High-resolution characterization of Stress Corrosion Cracking

This project is dedicated to characterize intergranular Stress Corrosion Cracking in Pressurized Water Reactor (PWR) primary circuit materials such as reactor grade stainless steels and nickel alloys via different electron microscopy (EM) methods. Materials exposed to the PWR environment are subjected to high temperatures, stress and pressure and develop microscopic defects that, after many years of operation, can lead to the catastrophic failure of entire reactor components. Stress Corrosion Cracking (SCC) is a yet not fully understood phenomenon that occurs as microscopic cracks within the material. 

The goal of this project is to understand the propagation of intergranular SCC in PWR primary circuit materials. Therefore, different EM methods have to be applied in order to achieve comprehensive results which may lead to a better understanding of the multitude of mechanisms involved. Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) are applied to gain an overview of the topography of the crack and distinguish between primary and secondary crack tips in order to select the most important features in the sample for further analysis. The crack tip region is crucial within this project, as it most likely contains valuable information on crack propagation (i.e. matrix composition, presence of defects, grain orientations and oxide distribution around the crack tip). For this purpose, high-resolution methods such as Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) will be applied. Analytical TEM is capable of resolving nanoscale features and using electron diffraction, oxide structures and their elemental composition can be determined. The high chemical sensitivity and atomic resolution of APT is unmatched by any other EM technique. However, due to huge efforts in sample preparation, APT has not been used to methodically study stress corrosion crack tips so far. One of the major milestones of this project is to achieve a standard preparation technique using the Focused Ion Beam (FIB) for the generation of APT samples containing crack tips.