High-resolution 4D visualization of deformation-induced martensitic transformation in a metastable austenitic steel by synchrotron X-ray nano-computed tomography

Synchrotron X-ray nano-computed tomography (nano-CT) was employed to achieve high-resolution four-dimensional (3D + deformation step) visualization of deformation-induced martensitic transformation (DIMT) in a metastable austenitic steel during interrupted tensile deformation. The identical region of interest was precisely aligned and tracked at each deformation step, enabling direct three-dimensional observation of nucleation, growth, and coalescence of individual martensite plates. With an effective voxel size of 40.4 nm and a spatial resolution of approximately 180 nm, needle-like martensite plates were resolved, and their evolution along multiple habit-plane traces was visualized. Complementary projection-type micro-CT provided a wide-field overview of the transformation distribution in the bulk specimen; the martensite phase was identified as darker grey objects owing to contrast between the austenite and martensite phases, and grey-value profiles enabled discrimination between martensite and microvoids. The combined micro- and nano-CT approach provides multi-scale 4D visualization of DIMT, linking overall transformation progression to high-resolution plate evolution in the same specimen.