Invited lecture
Dislocation structure and grain boundary motion induced by nanindentations on STO and W

In the present work, the dislocation structure evolution and indentation size effect has been studied for Strontium Titanate (STO) as well as W, using sequential polishing, etch-pits and High-Resolution-Eelectron-Backscattered-Diffraction (HR-EBSD) on pyramidal nanoindentation experiments. STO is used as a model material, allowing a detailed study and a quantification of the dislocation densities at small indentation depth. Nanoindentation load-displacement curves show multiple pop-in events, which relate to nucleation and extension of dislocation pile-ups around the indentations. Sequential polishing and etching revealed the three-dimensional dislocation etch-pit structure at various sub-surface depths. With HR-EBSD, the lattice rotation and thereby GND densities are determined, while the etch-pit technique revealed the total dislocation density. Based on the independently measured dislocation densities, we clearly show a depth dependent dislocation density, where for W and STO the GND densities increase with decreasing indentation depths, leading to the indentation size effect.

Furthermore, the dislocation structure below indentations at the grain boundary (GB) pop-in events were studied for tungsten using electron channeling contrast imaging (ECCI). During indentations in the vicinity of GBs so called GB pop-in events were identified as secondary jumps in the load-displacement curve. A significant hardness increase was observed before the GB pop-in event and the indentation experiments were stopped just after the displacement burst. The careful ECCI analysis on the sequential polished cross-sections clearly shows the dislocation pile-up in the vicinity of the GB along with transmitted dislocations in the adjacent grain. For specific GBs, a GB motion, as evidenced by a GB curvature was observed after dislocation transmission through the GB. The GB motion seems to be coupled with the GB pop-in, indicating a local GB yield process.