PSI - Issue 19

Fumiyosi Yoshinaka et al. / Procedia Structural Integrity 19 (2019) 214–223 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

222

9

Conclusions

This study investigated the fatigue behavior of a Fe-15Mn-10Cr-8Ni-4Si seismic damping alloy in the LCF and HCF regimes. Uni-axial fatigue tests were conducted and the post-fatigue microstructure was characterized using EBSD microscopy. As a result of the LCF test conducted under the axial strain control condition, the fatigue life of Fe-15Mn-10Cr-8Ni-4Si was found to be superior to that of the following reference alloys: Fe-28Mn-5Cr-6Si, SUS304, and LY225. The fatigue life of Fe-15Mn-10Cr-8Ni-4Si obtained at a total strain range of 2% obeyed a normal distribution, and the mean fatigue life was 1.2 × 10 4 , which was 15 times longer than the result of LY225. When the strain was divided into inelastic and elastic components, Fe-15Mn-10Cr-8Ni showed a longer fatigue life against the inelastic strain range compared to that of the other tested materials. On the other hand, the fatigue life of Fe-15Mn-10Cr-8Ni-4Si took a similar value to those of austenitic steels, Fe-28Mn-5Cr-6Si and SUS304, at a lower strain level against the elastic strain range. The cyclic hardening behavior was investigated at a total strain range of 1%, 2%, 4%, and 6%. In Fe-15Mn-10Cr-8Ni-4Si, the gradual increase trend up to the fracture was recognized at a total strain range of 1% and 2%, while the stress rapidly increased in the early stage for a higher Δ ε t of 4% and 6%. I n the case of Δ ε t = 6%, significant hardening was recognized at the stage of the fracture appearance. In summary, the hardening trend of Fe-15Mn-10Cr-8Ni-4Si was stable compared to that of the reference alloys. As a result of the HCF test conducted under the axial loading control condition, the fatigue limit (262.5 MPa) was comparable to the 0.2% proof stress (274 MPa). The surface observation of the specimen tested at a stress lower than the fatigue limit revealed that while surface relief was well-developed, no cracks of a size of at least several grains were found. The EBSD microscopy performed on the post-fatigue microstructure in the fractured specimens obtained by the LCF test demonstrated a total strain range of 2%, and by the HCF tests at the strain amplitudes of 325 MPa and 275 MPa revealed that deformation- induced ε -martensite was developed by cyclic loadings. In the specimen tested at a total strain range of 2%, most γ - austenite transformed into ε -martensite, while the amount of ε martensite was li mited in the HCF specimens. Around the secondary crack detected on the measurement section, α′ - martensite was found in the LCF specimen and ε -martensite was formed to trace the edge of the crack in the HCF specimen tested at 325 MPa, while no apparent difference between the regions near and far from the crack was recognized in the HCF specimen tested at 275 MPa. EBSD microscopy performed on the unfractured specimen showed that the ε -martensitic transformation can happen at a stress level lower than the fatigue limit. In addition to ε - martensite, γ -deformation twinning was observed in this specimen. One of our future tasks is to clarify the relationship between the deformed microstructure and the crack initiation and growth behaviors. The fact that the trend of the deformation-induced transformation was different between the regions near and far from the crack under relatively large stress/strain conditions indicate the importance of the local behavior at the crack tip.

Acknowledgments

The authors acknowledge the support of a Grant-in-Aid for Early-Career Scientists (JP19K14853) from JSPS, Japan.

References

Brechet, Y., Magnin, T., Sornette. D., 1992. The Coffin-Manson law as a consequence of the statistical nature of the LCF surface damage. Acta metallurgica et materialia 40, 2281 – 2287. Cladera, A., Weber, B., Leinenbach, C., Czaderski, C., Shahverdi, M., Motavalli, M., 2014. Iron-based shape memory alloys for civil engineering structures: An overview. Construction and building materials 63, 281 – 293. De Cooman, B. C., Estrin, Y., Kim, S. K., 2018. Twinning-induced plasticity (TWIP) steels. Acta Materialia 142, 283 – 362. Hatanaka, K., Shimizu, S., 1982. Fatigue Strength in Long Life Range and Non-propagating Fatigue Crack in SUS 304 Type Stainless Steel. Bulletin of JSME 25, 1859 – 1866. Jang, W. Y., Gu, W., Van Humbeck, J., Delaey, L., 1995. Microscopic observation of γ -phase and ε -and α′ -martensite in Fe – Mn – Si-based shape memory alloys. Materials Characterization 34, 67 – 72. Remy, L., Pineau, A., 1977. Twinning and strain-induced F.C.C. → H.C.P. transformation in the Fe-Mn-Cr-C system. Materials Science and Engineering 28, 99 – 107.