Dislocation interactions during low-temperature plasticity of olivine and their impact on the evolution of lithospheric strength
Dislocation interactions during low-temperature plasticity of olivine and their impact on the evolution of lithospheric strength
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DOI：
10.1016/j.epsl.2020.116349
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发表时间：
2020-08
影响因子：
5.3
通讯作者：
D. Wallis;L. Hansen;K. Kumamoto;C. Thom;O. Plümper;M. Ohl;W. Durham;D. Goldsby;D. Armstrong;C. Meyers;R. Goddard;J. Warren;T. Breithaupt;M. Drury;A. Wilkinson
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文献类型：
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作者：
D. Wallis;L. Hansen;K. Kumamoto;C. Thom;O. Plümper;M. Ohl;W. Durham;D. Goldsby;D. Armstrong;C. Meyers;R. Goddard;J. Warren;T. Breithaupt;M. Drury;A. Wilkinson
岩石圈的强度通常是基于稳态流动的本构方程来建模的。然而，应变硬化可能在岩石圈较冷的承载部分引起显著的强度演化。最近来自橄榄石低温变形实验的流变学数据表明，应变硬化是由于位错之间的远程弹性相互作用产生的温度无关的背应力的存在而发生的。这些解释为包含由背应力形成的硬化的流动规律提供了基础。在这里，我们通过检测在室温下变形的橄榄石样品的微观结构来验证这一位错-相互作用假设，分别在变形- dia仪器中≤4.3 GPa的差应力或在纳米压头中施加≥10.2 GPa的接触应力。高角分辨率电子后向散射衍射图显示，在两组样品中存在几何上必需的位错，其密度通常在10 - 14 m−2以上，并且在1 GPa量级的残余应力中存在晶内非均质性。扫描透射电子显微照片显示在滑移带中排列的直接位错，并与其他类型的位错相互作用，作为障碍。由此产生的位错在滑移面上的累积，以及相关的应力非均质性，与位错之间的远程背应力导致的应变硬化一致，从而支持低温塑性流动规律的形式。根据这些观测结果，我们预测位错之间的背应力会通过增加或减少有效应力来赋予变形岩石圈显著的力学各向异性。因此，应变历史及其相关的微观结构和微观力学演化是岩石圈强度模型的重要考虑因素。显微组织观察也为识别自然试样中反应力诱发的应变硬化操作提供了新的标准，从而为检验流动规律对低温塑性的适用性提供了一种手段。
The strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, strain hardening may cause significant evolution of strength in the colder load-bearing portion of the lithosphere. Recent rheological data from low-temperature deformation experiments on olivine suggest that strain hardening occurs due to the presence of temperature-independent back stresses generated by long-range elastic interactions among dislocations. These interpretations provided the basis for a flow law that incorporates hardening by the development of back stress. Here, we test this dislocation-interaction hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of≤ 4.3 GPa or in a nanoindenter at applied contact stresses of≥ 10.2 GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 10 14 m− 2 and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned in slip bands and interacting with dislocations of other types that act as obstacles. The resulting accumulations of dislocations in their slip planes, and associated stress heterogeneities, are consistent with strain hardening resulting from long-range back-stresses acting among dislocations and thereby support the form of the flow law for low-temperature plasticity. Based on these observations, we predict that back stresses among dislocations will impart significant mechanical anisotropy to deformed lithosphere by enhancing or reducing the effective stress. Therefore, strain history, with associated microstructural and micromechanical evolution, is an important consideration for models of lithospheric strength. The microstructural observations also provide new criteria for identifying the operation of back-stress induced strain hardening in natural samples and therefore provide a means to test the applicability of the flow law for low-temperature plasticity.