Microstructural evolution and strain-hardening of Fe-30Mn during tensile deformation

High Mn ferrous alloys have become of interest to the automotive community due to their outstanding combination of strength, formability and the high work hardening rates observed, arising from phase transitions or mechanical twinning during deformation. In the present work, the microstructural evolution - including the dislocation structure and phase transitions - and work hardening rate as a function of strain and temperature were examined for a low C Fe-30Mn alloy. At the deformation temperature of 293K, it was found that the alloy deformed by classic dislocation motion with only minor amount of phase transitions occurring at high strains. With suitable modifications to account for the stacking fault energy of the material, the classic Kocks-Mecking approach was applied successfully in describing the work hardening and dynamic recovery processes occurring during deformation. For the deformation temperature of 77K, it was found that the alloy deformed initially by dislocation motion and then deformed by a combination of dislocation and phase transformations (ε martensite formation) for flow stresses in excess of 930MPa. It was found that the Kocks-Mecking approach could be adapted by adding a term which includes the hardening contribution due to the formation of ε martensite during this portion of the deformation. In this way, the higher strengths and uniform elongations observed when deformation at 77K versus 293K were successfully explained.