Integrated solution for lokal, electron channeling based stress and strain measurement in the scanning electron microscope (εpsilator.X3)

• Applicant: Professor Dr. Christian Motz
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 445818037

The properties of many materials originate from their nano- and microstructure. While the geometric structures of these materials can be characterized by already established methods at the macro, micro and nano level, the local properties in many fields can be determined at the macro level but only partially at the micro level. Especially local mechanical stresses and deformations are not yet without great expenditure on equipment, e.g. XRD in the synchrotron, measurable on the micro and nano scale with sufficient reliability and spatial accuracy. But for microstructure development and characterization they are of considerable interest. In recent years, a large number of new methods for local deformation analysis in electron microscopy have been developed and enhanced, although most of these methods are still under development. In addition to digital image correlation (DIC) on SEM images, a major new methodology was the deformation analysis using electron back scatter diffraction (EBSD), in particular high-resolution (HR) EBSD. The latter represents a combination of DIC and EBSD and allows a spatially high resolved, highly accurate determination of the deformation state (strain and rotation) of the crystal lattice, but so far can only be performed using the commercial software CrossCourt. Additionally the software itself as well as the nature of the EBSD technology give rise to certain problems. The technique is therefore insensitive to hydrostatic strains and provides only deformation data relative to a local reference point. Regarding these limitations, the adaptation of the methods used here to the older selected area channeling (SAC) is promising. SAC provides data akin to EBSD in the form of electron diffraction patterns. However, SAC provides pattern in much higher quality and angular resolution. This enables more accurate deformation measurements with the methods underlying HR-EBSD. The adaptation of the Kikuchi bandlet method (KBM) originally developed for EBSD patterns further allows access to the hydrostatic strain parts. In addition, the use of dynamically simulated patterns as a reference point is conceivable which in turn allows absolute deformation measurements. Critical to all these methods is an effective indexing of the pattern, i.e. the determination of the associated crystal orientation and identification of relevant image details. Up to now, this has only been possible for SAC's patterns through relatively new dictionary indexing (DI). The methods listed here are in parts complex and computationally very intensive, but well documented and in the case of pattern simulation and dictionary indexing accessible via the open source software EMsoft. This Project aims to combine these methods to allow deformation measurement with an accuracy exceeding previous SEM based methods. For the necessary validation and calibration of this tool, suitable experiments will also be carried out.

• DFG Programme: Research Grants