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

Abstract

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 a parent phase to the HCP e martensite phase was observed in the samples with near 36Fe-24Cr-20Co-20Mn compositions. Bainitic transformation from the a phase occurred with aging. Although the a parent phase coexisted with the e martensite phase in the as-quenched specimen and transformation did not proceed with sub-zero treatment, the e martensite was completely induced by cold rolling. Furthermore, the FCC c phase was obtained by heavy deformation. These results indicate that multiple martensitic transformations from a to e and e to c occurred. The c ? e martensitic transformation was also observed for 48Fe-16Cr-16Co-20Mn. The phase stabilities of the a, e, and c phases were similar. The calculated T 0 lines successfully predicted the a ? e martensitic transformation when the e phase was more stable in the database. Thermodynamic analysis revealed that magnetic ordering in the a phase affected the phase stability, resulting in the novel e loop in addition to the conventional c loop.