**4. Conclusions**

In conclusion, an excellent combination of strength and tensile ductility was achieved in the ultrafine-grained Fe-Mn-Ni-Al-C alloy containing B2 intermetallic compounds as second phase particles. The tensile test with in situ X-ray diffraction measurement revealed a rapid stress partitioning between austenite phase and B2 phase at the very beginning of plastic deformation, which was associated with the Lüders band deformation of the specimen. In addition, through the stress partitioning analysis, it was found that the B2 particles, although they took only 0.09 (9%) volume fraction of the material, withstood very high phase stress during tensile deformation. More importantly, the B2 phase exhibited

a unique hardening behavior that could effectively slow down the decrease of the strain hardening rate of the whole specimen and delayed the onset of plastic instability, suggesting an importance of the hard B2 phase in the strength and ductility synergy of the material. Through the Williamson-Hall analysis, it was found that the dislocation accumulation rate in the B2 phase was enhanced during tensile deformation, which seemed to interpret the unique hardening rate of the B2 phase. Further studies should be focused on clarifying the interaction between B2 particles and austenite grains through microstructures' observations.

**Author Contributions:** Conceptualization, S.G. and N.T.; methodology, T.Y., W.M., Y.B., W.G., M.-h.P., H.A., M.S. and A.S.; writing—original draft preparation, S.G. and N.T.; writing—review and editing, N.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The present study was financially supported by JST CREST (JPMJCR1994), Elements Strategy Initiative for Structural Materials (ESISM, No. JPMXP0112101000), and KAKENHI (Grant-in-Aid for Scientific Research from JSPS; No.15H05767 and No.20H00306) all through the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The synchrotron radiation experiments (beam-time No. 2018B1760) at SPring–8 were performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). The alloy used in this study was kindly provided by Dmitri LOUZGUINE from Advanced Institute for Materials Research in Tohoku University. The authors gratefully appreciate all the support.

**Conflicts of Interest:** The authors declare no conflict of interest.
