in cones. The cone angle and CPO strength are observed to decrease with increasing stress. As stress increases, the fraction of grains with highly curved or lobate grain boundaries decreases and the fraction of polygonal grains with straight grain boundaries increases. Based on these observations, we propose that a transition in the dominant mechanism of CPO formation occurs with increasing stress, from GBM, which consumes grains with low Schmid factors at low stress, to lattice rotation caused by slip primarily on the basal slip system, which causes c-axes to rotate to become parallel to the shortening direction at high stress. Mapping out the transition from cluster (rotation-dominated) to cone (GBM-dominated) CPOs as a function of stress (this study) and temperature (future studies) allows for a robust extrapolation to, and a fundamental understanding of the CPOs formed at, glaciological stresses and temperatures. (C) 2017 Elsevier B.V. All rights reserved.
机译：在多晶冰的塑性变形期间，由于位错滑动，冰晶在基础滑移系统上，冰晶变得晶体地对齐。这种结晶优选取向（CPO）在冰中引入粘性各向异性，因此强烈影响冰川和冰盖流动的运动学。两个关键机制在CPO上发挥了不同的控制。在轴向压缩中，用晶格旋转支配的重结晶产生与压缩平行的C轴，并且由晶界迁移（GBM）支配的重结晶产生C轴的锥形分布，锥形轴与锥形轴平行于压缩。 CPO形成的这些主导机制之间的过渡并未被定量良好。在这项研究中，我们探讨了这种转变如何因压力而变化。使用高压，气体介质装置进行冰变形实验，以防止在相对高的应力下进行样品的压裂。在类似于-10℃的温度和10MPa的限制压力下，样品在单轴压缩中变形。具有类似于0.23mm或类似于0.63mm的起始平均晶粒尺寸的制造的冰上样品各自变形到类似于0.2的轴向应变，以1.2×10（-6）至2.4 x 10的标称恒定的应变速率。 （-4）S（-1），产生1.17至4.31MPa的流量应力。高质量的电子背散射衍射揭示了不同应力形成的晶粒尺寸，形状，粒子结构和CPO。所有变形样品都具有强，非随机的CPO，具有以锥体浓缩的C轴。观察到锥角和CPO强度随着压力的增加而降低。随着应力的增加，具有高弯曲或裂片晶界的颗粒的粒径降低，并且具有直晶界限的多边形颗粒的级分增加。基于这些观察结果，我们提出了CPO形成的主导机制的过渡，从GBM的压力增加，消耗低应力下的低脊髓系数的谷物，主要是在基底滑动系统上引起的晶格旋转，这使C轴旋转以在高应力下变得平行于缩短方向。根据压力的函数（本研究）和温度（未来研究）绘制从集群（旋转主导）到锥体（GBM主导的）CPO的过渡，允许强大的外推，以及对所形成的CPO的基本理解，冰川凝聚和温度。 （c）2017年Elsevier B.V.保留所有权利。

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