September 29-October 4, 2019
This study deals with in-situ micromechanical tests of micron-sized bi-crystals and observations coupling SEM, AFM and EBSD. Different FCC bi-crystals are obtained from FIB machining. A SEM in-situ compression test with a low strain is performed on a micron-sized bi-crystal in order to induce single slip deformation. Spatial variations in the surface step height due to dislocation activity in localized slip bands terminating at the grain boundary (GB) are measured by AFM. This allows the determination of the Burgers vector distribution and hence the dislocation positions in the pile-up as shown in Figure 1. Furthermore, local misorientation along slip bands is measured by high resolution EBSD in order to determine the deformation caused by the dislocation pile-ups. In parallel, an analytical approach based on the Leknitskii-Eshelby-Stroh (LES) formalism (1,2), which provides the elastic fields of straight dislocation pile-ups in anisotropic bi- and tri- materials (3) while considering (or not) free surface effects (4), are performed. The tri-material configuration allows considering a non-zero thickness in the nanometer range and a specific stiffness for the GB region. The configuration with two free surfaces could be used to study size effects. The effects of anisotropic elasticity, crystallographic orientation, GB stiffness and free surfaces are first studied in the case of a single dislocation in a Ni bi-crystal. Image forces may arise because of both dissimilar grain orientations, the presence of a finite grain boundary region and the presence of free surfaces. In particular, it is shown that the Peach-Koehler force projected along the dislocation glide direction can exhibit a change of sign with the dislocation position (5). The dislocation positions in a pile-up are calculated by an iterative relaxation scheme minimizing the Peach-Koehler force on each dislocation as shown in Figure 2. Both, GB stiffness and grains misorientation, influence pile-up length and the induced resolved shear stress in the neighboring grain, but the effect of misorientation is clearly predominant (5). Hence, the driving force for slip activation in the neighboring grain can be computed and compared to the observed GB resistance for slip transmission.
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Xiaolei Chen, Stéphane Berbenni, Christian Motz, and Thiebaud Richeton, "Effect of anisotropic elasticity on dislocation pile-ups at grain boundaries" in "Nanomechanical Testing in Materials Research and Development VII", Jon Molina-Aldareguia, IMDEA-Materials Institute, Spain Eds, ECI Symposium Series, (2019). https://dc.engconfintl.org/nanochemtest_vii/9