Exploring Dislocation Structures with Conventional EBSD

地球的内部不是冻结的，而是不断地转动。地球表面刚性板块的运动是由地球内部固体岩石的热对流驱动的。板块构造很可能在地球历史的早期就开始了。持续的变形在岩石中留下了记录，可以在地表取样。在模拟自然变形的实验条件下，在变形的矿物和岩石中也观察到这样的记录。这些记录包括形成岩石的矿物的大小和形状，称为微观结构，以及单个晶体中的特定缺陷。这些缺陷包含了矿物所经历的永久变形（称为塑性变形）的关键信息。晶体塑性变形的一个关键因素是被称为位错的线缺陷，这是在金属中首次被认识和广泛研究的。这些小的线性缺陷可以用电子显微镜成像。然而，由于它们的体积小，将它们与通常大得多的宿主晶体一起观察是具有挑战性的。在这里，研究小组开发了一种新方法，可以在代表天然岩石微观结构的区域上对位错进行成像。这种基于扫描电子显微镜的方法可以常规地应用于实验和自然变形的岩石。它为研究塑造地球的过程提供了一种既省时又经济的新方法。研究人员首先对晶体进行了基准测试，这些晶体已经经过实验变形，并通过一系列成像技术进行了广泛的研究。然后，他们将新的成像技术应用于自然变形的岩石，逐渐揭示它们的变形历史。该项目支持一名早期职业女性科学家和纽约州立大学新帕尔茨分校本科生的培训。其结果为科学界提供了改善地球材料微观结构研究的蓝图。大部分地壳和上地幔的变形是由位错蠕变引起的。实验和自然变形样品的晶粒内部结构通常通过位错氧化修饰或透射电镜成像来检测。前者不是一种常规的分析方法，不能解决位错的全部几何问题。虽然后者全面表征位错，但所研究的体积只有几微米，甚至是细粒度实验样品晶粒尺寸的一小部分。到目前为止，电子背散射衍射（EBSD）作图主要用于确定晶粒尺寸和晶格优选方向；但在过去十年中，EBSD索引的速度和质量都有了实质性的提高。自动化EBSD制图允许对相对较大的区域进行常规成像（达到薄片比例）。高分辨率的EBSD测绘已被证明能够成像橄榄石和石英中的位错结构。然而，与传统的EBSD相比，HR-EBSD需要大量的额外资源。在这里，研究小组研究了传统的EBSD映射是否可以提供足够准确的索引来表征位错类型/滑移系统，包括分布位错和亚晶界。为了验证该方法，研究小组绘制了先前实验变形的单晶图，并通过氧化修饰和透射电镜对其位错结构进行了全面评估。然后，该方法将应用于具有不同织物类型的天然样品，这允许以经济有效的方式索引其位错微观结构。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The interior of the Earth is not frozen but continuously turns over. The motion of rigid plates at the Earth’s surface is driven by the thermal convection of solid rocks in the planet interior. Plate tectonics likely began early in Earth’s history. The continuous deformation leaves a record within rocks that can be sampled at the surface. Such a record is also observed in minerals and rocks deformed experimentally at conditions reproducing natural deformations. The record includes the size and shape of the minerals forming the rocks, called microstructure, as well as specific defects within individual crystals. These defects contain critical information about the permanent deformation – called plastic deformation – that the mineral experienced. A key ingredient of crystal plastic deformation, first recognized and extensively studied in metals, are line defects called dislocations. These small linear defects can be imaged by electron microscopy. Yet, because of their small size, it is challenging to observe them together with their host crystal, usually much larger. Here, the team develops a new method to image dislocations over areas that are representative of the microstructure of natural rocks. The method, based on scanning electron microscopy, can be routinely applied to experimentally and naturally deformed rocks. It provides a new time- and cost-effective way to study processes that shape the Earth. The researchers first benchmark the method on crystals which have been experimentally deformed and extensively studied by a range of imaging techniques. They then apply the new imaging technique to naturally deformed rocks, gradually unveiling their deformation history. The project supports an early-career female scientist and the training of undergraduate students at SUNY College at New Paltz (NY). Its outcomes provide the scientific community with a blueprint for improving microstructural studies of Earth materials.Most of the Earth's crust and upper mantle deform by dislocation creep. Grain-internal structures of experimentally and naturally deformed samples are usually examined either by oxidative decoration of dislocations or TEM imaging. The former is not a routine analysis method and cannot resolve the full geometry of dislocations. While the latter comprehensively characterizes dislocations, the investigated volume is only a few microns, a fraction of the grain size even of fine-grained experimental samples. Up to now, electron backscatter diffraction (EBSD) mapping has primarily been used to determine grain sizes and lattice preferred orientation; but the speed and quality of EBSD indexing has substantially improved over the last decade. Automated EBSD mapping allows routine imaging of relatively large areas (up to thin section scale). High-resolution EBSD mapping has been shown to be able to image dislocation structures in olivine and quartz. HR-EBSD, however, requires substantial additional resources in comparison to conventional EBSD. Here, the team investigate whether conventional EBSD mapping can provide accurate enough indexing to characterize dislocation type/slip systems, for both distributed dislocations and sub-grain boundaries. To test the method, the team map single crystals previously deformed experimentally, for which the dislocation structures have been comprehensively evaluated by oxidative decoration and TEM. The method will then applied to natural samples with different fabric types, which allows indexing their dislocation microstructures in a cost-effective way.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.