Characteristics of Non-uniform Deformation Behavior in Fe-10%Mn-0.1%C Alloy with Ultrafine-grained Multi-phase Martensitic Structure

To clarify the characteristics of plastic deformation behavior in quenched Fe-10%Mn-0.1%C alloy (lOMn steel), the microstructure and tensile deformation behavior were investigated and the non-uniform deformation behavior was analyzed using digital image correlation (DIC) method. As a comparison material, Fe-5%Mn-0.1%C alloy with common lath martensitic structure (5Mn steel) was used. The 10Mn steel has an equiaxed ultrafine-grained (alpha'+epsilon+gamma) three-phase microstructure formed through a two-step martensitic transformation of gamma ->epsilon ->alpha during quenching. Tensile testing of 10Mn steel results in a stress-strain curve characterized by a clear yield point and significant work hardening. The yielding of 10Mn steel can be explained by the generation of plastic strain due to the stress-induced martensitic transformations such as epsilon ->alpha and gamma ->alpha transformations. Furthermore, the subsequent work hardening can be explained by the combined mechanism of the continuous epsilon ->alpha and gamma ->alpha transformations responsible for plastic deformation and the hard a' martensite responsible for stress. In 5Mn steel with lath martensitic microstructure, strain is concentrated in specific blocks during tensile deformation due to the priority of habit plane slip system, and the plastic deformation proceeds non-uniformly, whereas in 10Mn steel with equiaxed ultrafine grain microstructure, although small strain bands are generated, relatively uniform deformation tends to occur. The epsilon martensite and retained gamma dispersed in the microstructure are considered unlikely to be the cause of contributing to non-uniform deformation.