Micromechanical characterization of grain boundary slip: Towards a deformation mechanism map

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Grain boundary sliding (GBS) is one key deformation mechanism of polycrystalline materials, particularly for nanocrystalline materials and materials at elevated service temperatures. The immense number of different grain boundaries being present in polycrystalline materials as well as the occurrence of other slip mechanisms (e.g. dislocation slip) prevented a quantitative understanding of the fundamental processes during GBS so far. During the last decade, high temperature nanomechanical testing on focused ion beam (FIB) milled structures was established, and potentially allows for the isolation, characterization and quantification of GBS of one single grain boundary at the micron scale today. It is our objective to use small scale mechanical testing inside a scanning electron microscope (SEM) in order to decouple GBS from other mechanisms of plastic deformation. We propose to measure the shear stress vs. shear strain curve for GBS on individual grain boundaries located in micron and sub-micron sized compression pillars. The grain boundaries will be inclined by 45° with respect to the pillar loading axes. The shear stress is calculated using the applied load as well as the slip geometry (area and inclination of the boundary, direction of slip). The shear strain will be calculated from SEM images, analyzed by digital image correlation (DIC). As soon as a protocol for measuring the shear stress vs. shear strain curve of a GB is established, we will quantitatively assess the GBS mechanisms with regard to the subsequent fundamental questions: 1. What is the activation energy of GBS? 2. Is there an intrinsic size effect in Rachinger-type GBS? 3. What is the influence of grain boundary type for GBS? 4. What is the strain rate dependence on GBS quantitatively? 5. What is the importance of GBS for maintaining compatibility in (macro) polycrystals?