Dr. Christoph Gammer—Group leader at the Erich Schmid Institute of Materials Science Austrian Academy of Sciences

Abstract: The key strength of transmission electron microscopy (TEM) is not only its high resolution, but also the possibility to obtain different signals from the same specimen. Switching between imaging and diffraction mode, allows to obtain focused local information on the orientation and structure of multiphase materials. In addition, chemistry or even atomic bonding can be deduced from EDS and EELS signals. Still, TEM is traditionally used for atomic-resolution imaging, while quantitative diffraction measurements rely on X-ray techniques. In the present talk it will be shown how profile analysis of selected electron diffraction (SAD) – PASAD – can be used to quantify changes in coherently scattering domain size or dislocation density in nanocrystalline materials. This newly developed technique allowed to understand the deformationinduced disordering and temperature-induced reordering of nanocrystalline FeAl made by severe plastic deformation [1]. Recently, the use of scanning TEM (STEM) has become increasingly popular for analyzing defects in metals and alloys. While traditional STEM relies on the use of an annular detector that can be used to obtain chemical or diffraction contrast images, the development of ultrafast electron detectors enables to acquire the full diffraction pattern at each beam position in the STEM image (4D-STEM). Using a semi-converged electron beam (nanodiffraction mapping) yields a rich dataset, that can be analyzed and presented in multiple ways after the experiment. Importantly, the nanodiffraction patterns allow measurement of local strain even in highly-strained materials. This enables for the first time local strain mapping during in situ deformation in the TEM [2]. In crystalline materials the position of the Bragg peaks is used for strain mapping. Similarly, for amorphous materials the distortion of the first ring, enables to map elastic strain fields. The present talk will highlight how 4D-STEM can help understanding deformation in complex materials, where atomic-resolution imaging is not applicable. In addition, new developments are presented including the combination of energy-filtering and precession electron diffraction. This allowed to obtain sufficient resolution to map for the first time the elastic strain field in a single shear band formed during compression of a CuZrAl metallic glass, shedding light on the fundamental processes occurring during deformation in metallic glasses [3].

1 C. Gammer, C. Mangler, H.P. Karnthaler, C. Rentenberger. Scripta Materialia 65 (2011) 57.

2 C. Gammer, J. Kacher, C. Czarnik, O. L. Warren, J. Ciston, A. M. Minor. Applied Physics Letters 109 (2016) 081906.

3 H. Sheng, D. Şopu, S. Fellner, J. Eckert, C, Gammer. Physical Review Letters 128 (2022) 245501.

Biography: Christoph Gammer is group leader at the Erich Schmid Institute of the Austrian Academy of Sciences in Leoben, Austria. His research focuses on the mechanical and functional properties of novel nanomaterials, with a special focus on quantitative characterization techniques in the transmission electron microscope. This includes the development of new in situ techniques to directly image the effect of structure on physical properties at the nanoscale. Christoph received a PhD degree in physics from the University of Vienna, Austria in 2011, working on nanostructured intermetallic alloys. He was subsequently a research fellow at Lawrence Berkeley National Laboratory, where he pioneered strain mapping during in situ deformation using 4D-STEM. In 2016 he moved to Leoben, where he started to work on more complex materials, such as metallic glasses. His numerous awards include the FWF START price and the Fritz-Grasenick Award of the Austrian Society for Electron Microscopy. He has presented more than 30 invited talks and authored over 100 scientific papers in the fields of transmission electron microscopy, nanostructured materials and metallic glasses.