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

The construction of the 3D-SPED tool is inspired by similar and now mature approaches developed in the X-ray community in particular the DCT technique applied to synchrotron data. Challenges arise from dynamical effects that are orders of magnitude higher for electrons as compared to X-ray. Indeed, multiple scattering leads to exchanges between the transmitted and the diffracted beams that, in turn, produce contrast fringes so that none of these beams fulfill the projection requirement. To face this problem – and apart from the use of precession - an original and innovative combination of the diffraction reflection intensities is used as the physical signature that by-passes the reflection intensity oscillations produced by the multiple scattering events.
For sake of calibration/validation for the 3D-SPED output, Atom Probe Crystallography (APC) will be used to generate a 3D orientation map on an aluminum needle. APC refers to the various analysis methods which can be used to characterize the crystallographic information within atom probe datasets on top of the chemical composition. This information is particularly valuable for accurate calibration of the tomographic reconstruction as well as for directly correlating the crystallographic nature of defects with chemical segregation behaviour.

At date, a SPED diffraction pattern simulation tool has been implemented in Python. Also an optimization framework, which enables “fitting” experimental diffraction patterns as a linear superposition of pre-calculated diffraction patterns (corresponding to a regular, dense sampling of local orientation space) has been constructed.
Meantime, a first set of data was acquired by the Dusseldorf team. It consists of a collection of 12 orientation maps generated with the ACOM/TEM package for a FeC needle (APT compatible) for successive tilts angles covering the full rotation of the sample. It is the first time such a dataset is acquired with a high resolution/high sensitive camera (TVIPS TemCam-XF416).
The algorithms and software whose development is part of the project were applied to the FeC system although with no refinement. In particular, and automated procedure of ‘components’ extractions (grains or crystals) was applied and the components from successive tilt angles coupled to identify the overlapping grains. Virtual bright-field images of the grains are generated at each tilt position and successfully used for a first 3D reconstruction from diffraction patterns for the majority of the grains

The FeC sample considered as the main focus of the project displays far more intragranular misorientations than expected (> 5°). This makes the virtual bright-field construction less efficient as the diffraction pattern differs significantly from one part to the other of the grain.
The strategies considered to by-pass this limitation are two-fold:
- working on a different material. Ni-W alloy for which the misorientations are expected to be far less will be investigated. Samples were prepared by the German team and will be analysed soon
- strategies to refine the procedure in order to handle the intragranular misorientation (mosaicity) are currently under development

Reconstructing grains in 3D through 4D Scanning Precession Electron Diffraction
P. Harrison et al
Microscopy and Microanalysis 2021, August 1-5

A precession electron diffraction assisted orientation and phase mapping tool for transmission electron microscopes
EF Rauch et al
submitted to Symmetry/MDPI

In transmission electron microscopy acquiring a stack of diffraction patterns by moving the beam in a two-dimensional scan produces a four-dimensional dataset that contains the information related to all the overlapping grains, the through thickness boundaries as well as the embedded second phase particles. The coupling of such Scanning Precession Electron Diffraction (SPED) technique with sample tilts and tomographic post-processing routines is expected to enable the reconstruction of 3D phase and orientation maps and consequently to provide access to the interfaces or interphases geometric and crystallographic properties.
The first objective of this project is to finalize the development of such a tool by (i) optimizing the existing data acquisition and processing routes and (ii) constructing the complete set of user-friendly routines that will permit the 3D crystallographic characters of the sample to be investigated. 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 other issue for electrons is related to the orientation dependence of the diffraction signal. To handle this constrain, a curative algorithm is developed and implemented in the tomographic reconstruction tools initially developed for monochromatic beam X ray diffraction.
Our key objective is to correlate this 3D-SPED orientation mapping facility with atom probe tomography (APT) in order to couple the crystallographic information with the chemical composition at nanoscale. Atom Probe Crystallography (APC) will first be used to generate a 3D orientation map on a aluminum needle that will serve as a calibration/validation dataset for the 3D-SPED output. Then the correlative approach will be exploited to provide a full insight in grain boundary segregation for nanocrystalline materials. The Fe-C system, for which APC is not applicable, will serve as demonstrator: the correlative 3D-SPED/APT approach permit a large number of randomly oriented grain boundaries to be characterized in terms of both, their five crystallographic interface parameters, and their atomic-scale chemical composition. This will help to understand how the C segregation to grain boundaries stabilize the ferritic nanostructures and increases the grain boundary cohesion.