In Situ TEM Observation of Cooperative Grain Rotations and the Bauschinger Effect in Nanocrystalline Palladium

Abstract

We report on cooperative grain rotation accompanied by a strong Bauschinger effect in nanocrystalline (nc) palladium thin film. A thin film of nc Pd was subjected to cyclic loading–unloading using in situ TEM nanomechanics, and the evolving microstructural characteristics were investigated with ADF-STEM imaging and quantitative ACOM-STEM analysis. ADF-STEM imaging revealed a partially reversible rotation of nanosized grains with a strong out-of-plane component during cyclic loading–unloading experiments. Sets of neighboring grains were shown to rotate cooperatively, one after the other, with increasing/decreasing strain. ACOM-STEM in conjunction with these experiments provided information on the crystallographic orientation of the rotating grains at different strain levels. Local Nye tensor analysis showed significantly different geometrically necessary dislocation (GND) density evolution within grains in close proximity, confirming a locally heterogeneous deformation response. The GND density analysis revealed the formation of dislocation pile-ups at grain boundaries (GBs), indicating the generation of back stresses during unloading. A statistical analysis of the orientation changes of individual grains showed the rotation of most grains without global texture development, which fits to both dislocation- and GB sliding-based mechanisms. Overall, our quantitative in situ experimental approach explores the roles of these different deformation mechanisms operating in nanocrystalline metals during cyclic loading.

Quantitative Characterization by Transmission Electron Microscopy and Its Application to Interfacial Phenomena in Crystalline Materials

“…This is the first report of a specimen that is close to the bulk scenario. The deformation microstructures in various nanocrystalline materials were also quantitatively analyzed by utilizing a combination of ACOM and in situ straining experiments [191][192][193]. Details can be represented in Ref.…”

3D microscopy at the nanoscale reveals unexpected lattice rotations in deformed nickel

“…However, a recovery of plastic deformation during unloading has also been observed in bulk nanograined nickel and iron, albeit in that case indirectly through in situ x-ray diffraction profile analysis (18,24,31). One possibility to account for these rotations is the back stresses arising from plastically inhomogeneous deformation (25). To investigate this possibility, we used Wu's method (32)(33)(34) to assess the contribution of back stresses to the unexpected lattice rotations during unloading.…”

“…For coarse-grained metals, this has been achieved using x-ray synchrotron methods ( 3 ). For nanograined samples, previous observations have been limited to 2D measurements such as those on columnar nanograined palladium, in which a reversal of individual grain rotations on unloading has been reported ( 25 ). However, recent advances in the technique for 3D orientation mapping in a transmission electron microscope (3D-OMiTEM) ( 26 – 28 ) now allow rapid, nondestructive 3D mapping of the shapes and crystallographic orientations of hundreds of nanograins within a specimen.…”

Mapping Structural Heterogeneity at the Nanoscale with Scanning Nano-structure Electron Microscopy (SNEM)

“…4D-STEM has been used to obtain reliable structural information at the nm scale. 4D-STEM-like measurements often focus on well ordered systems, with clearly-present Bragg peaks [24,25] where matching diffraction patterns to a known structure to generate, for example, orientation, strain field or geometrically-necessarydislocation maps [26,27,28,29].…”