"Mapping structure and strain in metals and alloys using 4D-STEM"
Complex materials require transmission electron microscopy (TEM) to characterize the structure at the nanoscale. A key strength of TEM is the ability to obtain different signals from the same specimen. Switching between imaging and diffraction mode, allows to obtain focused local information of multiphase materials. In addition, chemistry or even atomic bonding can be deduced from EDS and EELS signals. Recently, the use of scanning TEM (STEM) has become increasingly popular for analyzing defects, such as dislocations, 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, containing information on the local structure and orientation of the specimen. Importantly, the nanodiffraction patterns allow measurement of local strain even in highly-deformed materials, where traditional strain mapping approaches based on atomic-resolution imaging fail. This enables local strain mapping during in situ deformation in the TEM [1]. 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 metals and alloys. In addition, new developments are presented, including the combination of energy-filtering and precession electron diffraction. This experimental approach enabled to map for the first time the elastic strain field in a single shear band formed during compression of a CuZrAl metallic glass, shedding new light on the fundamental processes occurring during deformation in metallic glasses [2].
1 C. Gammer, J. Kacher, C. Czarnik, O. L. Warren, J. Ciston, A. M. Minor. Applied Physics Letters 109 (2016) 081906.
2 H. Sheng, D. Şopu, S. Fellner, J. Eckert, C, Gammer. Physical Review Letters 128 (2022) 245501.
