Three-dimensional characterization of duplex stainless steel by means of synchrotron radiation X-ray diffraction imaging techniques

The combined use of X-ray diffraction contrast tomography (DCT) and X-ray phase imaging techniques like phase contrast tomography and holotomography enable non-destructive characterization of the three dimensional grain and phase microstructure in austenitic-ferritic duplex stainless steel. Phase contrast tomography highlights discontinuities of the refractive index inside a material and is therefore ideally suited for imaging fatigue cracks and phase boundaries. The acquisition of phase images at multiple propagation distances allows for the two-step procedure of phase retrieval and tomographic reconstruction of the refractive index via holotomography. Combined with appropriate regularization and segmentation techniques, this technique provides the sensitivity to discriminate the minute difference in electron density between the austenitic and ferritic constituent phases of duplex steel. X-ray diffraction contrast tomography on the other hand exploits X-ray Bragg diffraction signals of the individual crystallites and yields three-dimensional grain orientation maps for each of the constituent phases (austenite and ferrite). Merging the results of both imaging modalities, the fidelity of the inter-phase boundaries (derived from X-ray holotomography) can be used to enhance the spatial fidelity of the 3D grain orientation maps produced by DCT. We have combined this microstructure characterization scheme with time lapse observations of a propagating fatigue crack by means of repeated phase contrast tomography inspection during an interrupted fatigue test. Access to the crack growth history and the crystallographic microstructure allow for qualitative analysis of fatigue crack – microstructure interactions and provides valuable input for refinement and benchmarking of image based crystal plasticity finite element calculations.