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

We report direct evidence for cooperative grain rotations accompanied by a strong Bauschinger effect in nanocrystalline palladium thin films using in situ TEM nanomechanics coupled with HAADF-STEM imaging and quantitative ACOM-STEM analysis. HAADF-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. The cooperative grain rotation is explained by a strain gradient developing across grains, presumably due to geometric incompatibilities of the slip systems of individual neighboring grains, leading to dislocation pile-up at the corresponding grain boundaries. This stress buildup at a large number of grains also helps to explain the strong Bauschinger effect, which is to some extend accommodated by partially reversible grain rotation. A statistical analysis of the orientation changes of individual grains shows a rotation of most of the grains leading to a random texture. Unlike in the deformation of their coarse-grained counterparts, this grain rotation does not merely depend on the grain orientation with respect to the global deformation direction but is dominated by local cooperative effects and strain buildup leading to random grain rotation on a global level. Overall, our careful quantitative in situ experimental approach yields insights into dislocation driven, coupled grain rotations in nanocrystalline metals, which is dominated by the local interaction and stress buildup between grains.