Metastable austenitic CrMnNi steel undergoes a deformation-induced martensitic phase transformation from face-centered cubic (fcc) γ-austenite into body-centered cubic (bcc) α’-martensite via an intermediate hexagonal structure, known as ε-martensite (e.g. ). However, a pressure-induced formation of the hexagonal-closed packed (hcp) ε-iron phase was discovered in literature in α-iron with a bcc crystal structure at ambient temperature and quasi-hydrostatic pressures in the range of 5 to 16 GPa (e.g. ).
The aim of the present investigations is to demonstrate the difference between deformation-induced ε-martensite and the pressure-induced ε-iron phase in low carbon, high-alloy CrMnNi steel. Therefore, high-pressure quasi-hydrostatic experiments were performed by using multi-anvil systems at room temperature up to 18 GPa in combination with (i) in situ synchrotron X-ray diffraction measurements at Deutsches Elektronen Synchrotron (DESY) as well as (ii) post-mortem microstructural characterization by electron backscatter diffraction after experiments at Freiberger High-Pressure Research Centre (FHP).
The results of XRD and SEM studies revealed three different microstructural processes (see Fig. 1, ) during compression (pressure increase) and decompression (pressure release) path. First, the formation of deformation-induced ε1-martensite in the metastable austenite was observed during compression at pressures below 6 GPa. The ε1-martensite corresponded to deformation bands, which consisted of regular arrangements of stacking faults. Furthermore, the ε1-martensite transformed into α1‘-martensite (bcc) which appeared as small lentil-shaped nuclei with different crystallographic orientations. This transformation was caused by shear deformation due to deviations from ideal hydrostatic conditions. Second, the pressure-induced phase transformation from γ into ε2-iron started at about 6 GPa and was of martensitic nature. The ε2-iron phase appeared as a blocky microstructure with uniform crystallographic orientation since former fcc γ-grains transformed completely into the hcp ε2-iron. Third, the ε2-iron phase partially transformed into the bcc α2‘-martensite during decompression. The α2‘-martensite appeared as needles of uniform crystallographic orientation within ε2-grains. This indicated that the ε2-phase was not completely stable down to ambient temperature and pressure.
Finally, the deformation-induced γ–ε1 and ε1–α1‘ transformations and the pressure-induced γ–ε2 and ε2–α2‘ transformations can be clearly distinguished from each other by the different pressure ranges as well as by their characteristic morphologies and crystallographic orientations. The present experimental investigations confirmed statements in the literature (e.g. ) on the occurrence and differences of deformation-induced and pressure-induced phase transformations in high-pressure experiments if plastic deformation is present in the specimen due to non-hydrostaticity.
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