Stasis mechanism of γ → ε martensitic transformation in Fe-17Mn alloy

In the present study, we observed incomplete martensitic transformation in Fe-17Mn alloy that γ austenite → ε martensitic transformation stops at a certain temperature during continuous cooling and the ε phase fraction does not increase with decreasing temperature further. The mechanism of this transformation stasis (TS) was investigated through microstructural observation, thermodynamic calculation and tensile testing using Fe-17Mn specimens with various γ grain sizes. The TS phenomenon was observed in all specimens regardless of prior γ grain size, and the TS temperature was lowered with decreasing prior γ grain size. TS occurred due to the dynamic grain refinement that ε platelets subdivide a γ grain to make several subgrains. The γ/ε interphase boundaries surrounding subgrains revealed the higher suppression effect of γ → ε martensitic transformation than prior γ grain boundaries. Therefore, at the same level of subgrain size, the fine γ-grained specimen revealed more active γ → ε martensitic transformation due to the higher fraction of prior γ grain boundaries than the coarse γ-grained specimen. Considering the difference in boundary characteristic between prior γ grain boundaries and γ/ε interphase boundaries, the driving force for γ → ε martensitic transformation was newly calculated and used to explain the TS phenomenon.