Investigation of Phase Transformations in High-Alloy Austenitic TRIP Steel Under High Pressure (up to 18 GPa) by In Situ Synchrotron X-ray Diffraction and Scanning Electron Microscopy

In order to clarify the difference between the deformation-induced epsilon-martensite (epsilon (1)) and the pressure-induced epsilon-iron (epsilon (2)), high-pressure quasi-hydrostatic experiments were performed on a low-carbon, high-alloy metastable austenitic steel. In situ synchrotron X-ray diffraction measurements as well as post-mortem investigations of the microstructure by electron backscatter diffraction were carried out to study the microstructural transformations. Three processes were observed during compression experiments: first, the formation of deformation-induced hexagonal epsilon (1)-martensite, as well as small nuclei of deformation-induced bcc alpha'-martensite (alpha (1)') within the fcc gamma-matrix due to non-hydrostaticity in the experiments; second, the onset of the phase transformation from the metastable fcc gamma-austenite into the hexagonal pressure-induced epsilon (2)-iron phase occurred at around 6 GPa; third, during decompression, the hexagonal pressure-induced epsilon (2)-iron transformed partially into bcc alpha'-martensite (alpha (2)'). Completely different characteristics with regard to habitus as well as to orientation relationships were observed between the pressure-induced phases (epsilon (2)-iron phase and alpha (2)'-martensite) and the deformation-induced martensites (epsilon (1)- and alpha (1)'-martensite).