Utilising correlative 3D imaging to understand creep cavitation in stainless steel

The role of grain boundary orientation and secondary phase precipitation on creep cavitation in a stainless steel sample has been investigated using a correlative tomography1 approach. A number of different 3D imaging techniques are combined on the same sample in order to understand the initiation and progression of cavitation.

Correlative imaging has become an important tool in both biology2,3 and materials science4, where it provides 2D information of the same sample area at multiple length scales. Correlative tomography describes the extension of correlative imaging to three dimensions via a range of techniques, which also provides the opportunity to probe sub-surface volumes1.

The position, size and morphology of cavities on three grain boundaries in a stainless steel sample taken from a power station steam header were examined using X-ray computed tomography (CT) 5. X-ray CT demonstrates that the presence of cavities, as well as their size and shape, varies for each of the grain boundaries examined in this study (Figure 1). Subsequently, FIB-SEM slice and view provides a higher-resolution analysis of the same sample to resolve and identify the precipitates decorating the cavitated grain boundaries. Additionally, 3D electron backscatter diffraction (EBSD) mapping reveals the misorientation at grain boundaries and thus offers some insight in to why certain boundaries may possess cavities and whether grain boundary misorientation affects the size and shape of cavities.

Furthermore, a 200 nm diameter pillar was sectioned from one of the cavitated boundaries in order to perform scanning transmission electron microscope (STEM) – energy dispersive X-ray (EDX) tomography 6,7. STEM-EDX tomography reveals the distribution of elements at nanometre scale in three dimensions (Figure 2). This high-resolution chemical information aids understanding of precipitate formation and provides accurate characterisation of precipitate morphology.

The correlative 3D imaging approach applied here gives unprecedented insight in to cavitation in stainless steels and is also applicable to a wide range of other materials that display characteristic features at a number of different length scales.