Correlative Scanning Precession Electron Diffraction and Atom Probe Tomography to access the 3D polycrystalline grain microstructure of nanomaterials

在透射电子显微镜中，通过在二维扫描中移动准直光束来获取一叠衍射图，可以产生一个四维数据集。该数据集包含有关局部颗粒取向、相分布以及颗粒和相界位置的信息。在纳米材料的情况下，重叠的颗粒通常会影响数据分析，因为这会导致重叠的衍射图。即使有可能对这些进行索引，也不可能从沿单一视角的测量中辨别出哪个颗粒位于透射电子显微镜样品的上/下。为了克服这些局限性，我们提出了一种基于在不同倾角下的一系列2D-SPED测量的3D扫描进动电子衍射(SPED)方法。来自同步加速器社区的层析后处理程序将能够在3D中重建晶向和晶界图。在同一样品上额外使用APT将简化这一开发，并将提供关于元素三维分布的补充信息。这两种技术的结合将用于研究晶界偏析。然而，这一发展使人们能够以纳米分辨率对化学和晶体材料科学现象进行3D研究。在这个项目中，2D-SPED现有的数据处理工具箱和采集例程将进一步开发，以访问3D案例。一种专用的方法被用来提取信号，该信号绕过了由于动态效应而产生的强度调制，并且满足了投影要求。衍射信号的方位相关性将由治愈算法处理，并在层析重建工具中实现。在此基础上，将相关方法应用于APT不适用的纳米晶Fe-C系统的晶界偏析研究。这一主题非常重要，因为众所周知，C可以稳定铁素体纳米结构，并增加晶界内聚力。相关的3D-SPED/APT方法将有助于理解相关现象，因为大量随机取向的晶界将根据这两个参数、它们的五个晶界参数以及它们的原子尺度化学成分来表征。对于这项具有挑战性的任务，我们联合了来自互补领域的四位方法开发人员的专业知识：2D-SPED(Edgar Rauch，Grioble)、3D同步加速器衍射对比层析成像(Wolfgang Ludwig，Grioble)、相关的TEM/APT和晶界偏析(Michael HerBig，杜塞尔多夫)以及APT结晶学(Andrew Breen，杜塞尔多夫)。

In transmission electron microscopy acquiring a stack of diffraction patterns by moving a collimated beam in a two-dimensional scan produces a four-dimensional dataset. This dataset contains the information on local grain orientations, phase distributions, and grain and phase boundary locations. In case of nanomaterials overlapping grains often compromize the data analysis as this leads to superimposed diffraction patterns. Even if it is possible to index these, it is not possible to tell which grain is situated on the upside/downside of the TEM specimen from a measurement along a single perspective. Here we propose to develop a 3D Scanning Precession Electron Diffraction (SPED) method based on a series of 2D-SPED measurements at different tilt angles to overcome these limitations. Tomographic post-processing routines adopted from the synchrotron community will enable to reconstruct grain orientation and grain boundary maps in 3D. The additional use of APT on the same sample will simplify this development and will provide complementary information on the 3D distribution of elements. This combination of techniques will be used in this project to study grain boundary segregation. However, this development gives access to the 3D investigation of coupled chemical and crystallographic materials science phenomena with nanometer resolution in general.In this project existing data processing toolboxes and acquisition routines from 2D-SPED will be further developed to give access to the 3D case. A dedicated approach is used to extract a signal that by-passes the intensity modulations due to dynamic effects and fulfills the projection requirement. The orientation dependence of the diffraction signal will be handled by a curative algorithm and implemented in the tomographic reconstruction tools. After the development, the correlative approach will be applied to investigate grain boundary segregation in the nanocrystalline Fe-C system where APT crystallography is not applicable. This topic is of high importance as C is known to stabilize ferritic nanostructures and to increase grain boundary cohesion. The correlative 3D-SPED/APT approach will help to understand the related phenomena, as a large number of randomly oriented grain boundaries will be characterized in terms of both, their five crystallographic interface parameters, and their atomic-scale chemical composition. For this challenging tasks we unite the expertise of four method developers from complementary fields: 2D-SPED (Edgar Rauch, Grenoble), 3D synchrotron diffraction contrast tomography (Wolfgang Ludwig, Grenoble), correlative TEM/APT and grain boundary segregation (Michael Herbig, Düsseldorf) and APT crystallography (Andrew Breen, Düsseldorf).