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 (e1) and thepressure-induced epsilon-iron (e2), high-pressure quasi-hydrostatic experiments were performed on alow-carbon, high-alloy metastable austenitic steel. In situ synchrotron X-ray diffractionmeasurements as well as post-mortem investigations of the microstructure by electron backscatterdiffraction were carried out to study the microstructural transformations. Three processes wereobserved during compression experiments: first, the formation of deformation-induced hexagonalepsilon1-martensite, as well as small nuclei of deformation-induced bcc alpha'-martensite (a'1) within the fccgamma-matrix due to non-hydrostaticity in the experiments; second, the onset of the phase transformationfrom the metastable fcc gamma-austenite into the hexagonal pressure-induced epsilon2-iron phase occurred ataround 6 GPa; third, during decompression, the hexagonal pressure-induced epsilon2-iron transformedpartially into bcc alpha'-martensite (a'2). Completely different characteristics with regard to habitus aswell as to orientation relationships were observed between the pressure-induced phases (epsilon2-ironphase and alpha'2-martensite) and the deformation-induced martensites (epsilon1- and alpha'1-martensite)