High-resolution characterisation of stress corrosion cracking

The degradation of reactor grade stainless steels and their susceptibility to stress corrosion cracking (SCC) when exposed to the pressurised water reactor (PWR) primary water environment has been a topic of intense research for many decades. Nevertheless, our understanding of the underlying mechanisms of SCC remains incomplete to date. It has been generally accepted that only high-resolution (electron) microscopy techniques are capable of revealing the yet unidentified processes involved in SCC crack propagation. For this reason, one of the main objectives of this project was to make new techniques with improved spatial resolution accessible to SCC research. While low-keV energy dispersive X-ray spectroscopy (EDX) was used for the preliminary analysis of SCC cracks, transmission Kikuchi diffraction (TKD) and atom probe tomography (APT) were used for high-resolution studies of the microstructure and chemistry near the crack tip. In particular, TKD proved very beneficial for revealing the extent of the strain concentration around the crack tip. For the application of APT to SCC research, a novel method for preparing APT needles containing entire SCC crack tips was developed. The method was then used for acquiring very localised compositional measurements of the crack tip and GB oxide chemistry with extraordinary accuracy. The second objective of this thesis was to understand the impact of the SCC test temperature on the crack growth rate (CGR) in SUS316 stainless steel. It was found that after steady growth with increasing temperature, a peak in the CGR occurred at ∼ 320°C, followed by a substantial drop towards higher temperatures. The inhibition of the CGR with increasing temperature between 320° and 360°C and its impact on the microstructure were studied via analytical transmission electron microscopy (TEM) and TKD. Furthermore, the potential impact of thermally activated diffusion and mechanical response-based mechanisms was investigated. It appears that higher dislocation density and strain concentrations around the crack tips at lower temperature (i.e. 320°C) lead to possibly enhanced brittle-like fracture at the crack tip. An enhanced model for the ongoing processes involved in SCC crack propagation based on the experimental results is presented at the end of this work.