Kashiwar Ankush, Hahn Horst, Kübel Christian
Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
Department of Materials and Earth Sciences, KIT-TUD Joint Research Laboratory Nanomaterials, Technische Universität Darmstadt, Alarich-Weiss-Straße 2, 64287 Darmstadt, Germany.
Nanomaterials (Basel). 2021 Feb 9;11(2):432. doi: 10.3390/nano11020432.
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.
我们报道了纳米晶钯薄膜中伴随着强烈包辛格效应的协同晶粒旋转现象。使用原位透射电子显微镜纳米力学对纳米晶钯薄膜进行循环加载-卸载，并通过环形暗场扫描透射电子显微镜成像和定量环形暗场扫描透射电子显微镜分析研究其不断演变的微观结构特征。环形暗场扫描透射电子显微镜成像显示，在循环加载-卸载实验中，纳米尺寸的晶粒发生了部分可逆的旋转，且具有很强的面外分量。相邻晶粒组显示出协同旋转，随着应变增加/减少依次进行。结合这些实验的环形暗场扫描透射电子显微镜提供了不同应变水平下旋转晶粒的晶体取向信息。局部奈张量分析表明，紧邻晶粒内的几何必要位错密度演化显著不同，证实了局部非均匀变形响应。几何必要位错密度分析揭示了在晶界处形成了位错堆积，表明卸载过程中产生了背应力。对单个晶粒取向变化的统计分析表明，大多数晶粒发生旋转但没有全局织构发展，这与基于位错和晶界滑动的机制均相符。总体而言，我们的定量原位实验方法探索了循环加载过程中这些不同变形机制在纳米晶金属中的作用。
