5th International High Manganese Steel Conference - HMNS 2022

The transformation stasis (TS) phenomenon refers to the occurrence of incomplete transformation by exhaustion of a driving force while a phase transformation proceeds, which was observed during isothermal bainitic transformation. However, in the present study, the TS phenomenon was observed during athermal γ austenite → ε martensitic transformation in a high Mn alloy. Namely, γ → ε martensitic transformation stopped at a certain temperature during continuous cooling and the volume fraction of ε martensite did not increase with further cooling. The TS phenomenon occurred in all specimens regardless of prior γ grain size, and the TS temperature decreased with decreasing prior γ grain size. The TS phenomenon was highly related to dynamic γ stabilization during γ → ε martensitic transformation, which occurred most likely due to dynamic γ grain refinement that a prior γ grain is subdivided into several subgrains by ε platelets. As the size of γ subgrains decreased, the shear stress necessary for martensitic transformation increased, resulting in the exhaustion of the driving force for γ → ε martensitic transformation. In addition, the characteristics of boundaries surrounding subgrains also influenced dynamic γ stabilization. Compared to incoherent and curved prior γ grain boundaries, coherent and flat γ/ε interphase boundaries revealed the higher suppression effect of γ → ε martensitic transformation. Therefore, even if the subgrain size was similar, γ → ε martensitic transformation occurred more actively in the fine grain-sized specimen with a higher fraction of prior γ grain boundaries surrounding subgrains than the coarse grain-sized specimen. Considering dynamic grain refinement and boundary characteristic, the driving force for γ → ε martensitic transformation was newly calculated to explain the TS phenomenon.