Abstract
Creep cavitation in an ex-service nuclear steam header Type 316 stainless steel sample is investigated through a multiscale tomography workflow spanning eight orders of magnitude, combining X-ray computed tomography (CT), plasma focused ion beam (FIB) scanning electron microscope (SEM) imaging and scanning transmission electron microscope (STEM) tomography. Guided by microscale X-ray CT, nanoscale X-ray CT is used to investigate the size and morphology of cavities at a triple point of grain boundaries. In order to understand the factors affecting the extent of cavitation, the orientation and crystallographic misorientation of each boundary is characterised using electron backscatter diffraction (EBSD). Additionally, in order to better understand boundary phase growth, the chemistry of a single boundary and its associated secondary phase precipitates is probed through STEM energy dispersive X-ray (EDX) tomography. The difference in cavitation of the three grain boundaries investigated suggests that the orientation of grain boundaries with respect to the direction of principal stress is important in the promotion of cavity formation.
Highlights
Creep cavitation in an ex-service nuclear steam header Type 316 stainless steel sample is investigated through a multiscale tomography workflow spanning eight orders of magnitude, combining X-ray computed tomography (CT), plasma focused ion beam (FIB) scanning electron microscope (SEM) imaging and scanning transmission electron microscope (STEM) tomography
The carbides that are visible as slight changes in intensity along the grain boundary are present along the cavitated grain boundary, but their morphology cannot be determined due to the low X-ray contrast
The previous observations of Wahab et al.[30] that the relative frequency of creep void formation in gas reformer tubes is highest at boundaries decorated with carbides and inclined at 40° – 70° away from the direction of the principal stress, and that of Burnett et al.[21] that heavily creep cavitated boundaries are decorated with carbides and other precipitates and lie at approximately 45° to the apparent direction of the principal tensile stress, both suggest that shear at boundaries containing precipitates is conducive to the development of creep voids and that grain boundary sliding may play a significant role in void formation, as expounded in theoretical models of creep cavitation[21, 22, 24]
Summary
Creep cavitation in an ex-service nuclear steam header Type 316 stainless steel sample is investigated through a multiscale tomography workflow spanning eight orders of magnitude, combining X-ray computed tomography (CT), plasma focused ion beam (FIB) scanning electron microscope (SEM) imaging and scanning transmission electron microscope (STEM) tomography. Hierarchical structures underpin key processes in materials science, chemical engineering, biology and geology In all of these hierarchical materials the performance depends critically on features at length scales from the macroscale to the nanoscale. Three dimensional (3D) microstructural investigations covering a wide range of length scales have involved multiple instruments looking at different regions taken from the sample material[6,7,8]. Valuable, these uncorrelated data sets can fail to identify key relationships between nanoscale structures and macroscopic features. By incorporating transmission electron or atom probe tomography it is possible to extend the tomography workflow to nanometre resolution, allowing spatial correlation of volumes from metre length scales, down to less than 100 nm (Fig. 1)
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