Due to the continuously increasing global energy demand and the simultaneous efforts to reduce CO2 emissions, renewable and alternative energy sources are required in order to achieve these contrary goals. A promising candidate in the area of sustainable energy supply, whether for storage, transport or mobility, are hydrogen technologies. In order to enable a safe handling, it is essential to investigate the materials resistance against hydrogen degradation, as well as to improve and develop new alloys for future applications. Although the problem of hydrogen embrittlement in metals has been studied for many years, no consensus has been reached on the prevailing mechanisms. Furthermore, a number of component failures of nickel-base alloys, which have been attributed to hydrogen embrittlement, were reported over the years for applications in oil and gas industry. In order to shed some light on the underlying mechanisms and failure reasons, the scientific community is striving to improve existing experimental methods and to find new approaches to further develop promising material classes for application in hydrogen containing environments. Therefore, in this PhD thesis the technique of in-situ electrochemical nanoindentation is implemented, applied and further developed through combination with advanced measurement protocols. Besides, the determination of the fundamental mechanical properties of Young´s modulus and hardness, the main focus was on the investigation of the plastic deformation mechanisms. The experiments, which were conducted on the nickel-base alloy 725, showed a hydrogen-induced hardness increase for both investigated heat treatments. Furthermore, the influence of hydrogen on the strain rate dependent deformation parameters, i.e. strain rate sensitivity and activation volume, could be revealed. These experiments were complemented with detailed optical evaluation of the plastically deformed zone using scanning electron microscopy as well as laser scanning confocal microscopy. As a second approach, additionally to the determination of the influence of hydrogen on the mechanical properties, a concept of segregation-based grain boundary design was developed and applied to alloy 725. This approach is based on a simulation-based prediction and the experimental verification of the grain boundary segregation by means of atom probe tomography. An extraordinary quantitative agreement of the applied methods was achieved. The gained knowledge can be used in the future for the development and improvement of nickel-base alloys. The present work conducted with the above-mentioned methods should, therefore, contribute to a better understanding of the acting mechanisms and opens up the possibility to test the materials resistivity against hydrogen embrittlement. Furthermore, a targeted material improvement is possible to meet the high requirements for future applications under the influence of hydrogen.
|Translated title of the contribution||Elektrochemische in-situ Nanoindentation zur Untersuchung des plastischem Verformungsverhalten unter dem Einfluss von Wasserstoff|
|Publication status||Published - 2021|
Bibliographical noteno embargo
- in-situ electrochemical nanoindentation
- nickel-base superalloy
- hydrogen embrittlement