Superior strength-ductility combination achieved via double heterogeneities of microstructure and composition: An example of medium manganese steel

The double heterogeneities of microstructure and composition are introduced in a medium Mn steel via the partially austenite reversed transformation (PART). The role of these heterogeneities in the mechanical properties is revealed. A heterogeneous microstructure consisting of both the granular and lath-typed ferrite/austenite grains is obtained in the PART sample. During plastic deformation, the lath-typed microstructures bear more deformation than the granular ones due to their unique morphological geometry, leading to strain partitioning. This mechanical incompatibility gives rise to significant hetero-deformation induced (HDI) hardening and thus a remarkable enhancement of the mechanical properties. The compositional heterogeneity, i.e. Mn concentration in the lath-typed austenite is higher than that in the granular austenite, is also obtained in the PART steel. Such heterogeneity gives rise to different austenite stabilities and thus sustainable martensitic transformation at different stages of deformation, which also contributes to the enhancement of the mechanical properties. These results provide new insights into the design of advanced metallic materials with outstanding mechanical properties via the double heterogeneity of microstructure and composition.