Imaging Ghosts with 4D-STEM: Diffuse Scattering, Vacancies and Vanishing Dislocations

In situ transmission electron microscopy (TEM) experiments are typically recorded either in real space or diffraction space [1]. However, it would be ideal to have both real and diffraction space for when transient events occur that cannot be repeated exactly (ie- defect generation or irreversible phase transformations). Real space imaging provides context for these transient events by spatially-resolving microstructural features to one another while diffraction space provides better structural clarity about phase identification and lattice parameters. Four-dimensional scanning transmission electron microscopy (4D-STEM) [2], can come close to providing both simultaneous real-space imaging and diffraction analysis during <i>in situ</i> testing.<br/>Here, we use 4D-STEM with high precision bullseye apertures [3] to map the nanoscale strain landscape during <i>in situ</i> cooling of a NiTi through the phase transformation from the high temperature B2 cubic austenite phase to the low temperature B19’ monoclinic martensite phase. Using this method, we can map both the phase distributions and the strain as the sample approaches and then proceeds through the phase transformation, including the diffuse scattering ahead of the phase transformation.<br/>4D-STEM also provides an opportunity to map vacancy distributions with nanoscale resolution. Our experiment using a model Au film demonstrates the 4D-STEM method for measuring vacancy concentration by closely following the differential thermal expansion method [4] of measuring concentrations of point defects [5]. In a more complicated scenario, we have used vacancy mapping with 4D-STEM to also help explain the mechanism behind a new type of one-dimensional corrosion in metals [6]. Lastly, this talk will also describe our recent results utilizing in situ 4D-STEM during nanomechanical testing to provide insight into multiscale deformation phenomena in the CrCoNi medium entropy alloy (MEA). 4D-STEM can provide both real-space imaging and diffraction analysis during <i>in situ</i> testing, making it possible to classify defects during in situ deformation before they disappear. These vanishing dislocations help to explain the correlation between SRO and planar defects during deformation of the CrCoNi MEA.<br/><br/>References:<br/><br/>[1] F. Ross and A. M. Minor, “In situ Transmission Electron Microscopy”, in <u>Springer Handbook of Microscopy, </u>edited by Peter Hawkes and John Spence, Springer Nature Switzerland AG, 2019<br/>[2] C. Ophus, “Four-dimensional scanning transmission electron microscopy (4D-STEM): From scanning nanodiffraction to ptychography and beyond,” <i>Microsc. Microanal.</i>, <b>vol. 25</b>, no. 3, pp. 563–582, (2019).<br/>[3] Zeltmann, S. E., Müller, A., Bustillo, K. C., Savitzky, B., Hughes, L., Minor, A. M., & Ophus, C. (2020). Patterned probes for high precision 4D-STEM bragg measurements. <i>Ultramicroscopy</i>, <i>209</i>, 112890.<br/>[4] R. O. Simmons and R. W. Balluffi, “Measurement of equilibrium concentrations of lattice vacancies in gold,” <i>Phys. Rev.</i>, <b>vol. 125</b>, no. 3, p. 862, (1962).<br/>[5] S. Mills, et al, "Nanoscale mapping of point defects with 4D-STEM", <i>Acta Materialia</i>, <b>vol. 246</b>, pp. 118721, (2023)<br/>[6] Y. Yang, et al.,"One-dimensional wormhole corrosion in metals", <i>Nature Communications, </i>(2023)