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.
Highlights
In coarse-grained metals, the rotation of grains during plastic deformation is dominated by their orientation with respect to the straining direction, and the resulting texture can be predicted well by classical plasticity models, such as Taylor and Sachs models or self-consistent approaches [1,2,3,4,5]
The Taylor model is based on the assumption that each grain experiences the same strain as the surrounding bulk material, equal to the macroscopic plastic strain, and accommodates the strain via the five independent slip systems, which are required to describe the state of strain in face-centered cubic (FCC) metals [3,5,8,9]
Another indication of the role of dislocation activity in nc materials stems from the Bauschinger effect (BE), which has been attributed to microstructural heterogeneity leading to dislocation pile-up at the grain boundaries (GBs) and interfaces and their release during relaxation [30]
Summary
In coarse-grained (cg) metals, the rotation of grains during plastic deformation is dominated by their orientation with respect to the straining direction, and the resulting texture can be predicted well by classical plasticity models, such as Taylor and Sachs models or self-consistent approaches [1,2,3,4,5]. Suggested dislocation-based plasticity as a dominant mechanism even at grain sizes as low as 28 nm and were able to describe the texture development by classical models limiting the number of slip systems [21] Another indication of the role of dislocation activity in nc materials stems from the Bauschinger effect (BE), which has been attributed to microstructural heterogeneity leading to dislocation pile-up at the GBs and interfaces and their release during relaxation [30]. In situ nanomechanical testing inside an electron microscope can further provide high spatial resolution with reasonable statistics for reliable quantification of the grain structure, size distribution, crystallographic orientation, and various other microstructural parameters in order to experimentally follow the complex grain interactions and the nature of their deformation both individually and for a statistically meaningful ensemble These locally resolved in situ measurements allow for the direct visualization and analysis of the deformation processes occurring at individual grains or clusters of grains. Our experimental approach is a step towards a direct experimental visualization and quantitative understanding of the complex interaction between grains in nc metals during mechanical deformation, focusing on dislocation pile-up and grain rotation
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