Temperature and strain controls on ice deformation mechanisms: insights from the microstructures of samples deformed to progressively higher strains at-10,-20 and-30 °C
Temperature and strain controls on ice deformation mechanisms: insights from the microstructures of samples deformed to progressively higher strains at-10,-20 and-30 °C
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DOI：
10.5194/tc-14-3875-2020
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发表时间：
2020-11-10
期刊：
影响因子：
5.2
通讯作者：
Wheeler, John
中科院分区：
文献类型：
--
作者：
Fan, Sheng;Hager, Travis F.;Wheeler, John
为了更好地理解冰的变形机制，我们记录了冰的微观结构随应变的变化。我们包括在相对较低的温度（-20和-30摄氏度），其中的微观结构演变与轴向应变以前从未被记录的实验数据。在-10、-20和-30 ℃的温度下，在恒定的位移速率（应变速率类似于1.0 × 10（-5）s（-1））下使多晶纯水冰变形，逐渐增加应变（类似于3%、5%、8%、12%和20%）。微观结构数据产生的低温电子背散射衍射（cryo-EBSD）分析。所有变形的样品包含亚晶粒（低角度的取向差）的结构与取向差轴，主要位于基面，这表明位错蠕变（主要是在基面上的滑移），恢复和亚晶粒旋转的活动。晶界在所有的实验是叶状的，这表明应变诱导晶界迁移（GBM）的操作。变形的冰样品的特点是联锁大和小颗粒，平均而言，比未变形的样品更细的颗粒。二维EBSD图中相邻晶粒之间的取向差分析与三维体积中相同不规则晶粒的不同分支的一些二维晶粒是一致的。重复（即互连）晶粒的比例在高温实验中更大，表明晶粒具有更不规则的形状，可能是因为GBM在高温下更普遍。单位面积的晶粒数（占多次出现的相同的3-D晶粒）是在变形的样品比未变形的样品，它与应变增加，这表明，成核参与再结晶。在-20 ℃和-30 ℃的实验中出现了“核-幔”结构（小颗粒围绕大颗粒的环），这表明亚晶旋转再结晶是活跃的。在高于-20 ° C的温度下，c轴发展出晶体学优选取向（CPO），其特征在于围绕压缩轴的锥形（即小圆圈）。我们建议通过GBM在易滑移方向（即与缩短方向成45度）上选择性生长晶粒来形成c轴锥。c轴锥的张开角随应变而减小，表明应变引起的GBM通过晶粒旋转来平衡。此外，c轴锥的开度角随着温度而减小。在-30 ° C下，随着应变的增加，c轴CPO从窄锥变为与压缩平行的簇。这种封闭的c-轴锥被解释为一个更积极的晶粒旋转的结果，连同一个不太有效的GBM。我们认为，晶格旋转，促进晶内位错滑移的基面上，是占主导地位的机制控制晶粒旋转。低角邻对取向差，有关亚晶界，更广泛，并延伸到更高的取向差角在较低的温度和较高的应变支持位错活动的重要性相对增加。随着温度的降低，总的CPO强度降低，主要是因为小晶粒的CPO较弱。小晶粒之间的大角度晶界具有具有分布的晶体学取向的取向差轴。这意味着，与亚晶界相反，晶界取向差不受晶体学控制。再结晶过程中的形核不能单独用亚晶粒旋转再结晶来解释。更细晶粒的晶界滑动或产生具有随机取向的晶粒的不同形核机制可以解释细晶粒部分的较弱CPO和对高角度晶界的晶体学控制的缺乏。
In order to better understand ice deformation mechanisms, we document the microstructural evolution of ice with increasing strain. We include data from experiments at relatively low temperatures (-20 and -30 degrees C), where the microstructural evolution with axial strain has never before been documented. Polycrystalline pure water ice was deformed under a constant displacement rate (strain rate similar to 1.0 x 10(-5) s(-1)) to progressively higher strains (similar to 3 %, 5 %, 8 %, 12 % and 20 %) at temperatures of -10, -20 and -30 degrees C. Microstructural data were generated from cryogenic electron backscattered diffraction (cryo-EBSD) analyses. All deformed samples contain subgrain (low-angle misorientations) structures with misorientation axes that lie dominantly in the basal plane, suggesting the activity of dislocation creep (glide primarily on the basal plane), recovery and subgrain rotation. Grain boundaries are lobate in all experiments, suggesting the operation of strain-induced grain boundary migration (GBM). Deformed ice samples are characterized by interlocking big and small grains and are, on average, finer grained than undeformed samples. Misorientation analyses between nearby grains in 2-D EBSD maps are consistent with some 2-D grains being different limbs of the same irregular grain in the 3-D volume. The proportion of repeated (i.e. interconnected) grains is greater in the higher-temperature experiments suggesting that grains have more irregular shapes, probably because GBM is more widespread at higher temperatures. The number of grains per unit area (accounting for multiple occurrences of the same 3-D grain) is higher in deformed samples than undeformed samples, and it increases with strain, suggesting that nucleation is involved in recrystallization. "Core-and-mantle" structures (rings of small grains surrounding big grains) occur in -20 and -30 degrees C experiments, suggesting that subgrain rotation recrystallization is active. At temperatures warmer than -20 degrees C, c axes develop a crystallographic preferred orientation (CPO) characterized by a cone (i.e. small circle) around the compression axis. We suggest the c-axis cone forms via the selective growth of grains in easy slip orientations (i.e. similar to 45 degrees to shortening direction) by GBM. The opening angle of the c-axis cone decreases with strain, suggesting strain-induced GBM is balanced by grain rotation. Furthermore, the opening angle of the c-axis cone decreases with temperature. At -30 degrees C, the c-axis CPO changes from a narrow cone to a cluster, parallel to compression, with increasing strain. This closure of the c-axis cone is interpreted as the result of a more active grain rotation together with a less effective GBM. We suggest that lattice rotation, facilitated by intracrystalline dis- location glide on the basal plane, is the dominant mechanism controlling grain rotation. Low-angle neighbour-pair misorientations, relating to subgrain boundaries, are more extensive and extend to higher misorientation angles at lower temperatures and higher strains supporting a relative increase in the importance of dislocation activity. As the temperature decreases, the overall CPO intensity decreases, primarily because the CPO of small grains is weaker. High-angle grain boundaries between small grains have misorientation axes that have distributed crystallographic orientations. This implies that, in contrast to subgrain boundaries, grain boundary misorientation is not controlled by crystallography.Nucleation during recrystallization cannot be explained by sub- grain rotation recrystallization alone.Grain boundary sliding of finer grains or a different nucleation mechanism that generates grains with random orientations could explain the weaker CPO of the fine-grained fraction and the lack of crystallographic control on high-angle grain boundaries.