BCC-HCP-FCC Multiple Transformations and ε Loop in the Fe-Cr-Co-Mn System

Phase transformations and phase stability in the Fe-Cr-Co-20Mn (at.%) alloy system were investigated using optical microscopy, transmission electron microscopy, x-ray diffraction, and vibrating sample magnetometry. Thermal-induced martensitic transformation from the BCC α parent phase to the HCP ε martensite phase was observed in the samples with near 36Fe-24Cr-20Co-20Mn compositions. Bainitic transformation from the α phase occurred with aging. Although the α parent phase coexisted with the ε martensite phase in the as-quenched specimen and transformation did not proceed with sub-zero treatment, the ε martensite was completely induced by cold rolling. Furthermore, the FCC γ phase was obtained by heavy deformation. These results indicate that multiple martensitic transformations from α to ε and ε to γ occurred. The γ → ε martensitic transformation was also observed for 48Fe-16Cr-16Co-20Mn. The phase stabilities of the α , ε , and γ phases were similar. The calculated T 0 lines successfully predicted the α → ε martensitic transformation when the ε phase was more stable in the database. Thermodynamic analysis revealed that magnetic ordering in the α phase affected the phase stability, resulting in the novel ε loop in addition to the conventional γ loop.