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

Abstract

In order to clarify the difference between the deformation-induced e-martensite (e 1) and the pressure-induced e-iron (e 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 e 1-martensite, as well as small nuclei of deformation-induced bcc α′-martensite (α 1′) within the fcc γ-matrix due to non-hydrostaticity in the experiments; second, the onset of the phase transformation from the metastable fcc γ-austenite into the hexagonal pressure-induced e 2-iron phase occurred at around 6 GPa; third, during decompression, the hexagonal pressure-induced e 2-iron transformed partially into bcc α′-martensite (α 2′). Completely different characteristics with regard to habitus as well as to orientation relationships were observed between the pressure-induced phases (e 2-iron phase and α 2′-martensite) and the deformation-induced martensites (e 1- and α 1′-martensite).