Effect of Strain Rate on the Extremely Low-Cycle Fatigue of Fe-15Mn-10Cr-8Ni-4Si Bidirectional-TRIP Steel

Extremely low-cycle fatigue tests were conducted on the Fe-15Mn-10Cr-8Ni-4Si bidirectional-TRIP (B-TRIP) steel, up to an axial total strain amplitude of 10%. The fatigue life was about five times longer than that of SUS316 in the range of total strain amplitude of 4% or more. The improved fatigue life of the Fe-15Mn-10Cr-8Ni-4Si was attributed to the reversible bidirectional γ↔ε transformation during fatigue deformation that might mitigate the fatigue damage. On the other hand, the fatigue life tended to decrease with increasing strain rate when the strain rate was varied from 0.1 to 2.5%/s with the total strain amplitude of 10%. The fractography revealed that the fracture surface varied significantly with strain rate. At low strain rates, the crystallographic fracture surface characterized by facets and secondary cracks were observed, whereas these features were not observed at high strain rates. EBSD measurements on the postmortem microstructure showed that frequent ε-martensite formation occurred at low strain rates, whereas martensitic transformation was suppressed at high strain rates. The change in the specimen surface temperature was evaluated in terms of the Gibbs free energy difference between γ-austenite and ε-martensite ΔGγ→ε, and the effect of strain rate on the extremely low-cycle fatigue was discussed from the viewpoint of the deformation mechanism as follows. At low strain rate, ΔGγ→ε ≲ 0 (negative close-to-zero ΔGγ→ε), the condition for B-TRIP to work effectively, is maintained over the entire life span. At high strain rate, the deformation mechanism changes to one in which γ-austenite is dominant due to the increase in ΔGγ→ε caused by self-heating; the fatigue damage mitigation mechanism by B-TRIP is less likely to occur at high strain rates, resulting in a reduction in life. © 2023 The Society of Materials Science, Japan.