Low voltage electron back-scatter diffraction: enabling high resolution mapping of heavily deformed materials

现代工程材料通常具有复杂的微观结构，这使得很难准确预测它们在外力作用下的行为。材料变形行为的许多方面仍然知之甚少。由于变形发生在非常小的长度尺度，我们需要使用在相当高的放大倍率下操作的电子显微镜来成像该长度尺度下材料的内部行为。在这个研究项目中，先进的显微镜技术可以定量地确定一些金属和合金的局部变形；这些测量提供了洞察这些材料的行为，特别是当这些金属在我们现代社会的结构应用中使用时。这些算法可以通过出版物和开源代码提供给社区。开源教科书是为材料社区开发的，并为按需创建免费开源教科书提供了一种机制。一个多世纪以来，材料系统的热机械加工以及随后的退火和再结晶过程一直是许多实验和理论研究的主题，特别是对于金属系统。尽管在再结晶方面做了大量的工作，但由于难以定量和准确地表征材料的变形状态，这一过程的许多细节仍然不清楚；这种状态可以被认为是前驱态，为后续退火过程奠定了基础，因此理解这种状态是至关重要的。从变形样品中提取晶格应变具有挑战性，但由于电子背散射衍射（EBSD）探测器系统的最新改进，需要重新关注。将基于物理的EBSD预测模型与模式生成算法中局部变形张量的能力相结合。特别是在较低的显微镜加速电压下，即小于10 kV，有可能将EBSD测量的空间分辨率提高到几十纳米，特别是当与敏感的直接电子探测器结合使用时。EBSD模式与变形晶格的真实模拟模式的整体模式匹配使得在大样本区域内常规达到优于0.0001的应变灵敏度成为可能。所提出的研究包括计算模型开发和现场实验工作的紧密结合。这项工作的一部分是与UCSB和约翰霍普金斯大学的同事合作完成的。所有算法都以同行评审的出版物和开源代码的形式提供给社区。在教育方面，OpenChapters项目通过创建一个开源机制，通过众包的方式生成免费的教科书，解决了材料科学和工程教科书成本迅速上升的问题。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。

Non-technical SummaryModern engineering materials often have a complex microstructure that makes it difficult to accurately predict how they will behave under the influence of a applied forces. Many aspects of the deformation behavior of a material are still poorly understood. Since deformation occurs at a very small length scale, we need to use electron microscopes operated at a fairly high magnification to image the internal behavior of the material at this length scale. In this research project, cutting-edge microscopy techniques are advanced to quantitatively determine the local deformation in a number of metals and alloys; such measurements provide insight into the behavior of these materials, in particular when these metals are used in structural applications in our modern society. These algorithms are available to the community through publications and open source code. Open source text books are developed for the materials community and provide a mechanism for the on-demand creation of free open source text books.Technical SummaryThe thermomechanical processing of materials systems and the subsequent annealing and recrystallization processes have for more than a century now been the subject of numerous experimental and theoretical studies, in particular for metallic systems. Despite extensive work on recrystallization, many details of this process still remain unclear due to the difficulty to quantitatively and accurately characterize the deformed state of a material; this state can be considered as a precursor state, setting the stage for the subsequent annealing process, so understanding this state is of paramount importance. Extracting lattice strains from deformed samples is challenging but requires a renewed attention due to recent improvements in electron back-scattered diffraction (EBSD) detector systems. A physics-based EBSD prediction model is combined with the ability to incorporate the local deformation tensor in the pattern generation algorithm. In particular at lower microscope accelerating voltages, i.e., less than 10 kV, there is the potential to enhance the spatial resolution of EBSD measurements to a few tens of nanometers, especially when combined with sensitive direct electron detectors. Whole pattern matching of EBSD patterns against realistic simulated patterns for a deformed lattice make it possible to routinely reach strain sensitivities of better than 0.0001 over large sample areas. The proposed research consists of a tight combination of computational model development and in-situ experimental work. Portions of this work are carried out in collaboration with colleagues at UCSB and at Johns Hopkins University. All algorithms are made available to the community in peer-reviewed publications and as open source code. On the educational side, the OpenChapters project addresses the issue of rapidly rising cost of material science and engineering text books by creating an open source mechanism for the generation of free text books by a crowd-sourcing approach.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.