Homogenous and carbon graded Fe-22Mn alloys: Microstructure and mechanical properties

Among the advanced high-strength steels, high manganese steels have a superior combination of both a high UTS and a large uniform elongation. As a consequence of plastic deformation, these austenitic steels transform by creating mechanical twins (TWIP) or ε-martensite (TRIP). The deformation products depend significantly on the value of the stacking fault energy (SFE) which is in turn a function of the alloy composition. A Fe-22Mn-0.6C steel was decarburized under different atmospheres to obtain a homogenous Fe-22Mn-0C alloy as well as three carbon graded Fe-22Mn-C steels. The 0C alloy initially had a dual-phase microstructure of austenite and ε-martensite. The ε-martensite volume fraction increased significantly after deformation leading to a high initial work-hardening rate but relatively low uniform elongation. For the carbon graded steels, the initial microstructure was a mixture of austenite and ε-martensite where the ε-martensite amount increased from the core to the outer surface. The deformation products altered from twins at the core to ε-martensite at the surface of these steels. It was determined that the overall mechanical behavior of the alloy depended strongly on the volume fraction of the material with a C content exceeding 0.4wt%.