FIB Based Tomography of Dislocation Structures using Channeling Imaging

非技术概述：晶体材料的许多性质，如金属、陶瓷和电子材料，都是由晶体缺陷控制的。位错是一种晶体缺陷，它包含各种晶体结构的原子排列被破坏的长线。这些位错的运动负责控制结构金属和合金的强度、延展性和断裂行为，而在功能材料中，位错往往导致光学和电子性质的崩溃，通常导致寿命缩短。这项研究计划正在开发一种绘制位错图的新方法。该研究使用扫描电子显微镜中的电子沟道对比成像技术(ECCI)对近表面位错进行成像。这种成像与聚焦离子束(FIB)球磨相结合，通过感兴趣的体积切割一系列纳米级切片。每个截面的图像通过几十立方微米的体积被组合成位错的三维地图。绘制与纳米压痕相关的位错图有助于更好地理解晶体到晶体的位错运动，这是晶体阵列变形所必需的，也是导致晶体边界处断裂的原因。将这种增强的理解与位错运动的计算机模拟相结合，可以增强对材料断裂行为和寿命的预测。该研究计划支持对博士后研究助理进行广泛的专业培训，这是在正式的指导计划下进行的，与本科生研究相辅相成。利用博士后和本科生的研究支持，通过密歇根州立大学的扫描电子显微镜教育计划(MSU-SEMED)，使K-12学生和教育工作者接触科学和工程，该计划允许K-12学生和他们的老师通过电子显微镜的实践经验来接触材料科学。技术摘要：该研究项目正在开发一种新的方法，用于对小体积位错的纳米到微观尺度的分布进行层析成像。这项技术结合了聚焦离子束(FIB)球磨和电子沟道对比成像(ECCI)，ECCI是一种扫描电子显微镜(SEM)技术，允许在块状材料的近表面区域成像位错，通过感兴趣的体积收集位错图像的连续切片，然后将其重建为位错结构的3-D图。这种规模的测绘解决了我们目前技术中的一个空白，较小的体积可以用具有相似位错密度的透射电子显微镜(TEM)来评估，而X射线断层成像可以用低得多的位错密度来评估大得多的体积。开发的重点是三个技术任务：1)建立必要的FIB切片参数，以实现高质量FIB抛光表面和高FIB切片吞吐量的最佳组合；2)在优化的电子沟道条件下，通过变形体积收集连续的ECCI图像；以及3)开发图像识别技术，用于识别ECC图像中的位错并随后重建三维位错图。该技术正被用于绘制非立方金属中纳米压痕形成的塑性场中的位错结构，以便于对小规模塑性变形的晶体塑性有限元(CPFE)模型进行稳健的评估。人们正在研究晶界附近的压痕，以了解跨越晶界的塑性变形转移的性质，以便阐明与多晶中晶界破坏和损伤形核相关的机制。

NON-TECHNICAL SUMMARY: Many properties of crystalline materials, such as metals, ceramic, and electronic materials, are controlled by crystal defects. Dislocations are crystal defects containing long lines of disrupted atomic arrangements of their various crystal structures. The motion of these dislocations is responsible for controlling the strength, ductility, and fracture behavior of structural metals and alloys, while in functional materials dislocations are often responsible for breakdown in optical and electron properties, often leading to reduction in lifetimes. This research program is developing a new approach to mapping dislocations. The research uses electron channeling contrast imaging (ECCI) in scanning electron microscopy (SEM) to image near surface dislocations. This imaging is combined with focused ion beam (FIB) milling to cut a series of nano-scale sections through volumes of interest. The images of each section are combined into 3-dimensional maps of the dislocations through volumes on the scale of tens of cubic micrometers. Mapping of the dislocations associated with nano-indentations is facilitating a better understanding of the crystal-to-crystal dislocation motion necessary for deformation of arrays of crystals and responsible for fracture initiation at crystal boundaries. Combining this enhanced understanding with computer simulations of dislocation motion is enabling enhanced prediction of material fracture behavior and lifetimes. The research program supports the broad professional training of a postdoctoral research associate, carried out under a formal mentoring plan, in complement with undergraduate research. The postdoctoral and undergraduate research support is leveraged to expose K-12 students and educators to science and engineering through the SEM Education Program (MSU-SEMED) program at MSU, which allows K-12 students and their teachers to be introduced to materials science though a hands-on experience with electron microscopy.TECHNICAL SUMMARY: This research project is developing a novel approach for tomographic mapping of the nano- to micro-scale distribution of dislocations in small volumes. The technique combines focused ion beam (FIB) milling with electron channeling contrast imaging (ECCI), a scanning electron microscopy (SEM) technique that allows dislocations to be imaged in the near surface region of bulk materials, to collect serial sections of dislocation images through a volume of interest, which are then reconstructed into a 3-D map of the dislocation structure. Mapping at this scale addresses a gap in our current technology, with smaller volumes being assessable with transmission electron microscopy (TEM) with similar dislocation densities and x-ray tomography assessing much larger volumes with much lower dislocation densities. The development is focused on three technical tasks: 1) establishment of the FIB sectioning parameters necessary to achieve an optimal combination of high quality FIB polished surfaces with a high FIB sectioning throughput, 2) collection of serial ECCI images through deformed volumes under optimized electron channeling conditions, and 3) development of image recognition techniques for identifying the dislocations in the ECC images and subsequent reconstruction of the 3-dimensional dislocation maps. The technique is being used to map the dislocation structures in plastic fields developed under nano-indentations in non-cubic metals in order to facilitate a robust assessment of crystal plasticity finite element (CPFE) models of small-scale plastic deformation. Indentations near grain boundaries are being studied to understand the nature of plastic deformation transfer across the boundaries in order to elucidate the mechanisms associated with grain boundary failure and damage nucleation in polycrystals.