Wall Forum 2018

Up to now, it has been very challenging to directly observe defect creation by blistering. An even more difficult task is to identify criteria for the actual nucleation of blisters. We recently succeeded to create intragranular blisters in the transparent region of tungsten samples thinned for transmission electron microscopy. From a narrow zone without blisters near the central perforation of the sample, we were able to deduce a minimum W thickness of about 50 nm that is necessary for blister nucleation. We also observed a massive increase of the dislocation density due to blistering of about a factor 100 compared with the initial material. These dislocations predominantly exist close to observable cavities. A unique feature of creating blisters in electron-transparent samples is that this provides the opportunity to correlate the occurrence of blisters with pre-existing defects. In addition to observing blisters, we were also able to study the DSSL created during plasma exposure by TEM and investigate its defect structure. [mehr]

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The mechanical integrity of many structural materials is affected by the presence of hydrogen. Even minute amounts of a few ppm may give rise to fracture and embrittlement. From a modeling point of view, the descriptionof the impact of H on mechanical properties is challenging since hydrogen is known to interact with almost all microstructural and point defects. Thus, a quantitative description of how H impacts and interacts with the various defects requires to model large length and time scales while also having to take into account atomistic effects. In the talk it will be shown how by combining ab initio calculations with thermodynamic concepts the relevant mechanisms at atomic scale can be identified. Based on these insights atomistically informed mesoscale/macroscale concepts are constructed that allow to model H embrittlement on the length and time scales relevant for experiment. [mehr]

The mechanism of trapping and de-trapping of hydrogen atoms in tungsten is not yet fully understood. One method to investigate the underlying mechanism is to perform hydrogen isotope exchange in which trapped hydrogen atoms of one specific isotope (e.g. Deuterium) are replaced by those of a different isotope (e.g. Protium). Trapping and de-trapping of hydrogen atoms in tungsten can be described by two different models, the classical model and the fill-level model. Within those models the hydrogen isotope exchange proceeds differently. In contrast to the fill-level model the classical model should fail to explain hydrogen isotope exchange at low temperature where significant thermal de-trapping of hydrogen atoms is absent. The different hydrogen isotope exchange experiments at temperatures down to 150 K which were conducted to discriminate between the fill-level- and the classical model are presented and discussed. Furthermore, the experimental results are interpreted qualitatively in the context of a simple combinatorial model to highlight the fundamental exchange process, as well as quantitatively by applying the TESSIM-X diffusion-trapping code. [mehr]

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A method will be introduced to combine electron backscatter diffraction data from a scanning electron microscope and height profile data from a confocal laser scanning microscope. Oxidation and sputter experiments of tungsten were performed and results will be presented. [mehr]

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